window energy labeling in cooling season: fenestration … · 2016. 1. 19. · window uw heat...

LABORATORY OF BU ILD ING CONSTRUCT ION AND PHYS ICS -DEPARTMENT OF C IV IL ENG INEER ING

ARISTOTLE UNIVERSITY OF THESSALONIKI

Final report

TASK 1: Energy efficiency descriptors for labeling

Prof. Dimitris Bikas Katerina Tsikaloudaki

Thessaloniki, May 2009

WINDOW ENERGY LABELING IN COOLING SEASON: Fenestration & glazed structures

CONTENTS

Introduction 4

Chapter 1. Thernal transmittance coefficient: Definition - estimation 6 Chapter 2. Solar energy transmittance coefficient: Definition - estimation 18 Chapter 3. Visible transmittance coefficient: Definition - estimation 25 Chapter 4. Heating Performance Index: Definition - estimation 27 Chapter 5. Cooling Performance Index: Definition - estimation 30 Chapter 6. Daylight potential 33

4

INTRODUCTION

The purpose of a labeling scheme is to promote the use of thermally efficient products in certain climate zones within Europe (to reduce heat loss through the windows) or to reduce the need for artificial cooling systems (due to overheating) in other zones.

The thermal and optical behavior of a fenestration system can be described with the following parameters:

• Thermal transmittance coefficient or heat transfer coefficient (U-value),

• Solar transmittance coefficient or solar heat gain coefficient (g value) - Shading coefficient (SC)

• Visual transmittance ( vτ ).

These parameters apply both for glazing and windows and are defined in standards presented in Table 1.

Table 1. The conventional descriptors of thermal and optical performance of a fenestration system and the respective standards for their estimation

Element Parameter Description Standard

Glazing Ug Heat transfer coefficient EN 673, ISO 10077-1

gg Solar energy transmittance EN 410, ISO 9050

gvis ,τ Visible transmittance EN410, ISO 9050

Window Uw Heat transfer coefficient EN ISO 10077-1, -2, ISO 15099

gW Solar energy transmittance ISO 15099

wvis ,τ Visible transmittance ISO 15099

However, the energy performance of a window cannot be isolated from the performance of the building. Especially in cooling dominated climates, control of excessive solar heat gains is desirable, leading to fenestration with low g-values; for conventional glazings, visual transmittance would range in low levels, as well, and consequently, the use of artificial lighting instead of daylighting would increase the internal heat gains and finally the cooling loads. It is evident that a holistic approach is essential and therefore, descriptors, which characterize the impact of the fenestration systems on the energy performance of the building, are needed with regards to their thermal behavior and daylight potential.

Energy performance of the fenestration products is expressed through energy performance indices, EP, one representative of the heating season and one representative of the cooling season: The EPH,w, and EPC,w values are described in ISO 18292 and illustrate the energy needs per unit area of the window per year; that is the contribution of window to the energy need of the reference building for heating and cooling.

More specifically, heating energy performance is defined as the sum of the net heat loss through a window (w) for the heating mode, per m2 of (projected) window area Aw, per month, expressed in kWh/m2:

∑=

=12

1

,,,,,,

m

imwndHiwH qEP 1

where: EPH,,w,i is the energy performance value of the window facing i-orientation for the heating period, expressed in

kWh/m2 QH,nd,w,m is the net heat loss through the window, for the heating mode, per m2 of window area Aw, per month m,

expressed in kWh/m2 i is the orientation of the window w

5

m month

The cooling Energy Performance Index (EPc,w) is defined as the sum of the net heat loss through the window for the cooling mode over a period of time:

∑=

=12

1

,,,,,,

m

imwndCiwC qEP 2

where: EPC,w,i is the energy performance value of the window facing i-orientation for the cooling season, expressed in

kWh/m2 qC,nd,w,m is the net heat loss through the window, for the cooling more, per m2 of window area Aw, per month m,

expressed in kWh/m2

The parameters that describe the energy behavior of the fenestration products, as well as their impact on the building performance are presented in the following chapters in detail.

1. Thermal transmittance coefficient

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1. THERMAL TRANSMITTANCE COEFFICIENT DEFINITION - ESTIMATION

The thermal transmittance coefficient (U-value) is a measure of the heat transfer characteristics of a fenestration product under specific environmental conditions. When multiplied by the interior-exterior temperature difference and by the projected fenestration product area yields the total heat transfer through the fenestration product due to conduction, convection and infrared radiation. The U-Factor is the heat transmission in a unit time through a unit area of a test specimen and its boundary air films, induced by a unit temperature difference between the environments on each side in W/m2K (Btu/h·ft2·ºF).

In a window assebly, the following values should be determined:

• Center-of-glazing U-Factor (Uc): the U-Factor representative of the center-ofglazing area.

• Divider U-Factor (Ud): the U-Factor representative of the divider area.

• Edge-of-divider U-Factor (Ude): the U-Factor representative of the edge-ofdivider area.

• Edge-of-glazing U-Factor (Ue): the U-Factor representative of the edge-ofglazing area.

• Frame U-Factor (Uf): the U-Factor representative of the frame and sash area.

The total properties for window and door products are calculated by combining the various component properties weighted by either their respective projected areas or visible perimeter. The total properties are each based on total projected area occupied by the product, Aw. The projected component areas and the visible perimeter are shown in Figure 1.1.

a

b

Figure 1.1. Illustration of glazed area and perimeter of a window [ISO 10077-1].


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The procedure for calculating thermal transmittance is described in the following paragraphs. The effect of three-dimensional heat transfer in frames and glazing units is not considered.

The thermal transmittance of the fenestration product is given by:

w

ggffgg

wA

lUAUAU

∑ ∑ ∑ Ψ×+×+×= , where: 1.1. Ag is the projected vision area Af is the projected frame area Aw is the window area lg is the length of the vision area perimeter Ψg is a linear thermal transmittance that accounts for the interaction between frame and glazing or the

interaction between frame and opaque panel.

Figure 1.2. The various areas of a window component [ISO 10077-1].


8

The window area Aw is the sum of the frame area Af and the glazing area Ag. The frame area and the glazed area are defined by the edge of the frame, i.e. sealing gaskets are ignored for the purposes of determination of the areas. Figure 1.2. shows in detail the various areas of a window.

1.1. Thermal transmittance of the single glazing

The thermal transmittance of the single and laminated glazing, Ug, shall be calculated using equation 1.2.

∑ ++=

j

si

j

j

se

g

Rd

R

U

λ

1, where: 1.2.

Rse is the external surface resistance. Typical values of Rse are given in Table 1.1.

λj is the thermal conductivity of glass or material layer j. In the absence of specific information for the glass concerned the value λ=1.0W/m2K should be used.

dj is the thickness of the glass pane or material layer j

Rsi is the internal surface resistance. Typical values of Rsi are given in Table 1.1.

1.2. Thermal transmittance of multiple glazing

The thermal transmittance of multiple glazing Ug can be calculated by the equation 1.3.:

∑ ∑ +++=

j

si

j

js

j

j

se

g

RRd

R

U

,

1

λ

, where: 1.3.

Rse is the external surface resistance. Typical values of Rse are given in Table 1.1.

λj is the thermal conductivity of glass or material layer j

dj is the thickness of the glass pane or material layer j

Rsi is the internal surface resistance. Typical values of Rsi are given in Table 1.1.

Rs,j is the thermal resistance of air space j.

Typical values of the thermal resistance Rs of air spaces for double glazing are given in Table 1.2. the data apply for vertical windows, for spaces filled with air, with both sides uncoated or with one side coated with a low-emissivity layer and for a mean temperature of the glazing of 283K and a temperature difference of 15K between the two outer glazing surfaces. For triple glazing or for inclination other than vertical the procedure described in EN673 should be used.

For wider air layers in double windows or doors the calculation according to EN673 does not lead to correct results. For such cases more detailed equations are given in ISO 15099, or numerical calculation methods or measurements can be used.

Table 1.3. gives the thermal transmittance, Ug, of double and triple glazing filled with different gases, calculated in accordance with EN673. The values of the thermal transmittance in the table apply to the emissivities and gas

Table 1.1. the internal and external surface thermal resistances.

Window position Rsi [m2K/W] Rse[m2K/W]

Vertical, or inclination a of the glazing to the horizontal, such that 90o≥a≥60o (heat flow direction ±30o from the horizontal plane)

0.13 0.04

Horizontal, or inclination a of the glazing to the horizontal, such that 60o≥a≥0o (heat flow direction more than 30o from the horizontal plane)

0.10 0.04


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concentration given. For individual glazing units, the emissivity and or gas concentrations can change with time. Procedures for evaluating the effect of ageing on the thermal properties of glazed units are given in EN1279-1 and EN1279-3.

1.3. Thermal transmittance of windows with closed shutters

A shutter on the outside of a window introduces and additional thermal resistance, resulting from both the air layer enclosed between the shutter and the window, and the shutter itself (Figure 1.3.). The thermal transmittance of a window with closed shutters Uws is given by equation 1.4.

RU

U

w

ws

∆+=

1

1, where: 1.4.

Uw is the thermal transmittance of the window

∆R is the additional thermal transmittance due to the air layer enclosed between the shutter and the window and the closed shutter itself.

Table 1.2. The thermal resistance Rs of air spaces between double glazing.

Thermal resistance Rs One side coated with a normal emissivity of

Thickness of air space

[mm] 0.1 0.2 0.4 0.8 Both sides uncoated

6 0.211 0.191 0.163 0.132 0.127 9 0.299 0.259 0.211 0.162 0.154 12 0.377 0.316 0.247 0.182 0.173 15 0.447 0.364 0.276 0.197 0.186 50 0.406 0.336 0.260 0.189 0.179

Table 1.3. Thermal transmittance of double and triple glazing filled with different gases for vertical glazing.

Glazing Thermal transmittance for different types of gas spaces

Type Glass Normal

emissivity Dimensions Air Argon Krypton SF6 Xenon

4-6-4 3..3 3.0 2.8 3.0 2.6 4-8-4 3.1 2.9 2.7 3.1 2.6 4-12-4 2.8 2.7 2.6 3.1 2.6 4-16-4 2.7 2.6 2.6 3.1 2.6

Uncoated glass

0.89

4-20-4 2.7 2.6 2.6 3.1 2.6 4-6-4 2.7 2.3 1.9 2.3 1.6 4-8-4 2.4 2.1 1.7 2.4 1.6 4-12-4 2.0 1.8 1.6 2.4 1.6 4-16-4 1.8 1.6 1.6 2.5 1.6

One pane coated glass

≤0.2

4-20-4 1.8 1.7 1.6 2.5 1.7 4-6-4 2.6 2.3 1.8 2.2 1.5 4-8-4 2.3 2.0 1.6 2.3 1.4 4-12-4 1.9 1.6 1.5 2.3 1.5 4-16-4 1.7 1.5 1.5 2.4 1.5


≤0.15

4-20-4 1.7 1.5 1.5 2.4 1.5 4-6-4 2.6 2.2 1.7 2.1 1.4 4-8-4 2.2 1.9 1.4 2.2 1.3 4-12-4 1.8 1.5 1.3 2.3 1.3 4-16-4 1.6 1.4 1.3 2.3 1.4


≤0.10

4-20-4 1.6 1.4 1.4 2.3 1.4 4-6-4 2.5 2.1 1.5 2.0 1.2 4-8-4 2.1 1.7 1.3 2.1 1.1 4-12-4 1.7 1.3 1.1 2.1 1.2 4-16-4 1.4 1.2 1.2 2.2 1.2

Double glazing


≤0.05

4-20-4 1.5 1.2 1.2 2.2 1.2


10

Figure 1.3. Definition of air gaps [ISO 10077-1].

∆R depends on the thermal transmission properties of the shutter and on its air permeability. When the thermal resistance of the shutter itself, Rsh is known, the additional thermal resistance ∆R can be obtained using the appropriate expression in Table 1.4., depending on the permeability of the shutter.

Average air permeability applies typically to solid wing shutters, wooden Venetian shutters with solid overlapping slats, roller shutters with connecting slats made of wood, plastic or metal.

Table 1.5. gives some typical values of shutter thermal resistance and the corresponding values of ∆R, which can be used in the absence of values of Rsh obtained from measurement or calculation.

For the different types of shutters, the permeability criterion can be expressed in terms of an effective total gap, bsh, between the shutter and its surround as given in equation 1.5.

Bsh=b1+b2+b3, where: 1.5.

b1, b2, b3 are the average edge gaps at the bottom, top and side on the shutter. b3 is included for one side only, `since gaps at the side influence the permeability less than the gaps at the top and bottom.

Table 1.6. characterizes the relationship between permeability and effective edge gap between shutter and its surround. For permeability classes 2 and above, there should be no openings within the shutter itself. For shutters of tight permeability the following criteria apply:

- Roller shutters: the edge gaps at the sides and the bottom are considered equal to 0 if strip gaskets are supplied in the guide rails and the final lath, respectively. The gap at the top os considered equal to 0 if the entrance to the roller shutter box is fitted with lips or brush-type joints on both sides of the shutter, or if the end of the shutter is pressed by a device against a sealing material at the inner surface of the outer side of roller shutter box.

- Other shutters: effective presence of strip gaskets on three sides and the gap at the fourth side less than 3mm.


11

Table 1.4. The additional thermal resistance for windows with closed shutters.

Air permeability of shutter Additional thermal resistance ∆R

Very high 0.08

High 0.25Rsh+0.09

Average 0.55Rsh+0.11

Low 0.80Rsh+0.14

Tight 0.95Rsh+0.17

Table 1.5. Additional thermal resistance for windows with closed shutters.

Additional thermal resistance at specific air permeability of the shutters ∆R

[m2K/W] Shutter type

Typical thermal resistance of shutter

Rsh [m2K/W]

high or very high air permeability

average air permeability

Tight or low air permeability

Roller shutters of aluminium

0.01

Roller shutters of wood and plastic without foam filling

0.10

Roller shutters of plastic with foam filling

0.15

Shutters of wood, 25mm to 30mm thickness

0.20

Table 1.6. Relationship between permeability and effective total edge gap between shutter and its surround.

Class Air permeability of shutter bsh

1 Very high bsh≥35

2 high 15≤ bsh ≤35

3 Average 8≤ bsh ≤15

4 Low bsh ≤8

5 Tight 1bsh ≤3 and b1+b3=0 or b2+b3=0

An alternative method to establish that a shutter is class 5 is to verify by measurement that the air flow through the shutter does not exceed 10m3/(hm2) under a pressure drop of 10Pa.

1.4. Thermal transmittance of doors

1.4.1. fully glazed doors

the thermal transmittance, UD, of a door set of which the door leaf is fully glazed is obtained using the equation 1.6.

∑ ∑∑ ∑ ∑

+

Ψ×+×+×=

fg

ggffgg

DAA

lUAUAU , where: 1.6.

Ug is the thermal transmittance of the glazing

Uf is the thermal transmittance of the frame

Ψg is the linear thermal transmittance due to the combined thermal effects of glazing spacer and frame.


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1.5. Thermal transmittance of frame

The preferred methods for establishing values of thermal transmittance of frames are numerical calculation methods (e.g. finite element, finite difference, boundary element) in accordance with ISO 10077-2 and direct measurements using hot-box methods in accordance with EN12412-2.

Typical values for common types of frames are given in table 1.7. and figures 1.4 and 1.6., which can be used in the absence of specific measured or calculated information for the frame concerned. They are based on a large number of measured values as well as mathematically evaluated values determined using numerical calculation methods. The values of Uf in Table 1.7. and Figures 1.4 and 1.6. cannot be used for sliding windows.

1.5.1. Plastic frames

Table 1.7. gives approximate values for plastic frames with metal reinforcements. If no other data is available, the values can also be used for frames without metal reinforcements.

Table 1.7. Thermal transmittances for plastic frames with metal reinforcements.

Frame material Frame type Uf Polyourethane With metal core

Thickness of PUR≥5mm 2.8

Two hollow chambers

External internal

2.2

PVC hollow profiles Three hollow chambers

external internal

2.0

Figure 1.4. Thermal transmittances for wooden frames and metal-wood frames depending on the frame thickness df.

[ISO 10077-1].


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1.5.2. Wood frames

Values for wood frames can be taken from Figure 1.4. For Uf the values correspond to a moisture content of 12%. The definition of the thickness of frame is presented in Figure 1.5.

Internal: right-hand of frame section

2

21 ddd f+

=

External: left hand side of frame section

2

2

1 ∑≥

+

= jj

f

dd

d

Figure 1.5. Definition of the frame thickness df for various window systems [ISO 10077-1].

1.5.3. Metal frames

The thermal transmittance of metal frames can be determined by measurement using hot box methods in accordance with EN 12412-2 or by numerical calculation in accordance with ISO 10077-2.

Figure 1.6. Values of Rf for metal frames with thermal break [ISO 10077-1].


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If such data are not available, values of Uf can be obtained by the following procedure:

- Metal frames without a thermal break.

- Metal frames with thermal breaks corresponding to the sections illustrated in Figures 1.7. and 1.8., subject to restrictions on the thermal conductivity and widths of the thermal breaks.

For metal frames without a thermal break, Rf=0

For metal frames with thermal breaks, Rf is estimated with the help of Figure 1.6.

The thermal transmittance Uf is given by the equation 1.7.:

defefsedififsi

fAARAAR

U,,,, //

1

+= , where: 1.7.

Af,di, Af,de, Af,i, Af,e are the areas described in Figures 1.1 and 1.2.

Rsi is the appropriate internal surface resistance of the frame

Rse is the appropriate external surface resistance of the frame

Rf is the thermal resistance of the frame section

1.6. Linear thermal transmittance of frame/glazing junction

The thernak transmittance of glazing Ug us applicable to the central area of the glazing and does not include the effect of the glass spacers at the edge of the glazing. On the other hand, the thermal transmittance of the frame, Uf, is applicable in the absence of glazing. The linear thermal transmittance Ψg describes the additional heat conduction due to the interaction between frame, glazing and spacer, and is affected by the thermal properties of each of these components.

Figure 1.7. Section type 1 – Thermal break with a thermal conductivity less than 0.3W/mk [ISO 10077-1].

Figure 1.8. Section type 2 – Thermal break with a thermal conductivity less than 0.2W/mK [ISO 10077-1].


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The preferred method of establishing values of linear thermal transmittance is by numerical calculation in accordance with ISO 10077-2. Default values of Ψg for typical combinations of frames, glazing and spacers that can be used when the results of a detailed calculation are given in Table 1.8. for aluminium and steel spacers. In the case of thermally improved spacers, the linear thermal transmittance for glazing spacer bars is presented in Table 1.9.

It is worth mentioning that a thermally improved spacer is defined by the following criterion in equation 1.8.:

∑ ≤× 007.0)( λd , where: 1.8 d is the thickness of the spacer wall, expressed in m

λ is the thermal conductivity of the spacer material, expressed in W/Mk

The summation applies to all heat flow paths parallel to the principal heat flow direction, the thickness d being measured perpendicular to the principal heat flow direction.

When more detailed computation is needed, the environmental conditions should be taken into account. Moreover, the thermal transmittance of the glazed area can be found by simulating a simple environmental condition involving internal/external temperature difference, with or without incident solar radiation. Without solar radiation, the thermal transmittance is the reciprocal of the total thermal resistance.

t

gvR

U1

= 1.9

Rt is found by summing the thermal resistances at the external and internal boundaries and the thermal resistances of glazing cavities and glazing layers (Figure 1.9.).

Table 1.8. Values of linear thermal performance for common types of glazing spacer bars

Linear thermal transmittance for different types of glazing Ψg FRAME TYPE Double or triple glazing

uncoated glass air-or gas-filled

Double or triple glazing Low emissivity glass

air-or gas-filled Wood or PVC 0.06 0.08 Metal with thermal break 0.08 0.11 Metal without thermal break 0.02 0.05

Table 1.8. Values of linear thermal performance for common types of glazing spacer bars

Linear thermal transmittance for different types of glazing Ψg FRAME TYPE Double or triple glazing

uncoated glass air-or gas-filled

Double or triple glazing Low emissivity glass

air-or gas-filled Wood or PVC 0.05 0.06 Metal with thermal break 0.06 0.08 Metal without thermal break 0.01 0.04

Figure 1.9. Numbering system for glazing system layers [ISO 15099].


16

∑ ∑= =

+++=n

i

n

i

igvi

ex

th

RRh

R2 1 int

,

11, where 1.10

Rgv,i is the thermal resistance of the i-th glazing, equal to igv

igv

igv

TR

,

,

, λ=

Ri is the thermal resistance of the i-th space, where the first space is external environment, the last space

is internal environment and the spaces in between are glazing cavities, i

ibif

iq

TTR

1,, −−= , 1.11

where Tf,i and Tb,i-1 are the external and internal facing temperature of the i-th glazing layer.

When solar radiation is considered, then:

neni

I

gvTT

qU s

−= = )0int( , where: 1.12

qint(Is=0) is the net density of heat flow rate through the window or door system to the internal environment for the specified conditions, but without incident solar radiation, in W/m2

Tni is the internal environmental temperature

Tne is the external environmental temperature

The environmental temperature is a weighted average of the ambient air temperature and the mean radiant temperature Trm, which is determined for external and internal environment boundary conditions.

rcv

rmraicv

nhh

ThThT

+

+= , where 1.13

hcv the surface convective heat transfer coefficients. In winter conditions the internal and external surface convective heat transfer coefficients are equal to 3.6W/m2K and 20W/m2K respectively. In summer conditions the internal and external surface convective heat transfer coefficients can be regarded equal to 2.5W/m2K and 8W/m2K respectively.

hr the surface radiant heat transfer coefficients.

int,,

4

int,

4

int,int,

int,

)(

rminS

rmSS

rTtT

TTh

−

−=

σε, where: 1.14

εs,int total hemispherical emissivity of the glazing/frame

σ Stephan-Boltzman constant, 5.669 x 10-8

Ts,int internal surface temperature

Trm,int mean radiant temperature of the internal surfaces.

In order to convert the results of a two dimensional numerical analysis to thermal transmittance, it is necessary to record the rate of heat transfer from the internal environment to the frame and edge-glass surfaces (in the absence of solar radiation). The linear thermal transmittance Ψ values and frame thermal transmittances shall be calculated according to the following equations.

gvgvff

DlUlUL −−=Ψ 2 , where: 1.15

L2D is the thermal coupling coefficient determined from the actual fenestration system

f

pp

D

p

fl

lULU

−=

2

, where: 1.16

Lp2D is thermal coupling coefficient determined from the frame/panel insert system


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Up is the thermal transmittance of foam insert

lp is the internal side exposed length of foam insert

lf is the internal side projected length of the frame section

lgv is the internal side projected length of the glass section.

REFERENCES

ISO 10077.01. Thermal performance of windows, doors and shutters – calculation of thermal transmittance – Part 1: General.

EN ISO 10077-02: 2003. Thermal performance of windows, doors and shutters – calculation of thermal transmittance – Part 2: Numerical method for frames.

ISO 15099:2003. Thermal performance of windows, doors and shading devices – detailed calculations.

2. Solar energy transmittance coefficient

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2. SOLAR ENERGY TRANSMITTANCE DEFINITION - ESTIMATION

The solar energy transmittance (g) or the solar heat gain coefficient (SHGC) is defined as the ratio of the solar heat gain entering the space through the fenestration product to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and that portion of the absorbed solar radiation which is then reradiated, conducted or convected into the space.

The total solar energy transmittance of the total fenestration product is equal to:

t

ffgg

sA

AA∑ ∑+= τττ , where: 2.1

gτ is the solar energy transmittance of the vision area (glazing)

fτ is the solar energy transmittance of the frame area.

The above mentioned equation includes the assumption that the solar energy transmittance of the edge of the glass is the same as that of the centre of the glass area.

2.1. Determination of characteristic parameters of glazing and optical measurements

The characteristic parameters of the glazing are determined for quasi-parallel, almost normal radiation incidence. For the measurements, the samples shall be irradiated by a beam whose axis is at an angle not exceeding 10o from the normal to the surface. The angle between the axis and any ray of the illuminating beam shall not exceed 5o.

The characteristic parameters are as follows:

- The spectral transmittance τ(λ), the spectral reflectance ρο(λ) and the spectral internal reflectance ρi(λ) in the wavelength range of 300nm to 2500nm.

- The light transmittance τv, the external light reflectance ρv,o and the internal light reflectance ρv,i for illuminant D65

- The solar direct transmittance g

- The UV-transmittance τuv

- The general color rendering index Ra.

Optical measurements in transmission and reflection require special care and much experimental experience to achieve accuracy in transmittances and reflectance of about ±0.01.

Commercial spectrophotometers (with or without integrating spheres) are affected by various sources of inaccuracy when using reference materials obtained from metrological laboratories.

The wavelength calibration shall be performed by measuring glass plates or solutions which feature relatively sharp absorption bands at specified wavelengths. The photometric linearity shall be checked using grey filters with a certified transmittance.

For reflectance measurements reference materials having a reflection behavior similar to the unknown sample shall be selected.

Thick samples can modify the optical path of the instrument’s beam as compared to the path in air and therefore the sample beam hits an area of the detector having a different responsivity.


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A similar source of inaccuracy occurs in case of samples with significant wedge angles which deflect the transmitted beams. It is recommended to check the reproducibility by repeating the measurement after rotating the sample.

Additionally, in the case of reflectance measurements glass sheets cause a lateral shear of the beam reflected by the second surface, causing reflectance losses. This source of inaccuracy shall be taken into account particularly in the case of reflectance measurements through the uncoated side. In order to quantify and correct systematic errors, it is recommended to use calibrated reflectance standards with a thickness similar to the unknown sample.

In the case of diffusing samples, transmittance and reflectance measurements shall be performed using integrating spheres, whose openings are sufficiently large to collect the entire diffusely transmitted or reflected beam. The sphere diameter shall be adequate and the internal surface adequately coated with a highly diffusing reflectance material, so that the internal area can provide the necessary multiple reflections. Reference materials with characteristics similar to the unknown sample as specified above shall be used.

If the transmittance or reflectance curve recorded by the spectrometer exhibits a high level of noise for some wavelengths, the values to be considered for those wavelengths should be obtained after a smoothing of the noise.

2.2. Estimation of the total energy transmittance g

The total energy transmittance g is the sum of the solar direct transmittance τe and the secondary heat transfer factor qi towards the inside, the latter resulting from heat transfer by convection and longwave IR-radiation of that part of the incident solar radiation which has been absorbed by the glazing:

g=τe+qi 2.2

The incident solar radiant flux per unit area φe is divided into the following three parts (Figure 2.1. 1):

- The transmitted part τeφe

- The reflected part ρeφe

- The absorbed part aeφe, where:

τe is the solar direct transmittance

ρe is the solar direct reflectance

ae is the solar direct absorptance.

The relationship between the three characteristics is

τe+ + ρe + ae = 1 2.3

The absorbed part aeφe is subsequently divided into two parts qiφe and qeφe, which are energy transferred to the inside and outside respectively:

ae=qi+qe, where: 2.4

qi is the secondary heat transfer factor of the glazing towards the inside

qe is the secondary heat transfer factor of the glazing towards the outside.


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Figure 2.1. Division of the incident radiant flux for a double glazing unit [ISO 15099].

The solar direct transmittance τe of the glazing shall be calculated using the following formula:

∑

∑

=

=

∆

∆=

2500

300

2500

300

)(

nm

nm

nm

e

S

S

λλ

λλ

λ

λλττ , where: 2.5

Sλ is the relative spectral distribution of the solar radiation

τ(λ) is the spectral transmittance of the glazing

∆λ and integration procedure are

The corresponding vales Sλτ∆λ are given in Table 2.1.

It must be mentioned that contrary to real situations, it is always assumed that the solar radiation strikes the glazing as a beam and almost at normal incidence. In the case of oblique incidence of radiation, the solar direct transmittance of glazing and the total solar energy transmittance are both somewhat reduced. The solar control effect becomes greater in the case of oblique incidence of radiation.

For the calculation of the secondary heat transfer factor towards the inside qi, the heat transfer coefficients of the glazing towards the outside he and towards the inside hi are needed. These values depend mainly on the position of the glazing, wind velocity, inside and outside temperatures and furthermore on the temperature of the two external glazing surfaces.

The following conventional conditions have been adopted for simplicity:

- Position of the glazing: vertical

- Outside surface: wind velocity approximately 4m/s; corrected emissivity 0.837

- Inside surface: natural convection; emissivity optional

- Air spaces are unventilated.


21

Table 2.1. Normalized relative spectral distribution of global radiation [ISO 15099]..


22

Under these conventional, average conditions, standard values of he and hi are obtained:

he=23 W/m2K

hi=3.6+4.4εi/0.837 W/m2K, where: 2.6

εi is the corrected emissivity of the inside surface (for soda lime glass εi=0.837 and hi=8W/m2K). It is defined and measured according to ISO 10292.

The secondary heat transfer coefficient factor towards the inside, qi, of single glazing shall be calculated using the following formula:

ie

i

eihh

haq

+= , where: 2.7

ae is the solar direct absorptance

he, hi are the heat transfer coefficients towards the outside and inside respectively

The secondary heat transfer coefficient factor towards the inside, qi, for double glazing shall be calculated using the following formula:

Λ++

Λ+

+

=111

221

ei

e

e

ee

i

hh

a

h

aa

q , where: 2.8

ae1 is the solar direct absorptance of the outer pane within the double glazing

ae2 is the solar direct absorptance of the second pane within the double glazing

Λ is the thermal conductance between the outer surface and the innermost surface of the double glazing (Figure 2.2.). It shall be determined for a temperature difference of ∆Τ=15oC across the sample and a mean temperature of the sample of 10oC or by measuring methods using the guarded hot-plate method or the heat flow meter method.

he, hi are the heat transfer coefficient factors towards the outside and the inside respectively.

Figure 2.2. Illustration of the meaning of thermal conductance Λ [ISO 15099].


23

The total solar energy transmitted into the room per unit area of glazing φei is given by the relationship:

φei=φeg, where: 2.9

φe is the incident solar radiation flux per unit area, obtained from appropriate tables in meteorological literature

g is the total energy transmittance of the glazing.

If the room temperature Ti differs from the outside temperature To, an additional heat transfer occurs in addition to φei. This additional heat flow qz can be calculated as follows:

Qz=U(Το-Τι), where: 2.11

U is the U-value of glazing

If more detailed calculations are needed, then the total solar energy transmittance of the vision area can be determined for conditions involving internal/external temperature difference and any level of incident solar radiation. It is found by calculating the difference between the net heat flow rate into the internal environment with and without incident solar radiation.

s

Is

sI

gq )0int(int =−=τ , where 2.12

qint is the net density of heat flow rate through the window or door system to the internal environment for the specified conditions, in W/m2

qint(Is=0) is the net density of heat flow rate through the window or door system to the internal environment for the specified conditions, but without incident solar radiation, in W/m2

Longwave radiative exchange between glazing layers and conductive heat transfer within each glazing layer can be described using fundamental relations. Calculations dealing with convective heat transfer depend upon correlations based on experimental data.

Figure 2.3 shows the ith glazing in a sloped multilayer array. The values of four variables are sought at each glazing. These are the temperatures of the external and internal facing surfaces, Tft,i and Tb,i plus the radiant heat leaving the front and back facing surfaces, Jft,i and Jb,i. In terms of these variables, qi is:

1,,1,, ),( −− −+−×= ibiftibiicvi JJTTfthq 2.13

The solution is generated by applying the following four equations at each glazing:

1++= iii qSq 2.14

1,,,

4

,, , −++= ibiftiftiiftftift JrJTiJ τσε 2.15

1,,,

4

,,, +++= iftibibiibibib JrJTJ τσε 2.16

)2(2

1

,

,

,, ii

igv

igv

iftib Sqt

TT +×=− +λ 2.17

The effect of boundary conditions imposed by the environment on the window shall be specified. The internal and external temperatures Tft,n+1 and Tb,0 are:

int,1, ainft TT =+ 2.18

exaib TT ,0, = 2.19

The effect of long wave irradiance at internal and external glazing surfaces is included by setting

int,1, gvnft EJ =+ and exgvb EJ ,0, = , where 2.20


24

The effect of the convective heat transfer coefficients at the glazing surfaces is included by setting

exhcvh

hh

cv

cvncv

,1,

int,1,

=

=+ 2.21

Frame total solar energy transmittance

The frame total energy transmittance shall be calculated using the appropriate equation:

ex

f

s

f

ff

hA

A

Ua=τ , where: 2.22

As is the developed surface area

The external surface heat transfer coefficient at the frame, hex, is hex=hcv,ex+hr,ex.

Figure 2.3. Energy balance on glazing layer i [ISO 15099] .

REFERENCES

ISO 10077.01. Thermal performance of windows, doors and shutters – calculation of thermal transmittance – Part 1: General.

EN ISO 10077-02: 2003. Thermal performance of windows doors and shutters – calculation of thermal transmittance – Part 2: Numerical method for frames.

ISO 15099:2003. Thermal performance of windows, doors and shading devices – detailed calculations.

3. Visible transmittance coefficient

25

3. VISIBLE TRANSMITTANCE DEFINITION - ESTIMATION

The light transmittance (τv) or the visible transmittance (VT) is defined as the ratio of the visible light entering the space through the fenestration product to the incident visible light. The visible light entering a space is weighted by the photopic response of the eye.

The visible transmittance of the total fenestration product is equal to:

t

gvv

tA

A∑= ττ 3.1

The light transmittance τv of the glazing shall be calculated using the following formula:

∑

∑

∆

∆=

nm

nm

nm

nm

v

VD

VD

780

380

780

380

)(

)()(

λλ

λλλττ

λ

λ

3.2

Dλ is the relative spectral distribution of illuminant D65

τ(λ) is the spectral transmittance of the glazing

V(λ) is the spectral luminous efficacy for photopic vision defining the standard observer for photometry

∆λ is the wavelength interval

Table 3.1. indicates values for DλV(λ)∆λ for wavelenght intervals of 10nm.

In the case of multiple glazing, the spectral transmittance τ(λ) shall be obtained by calculation from the spectral characteristics of the individual components. Alternatively measurements on non-diffusing multiple units may be performed using an integrating sphere. This may be achieved after reducing the interspaces under conditions that allow the collection of the whole transmitted beam.

The calculation of the spectral transmittance τ(λ) shall be performed using methods such as algebraic manipulation, embedding techniques or by recursion techniques. For the calculation of τ(λ) as well as for the calculation of spectral reflectance the following symbols for the spectral transmittance and spectral reflectance of the individual components are used:

τ1(λ) is the spectral transmittance of the outer (first) pane

τ2(λ) is the spectral transmittance of the second pane

τn(λ) is the spectral transmittance of the nth (inner) pane

ρ1(λ) is the spectral reflectance of the outer (first) pane

ρ’1(λ) is the spectral reflectance of the outer (first) pane, measured in the opposite direction of incident radiation,

ρ2(λ) is the spectral reflectance of the second pane

ρ’2(λ) is the spectral reflectance of the second pane, measured in the opposite direction of incident radiation,

ρn(λ) is the spectral reflectance of the nth (inner) pane

ρ’n(λ) is the spectral reflectance of the nth pane, measured in the opposite direction of incident radiation.

For the spectral transmittance τ(λ) as a function of the spectral characteristics of the individual components of the unit, the following formulae are obtained.

3. Visible transmittance coefficient

26

For double glazing:

)()('1

)()()(

21

21

λρλρλτλτ

λτ−

= 3.3

For triple glazing:

)()(')()]()('1)][()('1[

)()()()(

3122

3221

321

λρλρτλρλρλρλρλτλτλτ

λτl−−−

= 3.4

For multiple glazing with more than three components relationships are found to calculate τ(λ) of such glazing from the spectral characteristics of the individual components.

REFERENCES

ISO 9050:2003. Glass in building – Determination of light transmittance, solar direct transmittance, solar energy transmittance, ultraviolet transmittance and related glazing factors.

4. Heating performance index

27

4. HEATING PERFORMANCE INDEX DEFINITION - ESTIMATION

The window energy performance for heating is the annual sum of the monthly contributions or the seasonal weighted average for the seasonal utilization factor method, from the window to the energy need for space heating.

The heating performance index can be calculated using the following equations:

Monthly method: ∑=

=12

1

,,,,,

m

imwndHwH qEP 4.1

Seasonal method: seaswndHwH qEP ,,,, = 4.2

Hourly method: ∑= iwndHwH qEP ,,,, 4.3

EPH,w,i is the energy performance value of the window facing i-orientation for the heating season

i is the orientation of the window

qH,nd,w,m is the net heat loss through the window, for the heating mode per m2 of window area Aw per month m

For situations where windows are places in more than one position:

i

i

iwH

wHN

EP

EP

∑=

,,

, , where: 4.4

Ni is the total number of orientations

The monthly contributions from the window to the energy needs for space heating are calculated according to:

)( ,,,,,,, wgnHwhtHmHwndH qfq η−= 4.5

qH,nd, w is the heat loss through the window for the heating mode per m2 of window area Aw

qH,ht, w is the overall heat transfer by transmission and infiltration through the window for the heating mode

qH,gn, w is the overall solar heat gain through the window for the heating mode

fH,m is the fraction of the month that is part of the heating season

ηH,gn dimensionless gain utilization factor for heating.

Values for the gain utilization factor for heating are obtained by curves depending on the heat balance ration for heating,γH and the time constant of the building (Figure 4.1.)

The heat balance ration for the heating mode γH is given by:

htH

gnH

HQ

Q

,

,=γ 4.6

The fraction of the month that is part of the heating season shall be calculated for each month m as

)( ,,,,

,,

,

mndCmndH

mndH

mHQQ

Qf

+= 4.7


28

Figure 4.1. Illustration of gain utilization factor for heating mode, for 8 hours, 1 day, 2 days, 1 week and infinite time constants, valid for monthly calculation method [ISO/CD 18292].

It is worth mentioning that since there are no internal heat gains coming in via the window, qH,gn, w is equal to qH,sol, w

4.1.1. Heat transfer by transmission and infiltration

The overall heat transfer by transmission and air leakage shall be calculated as:

( )100

,,int,

,,

,,

t

A

HUq avgeHset

w

wveH

wHwHht θθ −

+= , where: 4.8

UH,w is the thermal transmittance of the window for the heating mod

HH,ve,w is the heat transfer coefficient due to air leakage of the window, expressed in W/K

θint, set, H is equal to the set point temperature for heating expressed in oC

θe,avg is equal to the time-average external air temperature

t is the total length of the considered time period.

4.1.2. Solar heat gain

The overall solar heat gain through the window shall be calculated as:

100,,,,

tIgFq solwHobshwgnH = , where: 4.9

Fsh,ob is the factor due to glazing maintenance and shading effects during the heating season

gH,w is the dimensionless total solar energy transmittance of the window


Isol is the average solar irradiance for the considered time period on the window plane

4.1.3. Air infiltration and ventilation


29

Overall air infiltration (air permeability) rate L is measured for the whole fenestration product at the reference pressure difference and is denoted Lp,ref. The procedure to measure Lp,ref is specified in EN1026 or ISO 6613.

The heat transfer coefficient due to air infiltration of the window is given by:

refp

p

ref

wve LC

p

pH ,

3

2

,6.3

∆∆

=ρ

, where: 4.10

Hve,w is the heat transfer coefficient due to air infiltration of the window

∆p is the average pressure difference in the building, expressed in Pa (=6Pa)

ρCp is the thermal capacitance of air (=1.24kJ/m3K)

Lp,ref is the air leakage rate of the window at a pressure difference ∆pref

REFERENCE

ISO TC 163/SC 2: ISO/CD 18292. Energy performance of fenestrations systems – calculation procedure.

5. Cooling performance index

30

5. COOLING PERFORMANCE INDEX DEFINITION - ESTIMATION

The window energy performance for cooling is the annual sum of the monthly contributions or the seasonal weighted average for the seasonal utilization factor method, from the window to the energy need for space cooling.

The heating performance index can be calculated using the following equations:

Monthly method: ∑=

=12

1

,,,,,

m

imwndCwC qEP 5.1

Seasonal method: seaswndCwC qEP ,,,, = 5.2

Hourly method: ∑= iwndCwC qEP ,,,, 5.3

EPC,w,i is the energy performance value of the window facing i-orientation for the cooling season

i is the orientation of the window

qC,nd,w,m is the net heat loss through the window, for the heating mode per m2 of window area Aw per month m

For situations where windows are places in more than one position:

i

i

iwC

wCN

EP

EP

∑=

,,

, 5.4

The monthly contributions from the window to the energy needs for space cooling are calculated according to:

))(1( ,,,,,,,, whtClsCwgnCmHwndC qqfq η−−= , where: 5.5

qc,nd, w is the heat loss through the window for the cooling mode per m2 of window area Aw

cht, w is the overall heat transfer by transmission and infiltration through the window for the cooling mode

qc,gn, w is the overall solar heat gain through the window for the cooling mode

fc,m is the fraction of the month that is part of the cooling season

ηc,ls dimensionless loss utilization factor for cooling.

Values for loss gain utilization factor for heating are obtained by curves depending on the heat balance ration for cooling,γH and the time constant of the building (Figure 5.1.)

The heat balance ration for the cooling mode γc is given by:

htC

gnC

CQ

Q

,

,=γ 5.6

The fraction of the month that is part of the cooling season shall be calculated for each month m as

)( ,,,,

,,

,

mndCmndH

mndH

mHQQ

Qf

+= 5.7

It is worth mentioning that since there are no internal heat gains coming in via the window, qH,gn, w is equal to qH,sol, w


31

Figure 5.1. Illustration of loss utilization factor for cooling mode, for 8 hours, 1 day, 2 days, 1 week and infinite time constants, valid for monthly calculation method [ISO/CD 18292].

5.1.1. Heat transfer by transmission and infiltration

The overall heat transfer by transmission and air leakage shall be calculated as:

( )100

,,int,

,,

,,,

t

A

HUq avgeHset

w

wveC

wCwhtC θθ −

+= , where: 5.8

Uc,w is the thermal transmittance of the window for the cooling mode

Hcve,w is the heat transfer coefficient due to air leakage of the window, expressed in W/K

θint, set, c is equal to the set point temperature for cooling expressed in oC

θe,avg is equal to the time-average external air temperature


5.1.2. Solar heat gain

The overall solar heat gain through the window shall be calculated as:

100,,,

tIgq solwCwgnC = , where: 5.9

gc,w is the dimensionless total solar energy transmittance of the window


Isol is the average solar irradiance for the considered time period on the window plane

5.1.3. Air infiltration and ventilation


32

Overall air infiltration (air permeability) rate L is measured for the whole fenestration product at the reference pressure difference and is denoted Lp,ref. The procedure to measure Lp,ref is specified in EN1026 or ISO 6613.

The heat transfer coefficient due to air infiltration of the window is given by:

refp

p

ref

wve LC

p

pH ,

3

2

,6.3

∆∆

=ρ

, where: 5.10

Hve,w is the heat transfer coefficient due to air infiltration of the window

∆p is the average pressure difference in the building, expressed in Pa (=6Pa)

ρCp is the thermal capacitance of air (=1.24kJ/m3K)

Lp,ref is the air leakage rate of the window at a pressure difference ∆pref

REFERENCE

ISO TC 163/SC 2: ISO/CD 18292. Energy performance of fenestrations systems – calculation procedure


33

6. DAYLIGHT POTENTIAL DEFINITION - ESTIMATION

According to ISO 18292, the daylight potential of a window indicates its potential to supply a building with daylight and depends on the visible transmittance, the glazing to window area ratio and on the view factor from the glazing to the sky.

The daylight potential, DP, is expressed as:

w

gl

ggsgvisA

AFFDP ××+×= −− )2.0(τ , where: 6.1

visτ is the visible transmittance of the glazing

Fg-s is the view factor from the glazing to the sky

Fg-g is the view factor from the glazing to the ground

0.2 is the albedo of the ground

Agl is the visible glazing area of the window, expressed in m2

Aw is the area of the window, expressed in m2

The view factor from the glazing to the sky depends on the type of the rated window: façade window (vertical), roof window and skylight (sloped) or roof light (horizontal). If a movable window comprises a movable shading device that blocks direct solar radiation (e.g. Venetian blind), give values for the complete dynamic range (fully open/fully closed).

6.1. Calculation of lighting energy in buildings

An alternative method for expressing the daylight potential of a window is to calculate the amount of energy used for indoor lighting inside the reference building and provide a numerical indicator for lighting energy requirements.

There are two forms of installed power in buildings, luminaire power and parasitic power. Luminaire power, provides power for functional illumination, while parasitic power provides power for lighting control systems and for charging batteries for emergency lighting.

The total estimated energy required for a period in a room or zone shall be estimated by the equation:

tPtLt WWW ,, += , where: 6.2

An estimate of the lighting energy required to fulfil the illumination function and purpose in the building WL,t shall be established using the following equation.

( ){ }∑ +×= )]()[(), oNDoDcntL FtFFtFPW 6.3 An estimate of the parasitic energy Wp,t required to provide charging energy for emergency lighting and for standby energy for lighting controls in the building shall be established using the following equation.

∑ ×++−×= 1000/)}()]}([{{, ememNDypctP tPtttPW 6.4


34

6.2. Determination of the daylight dependency factor

The daylight dependency factor FD,n for the nth room or zone is defined as a function of the daylight supply factor FD,S,n and the daylight dependent electric lighting control factor FD,C,n and is given by:

nCDnSDnD FFF ,,,,, 1 ×−= , where: 6.5

FD,S,n is the daylight supply factor that takes into account the general daylight supply in the zone n. It represents the contribution of daylight to the total required illuminance in the considered zone n

FD,C,n is the daylight control factor that accounts for the daylight depending electric lighting control system’s ability to exploit the daylight supply in the considered zone n

In order to calculate FD,n the following steps can be made:

- Segmentation of the building into zones with and without daylight access

- Determination of the impact of room parameters, facade geometry, and outside obstruction on the daylight penetration into the interior space using the concept of daylight factor

- Prediction of the energy saving potential described by the daylight supply factor FD,S,n as a function of local climate, maintained illuminance and daylight factor

- Determination of the exploitation of the available daylight by the type of lighting control by the daylight control factor FD,C,n

- Conversion of annual value FD,n to monthly values.

6.3. Spaces benefiting from daylighting

Spaces have to be sub-divided into a daylight zone AD,j and a zone AND,j not receiving any daylight. If a zone receives daylight from several facades or roof lights, the more favorable case may be assumed for the superimposed daylighting zone. Alternatively, it is also permissible to superimpose the daylight factor that is used to classify the daylight supply exclusively for the respective type of daylight aperture.

The maximal possible depth of zone AD,j that receives daylight through facades results as follows:

)(5.2max, TaLiD hha −= , where: 6.6

aD,max maximum length of daylight zone

hli the height of lintel above floor

hTa height of task area

Here, the maximum depth of the daylight zone aD,max is calculated from the interior surface of the exterior wall, perpendicular towards the facade component. If the actual depth of the zone of calculation is smaller than the calculated maximum depth of the daylight zone, the space depth can be taken as the depth of the daylight zone aD.

Thus, the sub-area AD,j of the daylight space j results as follows:

DDDj baA = , where: 6.7

aD depth of daylight zone

bD width of daylight zone

Usually the width of the daylight zone corresponds to the interior width of the facade of the building zone or the sector of calculation. Internal walls may be neglected. If windows are placed only in parts of the facade, the width of the daylight zone allocated to this facade corresponds to the width of the facade section containing windows, plus half the depth of the daylight zone.


35

6.4. Daylight supply

Daylight supply of a zone benefiting from daylight depends on the geometric boundary conditions described by the transparency index IT, the depth index IDE and the obstruction index Io.

The transparency index IT of the part of the building which can benefit from daylight is defined by:

D

C

TA

AI = , where: 6.8

Ac area of the facade opening of the considered space

AD total area of horizontal work planes benefiting from natural light.

The depth index IDE of the space, which can benefit from daylight is equal to:

)/( TaliDDE hhaI −= 6.9

The obstruction index Io accounts for effects reducing light incident onto the facade. It can be obtained from the following equation.

GDFOCAOSFOOVOOBOO IIIIII ,,,,,= , where: 6.10

IO is the correction factor

IO,OB is the correction factor for linear obstructions

IO,OV is the correction factor overhang

IO,VF is the correction factor for vertical fins

IS,CA is the correction factor for courtyard and atria

IO,GDF is the correction factor for glazed double facades.

6.4.1. Correction factor for obstructions

The obstruction angle is determined from the middle of the considered carcass opening, measured at the outer plane of the building shell. The correction factor for linear obstructions can be obtained by:

)5,1cos( ,, OBOOBOI γ= for o

OBO 60,


36

6.4.3. Correction factor for vertical fins

The obstruction angle for side fins is determined by the middle of the considered carcass opening measured at the outer plane of the building shell. It can be obtained by:

300/1 ,, VFOVFOI γ−= 6.15

γΟ, vf is the vertical fin angle

6.4.4. Correction factor for courtyard and atria

Courtyards and atria are designed with many variations. The simplified model assumes 4 sided courtyards and atria. 3 sided and linear atria may provide better daylight supply in adjacent indoor spaces. This potentiality better daylight situation can always be determined with more detailed methods.

The courtyard and atrium geometry is described by the well-depth index:

)2/()(_ AtAtAtAtAtdi wIwIhw += 6.16

wi_d is the well-depth index

hAt is the height from floor level of considered adjacent spac to the top of atrium of the courtyard

wAt is the width of the atrium or courtyard.

The correction factor for courtyards and atria can be obtained by:

diCAO wI _, 85.01−= for courtyards 6.17

32,1,, ATATATAtCAO kkkI τ= 6.18

0, =CAOI for 18.1_ >diw 6.19

τAt is the transmission factor of atrium glazing for normal incidence

kAt,1 is the factor accounting for frames of atrium roof

kAt,2 is the factor accounting for dirt of atrium roof

kAt,3 is the factor accounting for not normal light incidence on facade

6.4.5. Correction factor for glazed double facades

The correction factor for glazed double facades in front of the considered space can be obtained by

3,2,1,, GDFGDFGDFGDGDFO kkFkI τ= 6.20

τGDF is the transmission factor of glazed double facades.

KGDF,1 is the factor accounting for frames of glazed double facades

KGDF,2 is the factor accounting for dirt of glazed double facades

KGDF,3 is the factor accounting for not normal light incidence on facade

6.5. Daylight factor

From the geometric indices IT, IDE and IO the access of the zone to daylight can be estimated for the carcass facade opening.

ODETc IIID )36.10.2013.4( −+= 6.21 Dc is the daylight factor for carcass facade opening


37

6.6. Daylight factor classification

The impact of the fenestration and shading system on the indoor lighting levels should be determined by using facade type dependent correlations of Dc with the expected energy demand, i.e. methods deriving the daylight supply factor FDs as a function of the facade system. Where these dependencies are not available, a simplified estimation, correlating the fenestration properties without shading system with the expected energy demand should be calculated as follows:

321 kkkDD Cτ= 6.22

D is the daylight factor

τ is the direct hemispherical transmission of fenestration

k1 is a factor accounting for frame of fenestration system (typically 0.7)

k2 is a factor accounting for dirt on glazing (typically 0.8)

k3 is the factor accounting for not normal light incidence on facade (typically 0.85)

The impact of the fenestration can be judged by using either Dc or D according to Table 6.1.

Table 6.1. Daylight penetration as function of the daylight factor

Classification

Dc D

Daylight penetration (access of the zone to daylight)

Dc ≥6% D ≥ 3% Strong

6% ≥Dc ≥ 4% 3% ≥Dc ≥ 2% Medium

4% ≥Dc ≥ 2% 2% ≥Dc ≥ 1% Weak

Dc < 2% Dc < 1% None

REFERENCES

ISO TC 163/SC 2: ISO/CD 18292. Energy performance of fenestrations systems – calculation procedure

prEN 15193:2006. Energy performance of buildings – energy requirements for lighting.

window energy labeling in cooling season: fenestration … · 2016. 1. 19. · window uw heat...

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