36789651-u-values

20 CIBSE Concise Handbook

A3.6.3 Frames and sashes (excluding glazing)

The U-values given in Table A3.25 are based on data givenin BS EN ISO 10077-1. Alternatively the U-value ofwindow frames can be calculated by software conformingwith BS EN ISO 10077-2.

A3.6.4 Spacer between panes (multipleglazing units)

In multiple glazing units, the thermal transmittance isincreased due to interaction between the glazing and theframe, including the effect of the spacer bars. This allowedfor by a linear thermal transmittance related to theperimeter length of the glazing.

The linear transmittance for particular glazing and framecombinations can be calculated by software conformingwith BS EN ISO 10077-2. Table A3.26 gives default valuesthat can be used in the absence of detailed information.

A3.6.5 Effect of blinds and curtains

A3.6.5.1 Internal blinds and curtains

Internal roller blinds or curtains provide additional insu-lation due to the air enclosed between the window and theblind. The degree of insulation depends strongly on thelevel of enclosure achieved. Roller blinds can achieveeffective entrapment provided they run in side channels

Table A3.23 U-values for vertical glazing

Type of Spacing U-value (/ W·m–2·K–1) for stated glazing / mm exposure of panes†

Normal Sheltered Severe(0.13/0.04) (0.13/0.06) (0.13/0.02)

Single — 5.75 5.16 6.49

Double 25 2.76 2.60 2.9020 2.74 2.60 2.9016 2.73 2.59 2.9012 2.85 2.70 3.029 3.01 2.84 3.206 3.28 3.08 3.51

Triple 25 1.72 1.67 1.7820 1.71 1.66 1.7716 1.78 1.72 1.8412 1.89 1.83 1.979 2.04 1.96 2.126 2.29 2.19 2.40

Coated double 20 1.85 1.78 1.92ε = 0.2 16 1.82 1.76 1.89

12 2.02 1.95 2.119 2.29 2.19 2.396 2.71 2.57 2.87

Coated double 20 1.60 1.55 1.65ε = 0.1 16 1.57 1.53 1.63

12 1.80 1.74 1.879 2.10 2.01 2.196 2.57 2.44 2.71

Coated double 20 1.45 1.41 1.49ε = 0.05 16 1.42 1.38 1.46

12 1.67 1.61 1.729 1.98 1.91 2.066 2.48 2.37 2.61

Coated double 20 1.65 1.60 1.71argon-filled 16 1.63 1.58 1.69ε = 0.2 12 1.76 1.70 1.82

9 1.98 1.90 2.066 2.35 2.24 2.46


9 1.75 1.69 1.816 2.16 2.07 2.26


9 1.61 1.56 1.666 2.05 1.97 2.14

† Internal and external surface resistances (m2·K·W–1) respectively aregiven in parentheses

Table A3.24 U-values for horizontal and roof glazing

Type of Spacing U-value (/ W·m–2·K–1) for stated glazing / mm exposure of panes†

Normal Sheltered Severe(0.13/0.04) (0.13/0.06) (0.13/0.02)

Single — 6.94 6.10 8.07

Double 25 3.32 3.11 3.5520 3.34 3.23 3.5816 3.37 3.16 3.6112 3.41 3.19 3.669 3.44 3.22 3.706 3.63 3.39 3.92

Triple 25 2.06 1.98 2.1520 2.09 2.00 2.1816 2.11 2.02 2.2012 2.14 2.05 2.239 2.17 2.08 2.276 2.46 2.35 2.59

Coated double 20 2.51 2.39 2.65ε = 0.2 16 2.56 2.43 2.69

12 2.61 2.48 2.759 2.67 2.53 2.826 2.95 2.79 3.14

Coated double 20 2.3 2.2 2.41ε = 0.1 16 2.35 2.24 2.46

12 2.41 2.3 2.539 2.47 2.35 2.66 2.78 2.64 2.95

Coated double 20 2.18 2.09 2.28ε = 0.05 16 2.22 2.13 2.33

12 2.29 2.19 2.49 2.35 2.25 2.476 2.68 2.55 2.84


9 2.33 2.22 2.446 2.52 2.40 2.66


9 2.09 2.02 2.196 2.32 2.21 2.42


9 1.96 1.88 2.046 2.19 2.10 2.29

† Internal and external surface resistances (m2·K·W–1) respectively aregiven in parentheses

Guide A: Environmental design 21

and are sealed at the top and bottom. With well-sealedblinds, further improvement can be achieved by using amaterial which has a low emissivity surface protected bylayer transparent to infrared radiation. Values for thethermal resistance of internal blinds and curtains aregiven in Table A3.27.

A3.6.6 Indicative U-values for conceptualdesign

At the concept design stage, it is convenient to useindicative U-values for typical window configurations toenable an initial evaluation of the heat losses and energyconsumption of the proposed building. Table A3.29provides such values for these purposes.

A3.6.7 Indicative U-values for energy rating

Tables A3.30, A3.31 and A3.32 provide indicative U-valuesfor windows, doors and rooflights for the purposes ofenergy rating.

Table A3.25 Thermal transmittances for various types of window frameand sash

Material Description U-value / W·m–2·K–1

Wood Average thickness 30 mm 2.30Average thickness 40 mm 2.15Average thickness 50 mm 2.02Average thickness 60 mm 1.90

Average thickness 70 mm 1.78Average thickness 80 mm 1.67Average thickness 90 mm 1.57Average thickness 100 mm 1.48

Plastic Without metal reinforcement: — polyurethane 2.8— PVC, two hollow chambers 2.2— PVC, three hollow chambers 2.0

Aluminium Thermal barrier† with:— 4 mm thermal break 4.4— 8 mm thermal break 3.9— 12 mm thermal break 3.5— 16 mm thermal break 3.2— 20 mm thermal break 3.0

Aluminium Without thermal barrier 6.9 or steel

† Thermal barrier must be continuous and totally isolate the interior sideof the frame or frame sections from the exterior side

Table A3.26 Linear thermal transmittance, Ψs , for conventional sealedmultiple glazing units

Frame type Linear thermal transmittance (/ W·m–1·K–1) for stated glazing type

Double or triple glazing Double or triple glazing,uncoated glass, air or gas low-emissivity glass filled (1 pane coated for

double glazing or 2 panes coated for triple glazing), air or gas filled

Wood or PVC 0.06 0.08

Metal with 0.08 0.11thermal break

Metal without 0.02 0.05thermal break

Table A3.27 Thermal resistance of blinds and curtains

Description Thermal resistance / m2.K.W–1

Conventional roller blind, curtain or 0.05venetian blind (vertical slats)

Closely fitting curtain with pelmet 0.07

Roller blind:— bottom only sealed 0.09— sides only sealed in channels 0.11— sides and top sealed 0.15— sides and bottom sealed 0.16— fully sealed 0.18

Low emissivity roller blind, fully sealed 0.44

Table A3.29 Indicative U-values for windows for conceptual design

Type Indicative U-value / W·m–2·K–1

Glazing only Window (including frame or sash)

Single 5.7 5.0Double 2.8 3.0Double (low emissivity) 1.8 2.2Triple 1.8 2.2

Table A3.30 Indicative U-values (/ W·m–2·K–1) for windows androoflights with wood or PVC-U frames, and doors (Crown copyright,reproduced with the permission of the Controller of Her Majesty’sStationery Office and the Queen’s Printer for Scotland)

Item Indicative U-value (/ W·m–2·K–1) Adjustment for stated gap between panes for rooflights

6 mm 12 mm ≥16 mm in dwellings†

Single glazing 4.8 — — +0.3

Double glazing(air filled): 3.1 2.8 2.7 +0.2— low-E, εn = 0.2[1] 2.7 2.3 2.1 +0.2— low-E, εn = 0.15 2.7 2.2 2.0 +0.2— low-E, εn = 0.1 2.6 2.1 1.9 +0.2— low-E, εn = 0.05 2.6 2.0 1.8 +0.2

Double glazing (argon filled[2]): 2.9 2.7 2.6 +0.2— low-E, εn = 0.2 2.5 2.1 2.0 +0.2— low-E, εn = 0.15 2.4 2.0 1.9 +0.2— low-E, εn = 0.1 2.3 1.9 1.8 +0.2— low-E, εn = 0.05 2.3 1.8 1.7 +0.2

Triple glazing: 2.4 2.1 2.0 +0.2— low-E, εn = 0.2 2.1 1.7 1.6 +0.2— low-E, εn = 0.15 2.0 1.7 1.5 +0.2— low-E, εn = 0.1 2.0 1.6 1.5 +0.2— low-E, εn = 0.05 1.9 1.5 1.4 +0.2

Triple glazing (argon filled[2]): 2.2 2.0 1.9 +0.2— low-E, εn = 0.2 1.9 1.6 1.5 +0.2— low-E, εn = 0.15 1.8 1.5 1.4 +0.2— low-E, εn = 0.1 1.8 1.4 1.3 +0.2— low-E, εn = 0.05 1.7 1.4 1.3 +0.2

Solid wooden door[3] 3.0 — — —

† No correction need be applied to rooflights in buildings other thandwellings

Notes: [1] The emissivities quoted are normal emissivities. (Correctedemissivity is used in the calculation of glazing U-values.) Uncoated glassis assumed to have a normal emissivity of 0.89. [2] The gas mixture isassumed to consist of 90% argon and 10% air. [3] For doors which arehalf-glazed the U-value of the door is the average of the appropriatewindow U-value and that of the non-glazed part of the door (e.g.3.0 W·m–2·K–1 for a wooden door).

22 CIBSE Concise Handbook

A3.8 Non-steady state thermalcharacteristics

A3.8.1 Admittance procedure

There are several methods available for assessing the non-steady state or dynamic performance of a structure. One ofthe simplest is the admittance procedure which isdescribed in detail in Guide A chapter 5. The method ofcalculation of admittances and related parameters isdefined in BS EN ISO 13786 and a summary is given inGuide A Appendix 3.A6.

A4 Ventilation and airinfiltration

A4.2 Role of ventilation

A4.2.1 Background

Ventilation provides fresh air to occupants, and dilutesand removes concentrations of potentially harmfulpollutants. It is also used to passively cool and distributethermally conditioned air.

Energy losses from ventilation and general air exchangecan account for more than half of the primary energy usedin a building. These losses comprise space heating andrefrigerative cooling losses as well as the electrical loadassociated with driving mechanical services.

A4.2.2 Minimum ventilation rates for airquality

The amount of ventilation required for air quality dependson:

— occupant density

— occupant activities

— pollutant emissions within a space.

BS EN 13779 provides basic definitions of air qualitystandards in occupied spaces and relates these to fresh airventilation rates required for each occupant. These aresummarised in Table 4.1.

Building Regulations Part F (2006) requires a minimumventilation rate of 10 L·s–1 per person for most non-domestic applications. This fits between classes IDA2 andIDA3 in Table 4.1.

For guidance on ventilation techniques, see CIBSE Guide B.

Table A3.31 Indicative U-values (/ W·m–2·K–1) for windows and fully-glazed doors with metal frames (4 mm thermal break) (Crown copyright,reproduced with the permission of the Controller of Her Majesty’sStationery Office and the Queen’s Printer for Scotland)

Item Indicative U-value (/ W·m–2·K–1) for stated gap between panes

6 mm 12 mm 16 mm or more

Single glazing 5.7 — —

Double glazing (air filled): 3.7 3.4 3.3— low-E, εn = 0.2[1] 3.3 2.8 2.6— low-E, εn = 0.15 3.3 2.7 2.5— low-E, εn = 0.1 3.2 2.6 2.4— low-E, εn = 0.05 3.1 2.5 2.3

Double glazing (argon filled[2]): 3.5 3.3 3.2— low-E, εn = 0.2 3.0 2.6 2.5— low-E, εn = 0.15 3.0 2.5 2.4— low-E, εn = 0.1 2.9 2.4 2.3— low-E, εn = 0.05 2.8 2.3 2.1

Triple glazing: 2.9 2.6 2.5— low-E, εn = 0.2) 2.6 2.1 2.0— low-E, εn = 0.15) 2.5 2.1 2.0— low-E, εn = 0.1) 2.5 2.0 1.9— low-E, εn = 0.05) 2.4 1.9 1.8

Triple glazing (argon filled[2]): 2.8 2.5 2.4— low-E, εn = 0.2 2.4 2.0 1.9— low-E, εn = 0.15 2.3 1.9 1.8— low-E, εn = 0.1 2.2 1.9 1.7— low-E, εn = 0.05 2.2 1.8 1.7

[1] and [2]: see footnotes to Table 3.30

Table A3.32 Adjustments to U-values in Table 3.31 for frames withthermal breaks (Crown copyright, reproduced with the permission of theController of Her Majesty’s Stationery Office and the Queen’s Printer forScotland)

Thermal break / mm Adjustment to U-value (/ W·m–2·K–1)

Adjustment for Additional adjustmentthermal break for rooflights angled

< 70° to horizontal

0 (no break) +0.3 +0.4

4 +0.0 +0.38 –0.1 +0.3

12 –0.2 +0.3

16 –0.2 +0.320 –0.3 +0.324 –0.3 +0.3

28 –0.3 +0.332 –0.4 +0.336 –0.4 +0.3

Table A4.1 Ventilation and indoor air quality classification (BSEN 13779)

Classification Indoor air Ventilation range Default valuequality / (L·s–1/person) / (L·s–1/person)standard

IDA1 High >15 20IDA2 Medium 10–15 12.5IDA3 Moderate 6–10 8IDA4 Low < 6 5

A4.2.3 Ventilation rate and metabolic carbondioxide

Carbon dioxide is emitted as part of the metabolic processand can be used as an estimate of the adequacy ofventilation. Guidelines related to CO2 concentrationsalmost always refer to sedentary environments. It takes afinite period for CO2 to reach a steady state level. TableA4.2 summarises CO2 concentrations above the ambientoutdoor concentration that reflect the air qualityclassifications of Table A4.1.

36789651-u-values

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