a critical review of long-term thermal...

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14TH CANAD IAN CONFERENCE ON BU I LD ING S C I ENCE AND T E CHNOLOGY

A CRITICAL REVIEW OF LONG-TERM THERMAL

PERFORMANCE OF VACUUM INSULATION PANEL

IN BUILDING ENVELOPE CONSTRUCTION

M. Morlidge

ABSTRACT

The higher level of insulation in building envelopes mandated by recent energy codes in Europe and North

America has provided a fresh impetus to the search for high performance thermal insulation. Among

various nonconventional insulations being introduced in the construction industry, vacuum insulation panel

(VIP) appears to be one of the most promising insulation materials with the highest thermal insulating

capacity (thermal resistance of VIP is up to 10 times or more than those of conventional thermal insulation

materials). This paper provides a thorough review on parameters that influence the long-term thermal

performance of VIP. The material properties that contribute primarily to VIP performance include: pore

structure and thermal properties of core materials, out-gassing of core materials, gaseous diffusion and

water vapour permeability of foil barrier materials and seam joints, moisture sorption and desorption

characteristics of core materials, seals and panel joints, thermal bridging and edge losses, and climatic

conditions (relative humidity, pressure, temperature and time). Built upon existing models that correlate the

long-term thermal resistance to basic material properties, an analytical model is developed to predict the

long-term thermal resistance of VIPs. The model is applied to predict the aging of five VIPs that were

tested under accelerated aging cycles in laboratory and the predicted aging yields close agreement with the

measured data.

INTRODUCTION

Recent upgrades of energy codes in Europe and North America have recommended higher levels of

insulation in building envelopes. Among various non-conventional insulations being introduced in the

construction industry, vacuum insulation panel (VIP) appears to be one of the most promising insulation

materials with the highest thermal insulating capacity (thermal resistance of VIP is up to 10 times or more

than those of conventional thermal insulation materials). Quite naturally, the application of VIP in building

envelope construction offers many advantages such as increased energy efficiency of exterior building

envelope, thinner wall thickness, optimum space use, reduced material consumption etc. Nevertheless real-

life applications of VIP in building envelope constructions are rare and sporadic. There are a number of

issues that are being raised by construction industry professionals and stakeholders and undoubtedly long-

term thermal performance of VIP is one of those issues. At present VIPs are available from different Asian,

European and North American manufacturing sources and come with wide range of choices in terms of

thickness, R-value/inch and size and users know very little or nothing about their long-term performance.

There have been numerous studies, theoretically and experimentally, to determine the thermal performance

of VIPs over time. Several numerical models have also been developed to predict the long-term

performance of VIPs. However, many of these models were implemented based on specific material

properties measured and few models holistically considered all the relevant properties of the material and

the change of properties over time under transient climatic conditions. This paper provides a thorough

review on parameters that influence the long-term thermal performance of VIP. Built upon existing models

that correlate the long-term thermal resistance to basic material properties, an analytical model was

developed to predict the long-term thermal resistance of VIPs. The model is applied to predict the aging of

13 CCBST 2014 Proceedings Book_v10 B6 439-496_Layout 1 14-10-17 4:09 PM Page 467

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five VIPs that were tested under accelerated aging cycles in laboratory and the predicted aging yields close

agreement with the measured data.

LITERATURE REVIEW

The Vacuum Insulation (VIP) systems consist of open-porous core insulation materials that are wrapped

within an exterior metallic layer. The air inside this membrane is mechanically removed and then sealed to

create the interior vacuum. By removing air from the insulation material, the thermal energy losses via air-

convection and air-conduction are reduced to zero. Ideally radiation and solid-conduction would be the

only two heat transfer mechanisms and are significantly less in comparison to its two counterparts. The

typically used core materials include foam, powder and fiber insulations which are open porous, making it

possible to evacuate air from the panels. This core must also be able to withstand atmospheric pressure in

order to maintain the vacuum, without compression failure occurring within the panel (Kwon, 2009). The

most commonly used system is a fumed silica core wrapped in a polymer based film and coated with

metallic foils. Metallic foils differ from aluminum foils in that they use multiple layers of thin aluminum

between polymers. Increasing the number of metal layers is to enhance the performance of the panel by

providing increased durability. Typically VIPs range in thickness from 8-36mm with dimensions ranging

from 10x10cm to 150x150cm.

In comparison to other non-conventional insulation materials including nanostructured materials and

aerogels, VIPs have been found to far outperform these materials in thermal resistance per unit material

thickness. However, their long-term performance is a concern given that the VIPs are evacuated and

gradual or instantaneous loss of vacuum cannot be totally ruled out. Research efforts have been made to

quantify the thermal performance of vacuum insulations. The parameters contributing primarily to VIP’s

performance include: core material properties, outgassing of core materials, gaseous/moisture diffusion

and permeability of the barrier envelope/foil, moisture sorption of core material, integrity of polymer

seal/joints, thermal bridging and edge losses, and climatic conditions. This section summarizes existing

studies on the thermal performance of VIPs and the prediction of their service life.

1. CORE MATERIAL PROPERTIES

In this review, the focus is on fumed silica as it is the most commonly utilized and researched core material

in building application (Baetens, 2010). The effective U value of VIPs is dependent on the ability to

eliminate air from the core, while also minimizing solid conduction. The way typical cores are fabricated

is by injecting blowing agents into the fumed silica substance to create air pockets and then air is evacuated

from the cells to reduce air conduction and convection (Kwon, 2009). Open cell insulation materials are

typically made by ensuring bubble growth from the injected blowing agents, followed by the cell wall

thinning and breaking. Several analyses have been conducted to determine how the core structure

contributes to the performance of the vacuum, and how core materials could be optimized during the

fabrication process to ensure the maximum thermal resistance.

Tseng and Chu (2008) investigated the variables which influence the radiative and conductive heat

resistance properties of vacuum insulation based on the cell formation of the foam; focusing specifically

on foam density, mean cell diameter, mean bead diameter and inter-bead porosity. The thermal conductivity

of 14 VIP samples with different broken and open cell structures were measured to evaluate the effect of

cell geometry on heat transfer. The study found that as the cell size increases, the heat conduction through

solid decreases while radiative heat transfer increases, resulting in an decrease in the overall thermal


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conductivity since solid conduction accounts for approximately 80% of the total core conductivity (Tseng

and Chu, 2009). An optimum overall thermal conductivity for the core can be achieved with cell sizes

between 100-300 µm. In a further study, Tseng and Chu (2009) investigated the influence of broken cell

ratio, average cell size and solid volume fraction by adding polyethylene to modify the cell structure and

improve the overall thermal conductivity. The broken cell ratio of a material accounts for how many cells

have physically broken within the material during the evacuation of air. The ratio primarily affects the

amount of heat transfer via radiation, as a broken cell ratio which is too high is often accompanied by

internal compression (causing cell walls to collapse on one another), while a broken cell ratio which is too

low does not allow for sufficient evacuation of air. The cell size and broken ratio therefore control an

optimum value to reduce radiative heat transfer, while the solid volume fraction is kept low to minimize

the conductive heat transfer. They concluded that the lowest conductivity achieved was 4.4 mW/mK;

which occurred with a broken cell ratio of 0.95 and a cell size of 170 µm. The addition of 2% polyethylene

to the product was the most effective combination to alter cell structure and reduce heat transfer and the

5% was the percentage where the material no longer improved, and should thus be considered the

maximum additive necessary.

Wong and Tsai (2006) and Wong and Hung (2008) also studied the effect of adding polyethylene to core

materials on the foam density, cell structure, and overall core thermal conductivity of VIPs. They

concluded that the addition of polyethylene within a composite can increase the foam intensity, which

allowed it to achieve a porous open-cell foam core at higher foaming temperature resulting in a lower initial

core thermal conductivity, however, potentially greater deficiency risks due to failure over time. The

modeling of heat transfer through VIPs at a cell-to-cell scale by Kwon, et al. (2009) found that the ranking

of factors affecting the thermal conductivity of the core from the highest to lowest are: density, mean cell

diameter, and inter-bead porosity.

2. OUTGASSING OF CORE MATERIALS

While the core material does not affect the thermal conductivity over time, its properties do contribute to

the amount of outgassing from the core material. The outgassing will produce a rise of internal pressure

that compromises the performance of the VIP as it provides a heat transfer medium in an otherwise

evacuated product. The outgassing rate is influenced by the properties of core materials, blowing agent, its

microscopic surface characteristics, and the exact fabrication process of each individual VIP panel and

temperatures, etc.. Kwon et al., (2011) analyzed the outgassing rate of a polycarbonate core by modeling

and measurements. Measurements on polyurethane foam by Yang and Xu (2007) showed that the

outgassing rate was significantly reduced within the first 84 days after manufacturing and begins to level

out within the first year. Although outgassing of core materials was identified as a significant contributor

to the initial performance deterioration in VIPs (Kwon et al., (2011), the impact on the long-term

performance is considered insignificant given that outgassing occurred within the first year.

3. GASEOUS DIFFUSION AND PERMEABILITY

Gaseous diffusion through the metallic and polymer coatings is the main cause for the degradation of the

VIPs. Gaseous conductivity in the VIP is determined by a number of gas molecules available in the

material as transfer medium. The mean free path of molecules is the average distance, which they travel

before encountering a collision with another gas molecule. At a high pressure, the mean free path of

molecules is smaller, thus collisions between gas particles occur more frequently, causing efficient heat

transfer. Removing air through evacuation reduces the gas pressure, and thus decreases the mean free path


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between molecules (Thorsell, 2010). Gaseous diffusion occurs through the metallic coatings and the rate

of diffusion is increased in areas where there are deficiencies in these coatings. Thorsell (2010) measured

the size, location, and density of deficiencies, which were characterized as pin holes that naturally occurred

during the fabrication and handling process of VIPs. The exterior and interior layers had different

deficiency sizes and distribution (i.e. interior mean defect diameter was 1.3 µm with density of 4x108

defects/m2; exterior mean defect diameter was 3.2 µm with density of 8.6 x 107 defects/m2). They

developed a numerical model based on the characterization of deficiency sizes and distribution to predict

the permeation rate of gases. They concluded that the permeability achieved was able to meet the

performance requirements of VIPs for 30-50 years with only 2 layers of metallization layers; this

performance was improved when utilizing additional layers. Thicker metallic coatings provide a greater

retardant for gaseous diffusion, however, they also cause increased conductivity through the panel edges.

The optimum thickness depends on the type of foil used, typically two or three metallic layers, depending

on the manufacturer’s preference although 3 layers are optimal. By analyzing the typical surface

deformation conditions using chemical analyses, optical microscopy and scanning electron microscopy,

Garnier et al, (2011) classified the deficiencies as either nano-defects (out of equilibrium growth

mechanisms) or macro defects (pinholes and micro-cracks). Their measurements concluded that small

holes in a barrier produce a higher permeation value than large holes with the same total area. The amount

of surface defects was observed to decrease as the thickness of aluminum coatings was increased; however

the addition of metallic coating also causes the effective conductivity to increase due to edge thermal

bridging effect. Although the permeation rate is different for different types of gases, their impact on the

conductivity of VIP does not differ significantly, therefore, typically permeation rate for all gases are

coupled as one transmission rate (Garnier et al, 2011).

4. MOISTURE PERMEABILITY AND WATER CONTENT

The presence of moisture can have a significant impact on the thermal conductivity of VIPs. There is the

initial moisture contained in the core materials and the moisture diffused through the panel during its

service life, a similar process to gaseous diffusion through defects and pin holes. Moisture contributes to

the heat transfer of the VIPs in two ways, one as the gaseous medium and the other the latent heat transport

process, i.e. liquid water evaporates at warmer side and condense at the colder side. Schwab, et al., (2005)

studied the effect of moisture content on the thermal conductivity of VIPs using a hot plate apparatus. The

panels were exposed to different levels of temperature and relative humidity to achieve various levels of

moisture content (MC). This study concluded that for the panels tested the increase of thermal conductivity

was proportional to the MC of the panel, approximately 0.5x10-3 W/mK conductivity increase per mass

percentage of water. For the panels tested they found that the rates of increase in MC were between 0.02-

3.8 % per year, which can contribute to a conductivity increase of approximately 1.9x10-3 W/mK annually.

Garnier et al., (2010) studied the influence of aluminum coating on the vapour permeability of VIPs. The

vapour permeance was measured under different temperatures and relative humidity. They found that the

permeance was inversely proportional to the coating thickness and proportional to the surface fraction of

pin-holes. Through a hygrothermal aging process conducted in a climatic chamber at 70°C, 90% RH,

Garnier et al., (2011) noted that layering multiple sheets of polymer films had no further effect on vapour

diffusion and the increase of water content in the VIPs would cause the degradation of the polymers over

time due to a hydrolysis reaction which results in the delamination of various barriers. Similarly

Heinemann (2008) studied the effect of moisture on the overall thermal conductivity of VIPs using hot

plate apparatus. It was observed that most moisture was adsorbed in the core material while only a small

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amount present in a gas state. With the increase of temperature difference between the hot and cold plate,

the moisture content level increases at the colder side, which is an indication of moisture transport under

temperature gradients.

5. POLYMER SEALS AND PANEL JOINTS

The manufacturing technique is an important component contributing to the long-term performance of

VIPs. This is particularly the case when it comes to sealing and evacuating the polymer seals and

connecting the panels’ joints. The moment the seal is broken, air is allowed easy passage back into the

panel, eliminating the benefits of the system. Malsen et al. (2008) studied the effect of heat seals on metal

films. The composite investigated comprised a base layer of polyethylene sealant, followed by an

aluminum layer for the metallic coating and lastly a layer of polyethylene terephthalate (PET) for

protection. The heat seal where these layers are connected is considered to be the weakest part of the

coating. This study compared the bond of seals at extremely low temperatures as well as at room

temperature. The temperature at which the polymers are sealed and the amount of time they are heated,

largely determines the strength of the bond. When the films are heated, a small amount of pressure is

applied and the layers fuse. There are various types of failures that can occur as a result of sealing methods.

Peeling occurs when two fused laminates are completely de-bonded. Tearing is a rupture that occurs in the

film in a non-sealed area. The third failure is a combination of peeling and tearing, often resulting is tearing

along the seal. It is also possible for delamination to occur at areas other than the seal. Four types of panels

with different metallic layers were tested. At room temperatures, it was found that the most common type

of panel failure was delamination along the seal. Overall, seals tend to perform better at colder

temperatures.

6. THERMAL BRIDGING AND EDGE LOSS

Since the foil has a much greater thermal conductivity value than the core insulation material, the rate of

heat transfer is significantly increased along the edge of the panel as well as the increased potential for

moisture accumulation in micro-regions of the panels. When two panels are joined, the thermal bridging

effect will be increased as two metal barriers in contact increase the potential for heat transfer. Tenpierik

et al., (2007) developed a numerical model to evaluate the thermal bridging and edge effects of VIPs using

four different types of seaming methods with three types of metallic films. The modeling results were

compared to laboratory measurements for validation. They concluded that the significance of the edge

effect of VIPs depends on the thermal conductivity of core materials, thermal conductivity of the laminate,

thickness of aluminum, and the edge seaming methods resulting in different seaming thickness. Wakili et

al, (2004) carried out hot plate tests to measure the thermal conductivity of VIPs with edge effects included.

The test samples had fumed silica cores with three different seaming methods. They found that the heat

transfer via the edge effect is significant and is dependent on the geometric relation between the volume of

panel and surface area of the foil, thus larger, square panels will provide a lower conductivity value than

smaller, rectangular panels. Through numerical modeling Tenpierik and Cauberg (2010) studied the effect

of eliminating the thermal bridging in VIPs by encapsulating VIPs with an additional layer of expanded

polystyrene (EPS). They found that by integrating VIPs the thermal resistance of EPS could be

significantly improved (up to 35% with 30mm panels, 248% with 100mm.) The addition of EPS may also

contribute to the durability of the VIP and potential reduction of deficiencies in the metallic barriers.


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7. PERFORMANCE OF VIP ASSEMBLIES

The majority of studies on VIPs focused on factors influencing the overall thermal conductivity of the

panel itself. However, in building applications the actual performance of VIPs will be influenced by its

construction and interaction with other materials/elements within the wall assemblies. Nussbaumer et al.,

(2006) tested the thermal performance of VIPs assemblies encapsulated with EPS using a hot box

apparatus. Six sample panels were mounted to a concrete wall and monitored with thermocouples.

Damaged VIPs were also included in the test samples. The damaged area was characterized by the length

of tear in foil seam. The study concluded that the U-value of a 60mm EPS containing VIPs outperforms

120mm conventionally used insulation. The U-value of the 60mm EPS-VIP assembly was slightly higher

than the 120mm insulation with all VIPs damaged. Grynning et al, (2010) carried out hot box

measurements on a complete wall assembly. They studied the single layer application of VIPs versus

double layer configurations, while considering different panel thicknesses, edge effects, and the effects of

staggering panels and taping joints. Temperature measurements showed irregularities in certain areas due

to the compression of panels during installation. A 19% higher overall U-value was found in measurements

when compared to calculations based on the nominal thickness of VIP panel. This study concluded that the

size of the panels had an impact on their overall thermal resistance, as did the orientation and

configurations. It was speculated that this was due to possible convection in gaps between the hot and cold

surfaces of the VIP.

8. SERVICE LIFE PREDICTION MODELS

Several studies had focused on developing models to predict the service life of VIPs. The definition of

service life has not yet been standardized, however, it is generally agreed upon within the academic

community that the service life of a VIP is defined by the amount of time it takes for the conductivity to

exceed 8.0 x 10-3 W/mK (Schwab et al, 2005). To perform as an acceptable insulation within a building

envelope, the service life must exceed a minimum of 50 years (Mukhopadhyaya et al, 2011). In Simmler

and Brunner’s model (2005) the overall change in the conductivity was expressed as the change in

conductivity due to internal gas pressure change and moisture content change. The rate of pressure change

is primarily determined by the air permeance, which is influenced by temperature, relative humidity,

surface area and panel length, and manufacturing methods. Similarly the accumulation of moisture within

the core materials is determined by the vapour pressure difference between the interior and exterior of the

panel and the water vapour transmission rate. The thermal bridging and edge effect of the panel was

included in the initial conductivity. Their study concluded that VIPs can provide a sufficient service life;

however special care should be taken in envelope design to prevent exposure to excessive humidity and

minimize the potential for condensation, as moisture accumulation proved to be the largest contributor to

the increased thermal conductivity.

Schwab et al., (2005) carried out comprehensive analyses of VIP performance based on measurements and

developed a service prediction model. The initial model was broken down into the conductivity due to solid

and gaseous conduction, radiation, gaseous diffusion and moisture convection. The contribution of each

term to the overall conductivity over time was described and evaluated individually. The two main

parameters were the change in conductivity due to gas permeation and water vapour transmission. A

mathematical expression of the change of internal pressure over time was formulated based on the

measured gas transmission rate and the ambient temperature, pressure, effective pore volume of the core

materials. The final prediction model included the initial conductivity of the VIPs with two terms


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accounting for the change in conductivity due to gas permeation and change in moisture content. While the

gas portion of this model is consistent with others’ work, this model lumps moisture accumulation and

water vapour transmission as one variable. Upon examining Schwab’s work thoroughly, Wegger et al.,

(2010) proposed an alternate model that separated the water accumulation and water vapour transmission.

Theoretical analyses by Wegger et al. showed that for five types of commonly used VIPs with an initial

thermal conductivity of 4.0 x10-3 W/mK, four laminates can maintain a thermal conductivity of less than

8.0 x 10-3 W/mK for a minimum of 50 years.

All the models reviewed require panel specific performance data such as gas transmission rate and water

vapour transmission rate in addition to basic material properties. However, these material properties are not

yet standardized within the industry and makes it difficult to synthesize and apply these models to make

prediction without conducting specific measurements.

AN ANALYTICAL MODEL


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Assumptions and Excluded Variables

• A two-dimensional steady state heat transfer is assumed. While the edge effect is accounted for by a

linear thermal transmittance, the 3D effect at corners was simplified.

• This model does not account for structural and physical deteriorations. For example, exposure to

ultraviolet radiation may cause the foil to weaken thereby causing higher transmission rates of gas and

moisture. The oxidation of the metallic foil also has an impact on the transmission rate of air as

deficiencies within the foil increase. The model does not account for exposure to pollutants and acidity

that could contribute to degradation of materials over time.

• The outgassing and its contribution to the change in conductivity of core materials was also excluded

in this model. Previous studies showed that the majority of outgassing occurs within the first year after

panel fabrication and therefore on a scale of a 50 year service life, this value is negligible.

• This model applies to fumed silica core materials only.


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Categorization of Variables and Determination of Generic Properties

The model is used to predict the thermal conductivity over time under the same accelerated aging

conditions as NRC’s aging tests in order to evaluate the accuracy of the prediction model. The specific

material properties of the panels tested at NRC were unknown, therefore, a generic material database was

assembled from literature and the gas transmission rate and water vapour transmittance rate were taken

from literature. Six types of commonly used VIPs are chosen. Three of the panels were of the aluminum

foil variety (AF) and the others were metalized foils (MF). Typical examples of these foil compositions are

shown in Figure 1. It should be noted that the material properties for these foils were taken from typical

foils that may be used in the industry, and are not representative of any specific VIP products.

Based on the analysis of material properties assembled from numerous sources, these material properties

were categorized as associated with high, average and low quality VIPs. These material qualities were

determined primarily by the foil thickness, the gas transmission rate, the water vapour transmission rate

and core material density; which are the most influential factors on the VIP performance over time. The

high quality materials typically have greater foil thickness, low gas transmission and water vapour

transmission rates and high material density. Low quality materials typically have smaller foil thickness,

high gas transmission and water vapour transmission rates with lower material density. The average

material was determined based on average values. Tables 1 and 2 list the constants and material properties

used in the calculation. Table 3 lists the climatic conditions for accelerated aging tests by NRC. The values

here represent the initial cycle, as the pressure values inside the panel would increase with each cycle.

FIGURE1: TYPICAL SECTIONS OF ENVELOPE MATERIALS FOR VIPS. (TENPIERIK ET AL, 2007)


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TABLE 1: LIST OF CONSTANTS USED IN THE CALCULATIONS.

TABLE 2: LIST OF MATERIAL DEPENDENT VARIABLES FOR EACH VIP TYPE CALCULATED.

MF=METALIZED FOIL, AF=ALUMINUM FOIL.


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TABLE 3: CLIMATE CONDITIONS USED TO SIMULATE ACCELERATED AGING (7 DAYS PER

CYCLE).

RESULTS AND DISCUSSION

The experimental results published by the National Research Council of Canada (Mukhopadhyaya, 2011)

were used to compare with the model predictions. Five types (3 samples for eack type, total 15 samples)

of VIPs with initial thermal resistances ranging from RSI 4.29 to 4.85 m2K/W were subjected to accelerated

aging in the lab. These panels were identified by 482-171, 487-61, 482-88, 487-115, and 499-106. During

the first half of each cycle, the panels were kept under conditions of 23°C, 95% RH for seven days. For the

next seven days the panels were kept under conditions at 70°C, 5%RH. The panels went through these

fourteen-day cycles nine times. The thermal resistance of each panel was measured at the end of each half

cycle. Since the initial thermal resistances varied, these values were normalized to their initial RSI in order

to compare the change over time in each panel. As shown in Figure 2, three VIP products behaved in a

similar manner (487-61, 482-171 and 487-115) where their thermal resistance was reduced to 95% of the

initial value. Product 499-106 had a much faster rate of reduction in thermal conductivity over the

accelerated testing. Results for the product 482-88 showed an initial thermal resistance increase after the

first elevated temperature (70°C) exposure, due to enhanced getter/desiccant performance, before reducing

at a rate similar to the average cases.

Figure 3 shows the results of the calculations under the same climatic conditions used in the accelerated

aging process. When comparing the results in Figure 3 to those presented by the NRC tests, it can be

observed that the average metalized foil VIP (MF avg.) follows a very similar trend to products 487-61,

482-171, and 487-115. It also appears that MF low follows a similar trend to product 499-106. Although

the predicted results are based on assumed material properties (which are assembled from commonly used

materials and construction methods), the good agreement achieved between prediction and measurements

indicates the validity of the prediction model and the applicability of the model to commonly used VIPs.

The other observation that can be made from the normalized thermal resistances is that the rate of change

in thermal resistance among the metalized foil products varies greatly in comparison to the aluminum foil

films, and they generally decrease in thermal resistance more quickly than the aluminum foils. This is

likely because the aluminum foils tend to have lower gas and water vapour transmission rates.


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FIGURE 2: MEASURED RESULTS PROVIDED BY NRC OF ACCELERATED AGING OF 5 VIP

PRODUCTS. (MUKHOPADHYAYA, 2011)

FIGURE 3: NORMALIZED THERMAL RESISTANCE OF SIX TYPES OF COMMONLY USED VIP

PANELS USING THE PROPOSED MODEL UNDER NRC’S ACCELERATED AGING TEST

CONDITIONS (20CM THICKNESS).

CONCLUSIONS

This paper provides a thorough review on parameters that influence the long-term thermal performance of

VIPs. The critical material properties that contribute primarily to the aging of VIPs are the gas permeation

and water vapour transmission through the foil barrier and seam joints. The literature reviewed covered

most of the known and relevant findings regarding this material. These findings allowed the development

of generic material properties based on previously conducted experiments. A simplified numerical model

built upon existing models was proposed and used to predict the performance of five types of commonly

used VIPs under accelerated aging test conditions. The predictions generally agree well with

measurements, which indicated that the proposed model can be used as a viable tool to predict the

approximate long-term performance of VIPs.


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