Vacuum insulated panels for sustainable buildings: a review of research and applications

Authors

  • Sultan Sanat Alotaibi,

    Corresponding author
    1. Institute of Sustainable Energy Technology, Department of Architecture and Built Environment, University of Nottingham, Nottingham, UK
    • Correspondence: Sultan Sanat Alotaibi, Institute of Sustainable Energy Technology, Department of Architecture and Built Environment, University of Nottingham, Nottingham, NG7 2RD, UK.

      E-mail: laxsa9@nottingham.ac.uk

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  • Saffa Riffat

    1. Institute of Sustainable Energy Technology, Department of Architecture and Built Environment, University of Nottingham, Nottingham, UK
    Search for more papers by this author

SUMMARY

New research has identified vacuum insulation panels (VIPs) as highly efficient insulators for use in building construction. They are reported to be several times more effective than conventional materials of a similar thickness in terms of thermal conductivity. Because of their smaller space requirement, VIPs maximize the internal usage area of buildings and so reduce the cost of construction. There are however some obstacles that have hindered the application of VIPs, notably their high cost, susceptibility to perforation and the long-term water and gas effects that worsen their performance. This paper reviews the contemporary research on VIP as a state-of-the-art material for building insulation. The main components and physical principles of VIP performance are discussed. Finally, the review of VIPs available on the market and their performance is provided. © 2013 The Authors. International Journal of Energy Research published by John Wiley & Sons, Ltd.

1 INTRODUCTION

In recent years, many developed countries have introduced programmes directed at decreasing energy consumption and improving carbon performance of buildings [1-3]. Development and introduction of energy-efficient materials to the market play a significant role in achieving these goals, because they help decrease building heat losses to the environment, thus reducing the necessary amount of energy used to support the desired heat comfort levels.

Traditional insulation materials, such as cork, mineral wool, cellulose, polystyrene or polyurethane (PUR), are capable of preserving energy to certain extents. Research also indicates use of nontraditional materials for insulation, such as wool-hemp, date palm fibre and phase-change materials [4-6]. However, supporting the required thermal levels in buildings often require the increased thickness of these materials, which is not always feasible with respect to material economy, transportation and use. Recent developments in the physics of construction materials have led to the creation of new building insulation materials that provide lower thermal conductivity rates than the traditional ones. Contemporary research mentions several types of such materials: gas-filled panels (GFPs), polymer skins, aerogels and vacuum insulation panels (VIPs).

Gas-filled panels use a combination of thin polymer films and low-conductivity gases to achieve lower thermal conductivity rates. Visually, GFPs are hermetic plastic bags of different shapes and sizes that are filled with an inert gas having low thermal conductivity, such as argon, xenon or krypton. Inside the outer barrier of GFP is a baffle – a cellular structure that suppresses gas convection and radiation. GFPs, as thermal insulators, have been actively studied in past two decades [7-10]. So far, however, experimental thermal conductivities achieved from GFP (40 mW/mK) have only been comparative with those of the traditional materials, although theoretical investigations predicted values as low as 10 mW/mK [11].

Polymer skins are large films, most commonly ethylene tetrafluoroethylene, that are parts of a pneumatic cushion assembly that inflates or deflates the space between the films to increase or decrease the thermal resistance [11]. As such, polymer skins are dynamic thermal insulators that can be adjusted on the basis of the thermal conductivity requirements. Figure 1 schematically shows how the polymer skins are used. They are often designed as a part of a pneumatic cushion assembly, with skins held between the structural frames. The upper picture demonstrates the cushions in an inflated state, and the lower picture in a deflated state. Cushions in an inflated state increase the thermal resistance, which is useful in the cold weather. Polymer skins have been successfully applied in large construction projects, including the Allianz Football Arena in Munich, Germany, and the Beijing National Aquatic Centre in China. However, studies on applicability of polymer skins at smaller building levels are not available [3].

Figure 1.

Polymer skins in action [3].

Aerogels are synthetic porous materials derived from drying gels under critical conditions. Gas replaces liquid components of gel, which results in a highly porous structure (over 85% of the total volume) with extremely small pore diameters (5–100 nm) (Figure 2) [12]. This structure not only makes aerogels one of the lightest known solid materials but also limits the three major mechanisms of thermal transport, which results in very low thermal conductivity rates [3]. Aerogels have been relatively popular among the researchers of thermal insulation materials [13-16]. Commercially available aerogels have the thermal conductivity of 13–14 mW/mK at ambient pressures; although under pressures of 50 mbars and with the use of carbon to limit radiative transfer, it is possible to achieve thermal conductivities as low as 4 mW/mK [17, 18].

Figure 2.

Aerogel under electronic microscope [12].

Although aerogels are very promising materials for thermal insulation in buildings, their commercial applications are limited because of extremely high cost of production (€214*/m2 on average as reported by [19]) and fragility because of low tensile strength.

Vacuum insulation panels have been proposed by a number of researchers as the most promising new age insulation solution for sustainable building [1-3, 18, 20]. VIPs have a lower k-value than both conventional and state-of-the-art insulation materials, meaning that wall construction can be thinner. One analysis indicated that a traditional mineral wool or PUR foam insulation board with a thickness of 185 is equivalent to a VIP only 20 mm thick [1]. There is potential for mass-scale application of VIPs in buildings because of their low k-value, and hence, the small wall thickness required. A high barrier laminate covers the microporous core of the VIP, and the vacuum inside the core minimizes thermal conductivity. However, despite the growing number of studies on these potentially auspicious materials, the research on their application for buildings remains somewhat disorganized and localized. The main contributions of this paper to the state-of-the-art insulation systems research for buildings include the following: (i) systematisation and organization of information regarding VIPs as an insulation material for sustainable buildings; (ii) identification of the most perspective VIP components; (iii) practical review of the current market for VIPs in buildings; and (iv) providing an outlook for VIP potential within the new energy initiatives in the European Union.

2 VACUUM INSULATION PANELS

2.1 Vacuum insulation panel overview

Vacuum insulation panels represent an evacuated, open-porous material that is enveloped into a multilayer film. A special structure of VIP makes it the best material in terms of thermal conductivity in pristine condition: 3–4 mW/mK [18]. Figure 3 demonstrates a VIP with its key components and shows how it compares to a traditional insulation material based on thermal conductivity [21, 22].

Figure 3.

Visual presentation of a vacuum insulation panel (left) and a comparative material thickness with a traditional insulation (extruded polystyrene ) achieving the same thermal conductivity level (right) [21, 22].

In recent years, research interest in VIPs increased, as the studies covered such topics as thermal conductivity and gas pressure in VIPs [23-26], analytical and optimisation models [27, 28], application of various materials in VIP manufacturing [1, 29-32] and applications of VIPs in building construction became growing areas of research [3, 20, 33, 34].

As any other material for thermal insulation of buildings on the market, VIPs have its advantages and disadvantages. On the positive side, they have the lowest thermal conductivity rate, and they allow for significant space economy. On the negative side, they are relatively fragile, their performance significantly decreases with time and they are not adaptable for construction sites without losses in thermal conductivity. VIPs are also relatively expensive to produce, costing €168/m2 on average [19, 20]. Table 1 compares VIPs with the traditional and other state-of-the-art insulation materials.

Table 1. Comparison of vacuum insulation panel to the traditional and other state-of-the-art thermal insulation materials.
MaterialThermal conductivity (mW/mK)Cutting to adapt for constructionResistance – fire, water and chemicalsResistance – physical damagePerformance if perforatedCost per thermal resistanceEnvironmental impact of production and use
  1. Sources: [6, 19, 20]

Vacuum insulation panel4–8NoLowLowWorseHighModerate
Traditional materials      
Cellulose40–50YesLowLowSameLowLow
Fibreglass30–40YesHighHighSameLowModerate
Expanded polystyrene30–40YesLowModerateSameLowHigh
Extruded polystyrene30–32YesModerateModerateSameHighHigh
Polyurethane20–30YesModerateHighSameHighHigh
State-of-the-art materials      
Gas-filled panel10–40NoLowLowWorseHighModerate
Aerogels13–14YesModerateLowSameHighModerate

Despite the current difficulties that VIPs are facing in becoming a thermal building insulation material of choice, they represent one the most promising thermal insulation technologies on the market today. Even if it will not be possible to create a market for them as for a standalone material for building insulation, it is likely that using VIPs in combination with other materials will be beneficial. Theoretical and practical research on VIPs is also likely to contribute to development of the optimal thermal insulation solutions for the future.

2.2 The scheme of vacuum insulation panel work

A typical VIP represents an insulation system that includes an inner core material, barrier envelope (film), a getter and/or desiccant and a heat seal (Figure 4).

Figure 4.

The structure of a vacuum insulation panel.

Core materials serve as the main filler of VIPs. Barrier films serve as protectors from environmental and physical damages resulting from the panel handling. Getters and/or desiccant adsorb water vapours and other gases that may break through the barriers. As a system, a VIP's performance depends on the performance of each of these components. This section continues with the discussion of various materials used for inner core, barriers and getters in VIPs and the physical processes that determine performance of VIPs.

2.3 Core materials for vacuum insulation panels

The major function of core materials in VIPs is to support the required vacuum level to ensure low thermal conductivity of a panel. To meet this objective effectively, a core material has to possess a number of specific characteristics. Firstly, as was discussed previously, core materials with smaller pores are preferable, because they allow to decrease the gaseous conductivity to insignificant levels even at normal pressure conditions. Secondly, core materials with open cell structures are preferable because they allow evacuation of gas [20]. Thirdly, specific geometry of skeleton is required of core materials to maintain small contact points between the structures, consequently minimizing conductive heat transfer [3]. Finally, core materials have to ensure the lowest possible radiation transfer between the panels themselves [3, 20]. Some of the most common types of core materials used for VIPs are briefly discussed in the succeeding texts and compared on the basis of these requirements.

2.3.1 Fumed silica

Fumed silica is the most commonly used core material for VIPs [3]. It is a porous material produced in a flame where silicon chloride (SiCl2) is transformed into silicon dioxide (SiO2) aggregates (Figure 5).

Figure 5.

Fumed silica under electronic microscope [3].

Fumed silica is a stable material that endures pressures of up to 10 t per square metre, which is required for evacuation of the core [3]. At the same time, this material has a relatively low density of 150–200 kg/m3 and surface area of 50–600 m2/g, which allow it to achieve a remarkably low thermal conductivity levels of 3–6 mW/mK at pressures in the range of 20–100 × 10−3 bar [2, 35]. Finally, fumed silica material achieves low radiative heat conductivity of 0.001–0.004 W/mK at 1 mbar gas pressure levels depending on temperature [20].

2.3.2 Silica aerogel

Silica aerogels are the aerogels derived from silicate. Like other aerogels, this material is notable for its extremely low density (up to 3 kg/m3), high internal surface area (600–1000 m2/g) and small pore size (1–100 nm) [20]. This allows silica aerogels to achieve very low thermal conductivity rates: 1–3 mW/mK under evacuated and opacified conditions and certain temperatures and up to 4 mW/mK under 50 mbar pressure or less, whereas values of 13.5 mW/mK are possible at regular pressures [36]. Silica aerogels are also nonreactive and nonflammable materials, and they have the tendency to absorb infrared radiation. The major disadvantage of these materials for VIP applications remain relatively the high cost of production: with €25/m2 and €3000/m3, they cost nearly 10 times more than the traditional insulation materials of the same thermal resistance [36]. Further, silica aerogels, like other aerogels, are also very fragile because of poor tensile strength.

2.3.3 Expanded polystyrene and polyurethane foams

Open cell expanded polystyrene (EPS) and PUR foams represent another core materials applied in VIPs [29]. The low cost, small pore size (30–250 nm) and low density (60–100 kg/m3) of these foams make them suitable for use in VIPs [29]. PUR foams are nearly three times more expensive than EPS foams (€7.10 vs €2.81/m2), but they offer higher thermal resistance [20]. However, to maintain the acceptable low thermal conductivity of both PUR and EPS foams as core materials, relatively low gas pressures are required (10−4 bar and lower). As [37] showed, at this pressure, the total thermal conductivity of PUR foams with 100 nm pore size can reach as low as 7.8 mW/mK. However, pressures that low are extremely difficult to maintain over the life time of a VIP. The same study demonstrated that even if the gaseous thermal conductivity can be eliminated, EPS and PUR foams at densities of 70 kg/m3 will have total thermal conductivities of 5.7–9.7 mW/mK [37], which are higher than the values demonstrated by fumed silica.

2.3.4 Fibreglass

As a core material for VIP, fibreglass has advantages of high temperature applications, which are possible because of its high thermal stability (over 1000 °C) and density. The internal structure of fibreglass (Figure 6) ensures small contacts between the particles, which leads to low conductive heat transfer rates even when the material is compressed. Average cost of fibreglass is about €2.81/m2 [20]. [37] experimentally demonstrated the solid thermal conductivity and the radiative conductivity of 2.1 and 0.7 mW/mK, respectively, for fibreglass of density 250 kg/m3 and diameter of fibre 0.5–0.7 nm at 300 K. [37] also showed that the theoretical total thermal conductivity of 3.6 mW/mK is possible; although, as with the foams, it would require low pressure at 0.1 × 10−3 bar. Therefore, fibreglass as a core material for VIP has the same disadvantage in terms of long-term applications.

Figure 6.

Fibreglass under electronic microscope [3].

2.3.5 Staggered beam

Kawaguchi and Nagai have proposed by using the staggered beam structure (Figure 7) in vacuum insulation for reducing the solid thermal conduction, thus decreasing the total conduction value [37]. The structure makes the use of parallel beams having rectangular cross-sections to make the thermal conduction path as long as possible.

Figure 7.

Multilayered staggered beam with radiation shields redrawn from [37].

[37] experimentally demonstrated that a polycarbonate staggered beam structure allows to achieve very low solid thermal conductivity of about 0.9 mW/mK; however, because of large pore size, the gaseous conductivity and the radiative conductivity of the staggered beam is higher than that of the other VIP core materials presented previously.

2.3.6 Composite materials

National Research Council Institute for Research in Construction researchers have developed an alternative type of core material, which they refer to as low cost. Thin slices of fibre insulation board and fine insulating powdered material are layered together in a composite material [38]. Figure 8 shows how the movement of powders inside the fibre structures results in smaller pore sizes in the fibre-powder composite structure.

Figure 8.

Fibre pore structures packed with powder particles (redrawn from [38]).

Although the total thermal conductivity of the composite materials has been found comparative with that of precipitated silica for pressure ranges 0.25–100 × 10−3 bar [38], there is still little research available to report on the suitability of using these materials for VIP cores.

2.4 The envelope

2.4.1 Function and materials

Envelopes preserve vacuum in the VIP by creating barriers for water vapour and other gases entering the panels and protect the panel from the damages caused by the environment and handling procedures. VIP envelopes usually consist of several layers: the outer protective layer protects the panel from handling and environmental stresses, the barrier layer protects the core material from water vapour and gas transmissions and the inner layer seals the core material of the panel [2]. The envelope is formed by joining these layers with an adhesive, most commonly, PUR [2]. Double envelopes that use additional porous material between the envelopes have been investigated [32], although their application may not be feasible because of the greater panel thickness and additional costs of production.

Vacuum insulation panel envelope layers are made of films, which are generally 100–200 nm thin [20]. Plastics and metal foils are common materials used for the films. Metal foils, such as aluminium, have advantage of sturdiness and extremely low gas and water permeating rates; however, their disadvantage is high thermal conductivity rates. It is typical, therefore, for VIP manufacturers to use plastics and metals in combination by producing metallized plastics and multilayer laminated foils. Three types of multilayer films are available on the market today: (i) metal foils, which consist of an outer polyethylene terephthalate (PET) layer, aluminium barrier layer and a PE inner layer; (ii) metallized films consisting of up to three layers of aluminium-coated PET for the outer and barrier layers and a PE sealing layer; and (iii) polymer films based on nylon, polyester or polypropylene. However, the application of the polymer films on their own is limited because of higher gas and vapour permeability rates. Figure 9 demonstrates some of the most commonly used films for VIP.

Figure 9.

Some typical films for vacuum insulation panel: a metal film (AF), a single layer metallized film (MF1) and three layer metallized films (MF2, MF3) [20].

Because it is common to use a combination of film types in VIP envelopes, there is no specific envelope permeance rate. Instead, two empirical values are used to determine envelope material properties: the gas transmission rate (GTR) and the water vapour transmission rate (WVTR).

2.4.2 Gas transmission rate

Gas transmission rate can be defined as the total volume of gas that passes through a unit area of material per unit of time under unit partial pressure difference. [20] Measured GTR is measured as follows [39]:

display math(1)

where

GTRtot

is the total gas transmission rate of material;

GTRA

is the GTR of the surface of the laminate cover per panel area;

A

is the total area of the panes, including front and back sides;

GTRL

is the length-related GTR along the panel's circumference;

L

is the panel circumference.

The total GTR relates to the laminate permeance through the pressure difference across laminate barrier:

display math(2)

where

Qtotg

is the laminate permeance;

∆pg

is the pressure difference across laminate barrier.

Using the law of mass conservation and the ideal gas equation, the relationship between the gas pressure increase and the total GTR can be written as follows:

display math(3)

where

inline image

is the conversion factor from standard to measurement conditions (0–m);

Veff

is the effective pore volume in the panel.

If assumed that the initial internal pressure is negligible, then pressure difference across laminate barrier would initially be equal the atmospheric pressure. Consequently, using (3), gas pressure increase over time can be defined as follows:

display math(4)

Figure 10 shows this dependence on a graph for the envelope materials identified in Section 2.3.1. The lines marked 50 are for the panel sizes 50 × 50 × 1 cm3, and the lines marked 100 are for the panel sizes 100 × 100 × 2 cm3. This is a simplified version based on [39], which assumes the initial internal air pressure 0 bar, no getters are used and that the envelope properties do not change over time.

Figure 10.

Increase in internal gas pressure over time for some envelope materials and panel sizes [20].

2.4.3 Water vapour transmission rate

Water vapour transmission rate refers to the total volume of water vapour that passes through a unit area of material per unit of time. It is measured in the following way [40, 41]:

display math(5)

where

inline image

is the panel's mass increase over time;

Qwtot

is the total water permeance;

∆pwv

is the water vapour pressure across the film.

Applying the inverse function of sorption isotherm (ϕ(Xw)):

display math(6)

where

pw

is the partial water vapour pressure;

pwsat(T)

is the saturation of the water vapour based on temperature;

ϕ(Xw) 

is the water vapour saturation pressure based on relative humidity level.

Then, the change in water content over time can be expressed as [40]:

display math(7)

where

md

is the mass of VIP dry;

pwout and pwin

are the water vapour pressures outside and inside of VIP, respectively;

ϕout and ϕin

are the relative humidity outside and inside of the panel. respectively.

Considering the sorption isotherm linear as Xw = kϕ, where k is the coefficient representing the degree of slope, Equation (10) can show water saturation over time as follows:

display math(8)

Figure 11 shows a graphical representation of the function for the envelope materials identified earlier in the paper. The lines marked 50 are for the panel sizes 50 × 50 × 1 cm3, and the lines marked 100 are for the panel sizes 100 × 100 × 2 cm3. This is a simplified version based on Schwab et al. (2005), which assumes that no getters are used and that the envelope properties do not change over time.

Figure 11.

Increase in water content over time for some envelope materials and panel sizes [20].

2.4.4 Factors affecting gas transmission test and water vapour transmission rate

As follows from Equations (4)(11), GTR and WVTR in VIPs depend on temperature and panel size, whereas WVTR is also dependent on the factor of humidity. Figures 12 and 13 demonstrates the differences in GTR and WVTR based on panel sizes: smaller size VIPs had higher degrees of both. Temperature influence on both GTR and WVTR is commonly expressed as an Arrhenius equation type [21, 34]:

display math(9)

where

Ea

is the activation energy;

R

is the gas constant.

Figure 12.

Vacuum insulation panel envelope films' gas transmission rate (GTR) as a function of temperature. On the basis of the empirical numbers provided by[39].

Figure 13.

Vacuum insulation panel envelope films' water vapour transmission rate (WVTR) as a function of humidity. On the basis of the empirical numbers provided by[40].

Different types of films have different levels of sensibility to temperature changes. Figure 12 shows how the film types described in the previous sections react to temperature changes in terms of GTR.

As is seen, a single layer metallized film (MF 1) is the most sensitive to temperature changes, which makes them less suitable for VIP applications. Metal foils (AF) and three layer metallized films with 20 nm aluminium coatings (MF 2) have comparable values. However, thermal conductivity of AF is higher, which makes MF 2 films the most promising multilayer material for VIP use on the basis of temperature increases.

The effect of humidity levels on WVTR were reported by [40, 41] and [20]. At the fixed temperature of 25 °C, the differences between various barrier films' WVTR are demonstrated in Figure 13 for humidity levels of 45% and 75%. The graph shows that a single-layer metal foil barriers (MF 1) are, again, inferior because of higher initial level of WVTR and a stronger spike in WVTR at the increased humidity level in comparison with the three-layered metallized films (MF 2) and metal foil films (AF).

2.5 Getters and desiccants

Getters and desiccants are the chemicals added to the core materials to adsorb gases and water vapour. By doing so, they prevent the higher gas and water vapour pressures, thus decreasing the thermal conductivity of VIPs and their lifetime [2]. Silica core materials act as desiccants themselves, but other core materials require additional chemical protection. Silica gels and, sometimes, other chemical such as synthetic zeolites are added to nonsilica core materials for gas adsorption purposes [20]. For silica core materials, opacifiers, such as silicon carbide powder, black carbon, iron oxide (Fe3O4) and titanium dioxide (TiO2) are added to reduce the material's radiative thermal conductivity [20].

3 MEASURING VIP PROPERTIES

3.1 Thermal conductivity

The major property of an insulation material for buildings is the thermal conductivity, which is determined as the material's thickness divided by its thermal resistance.

display math(10)

The materials with the lowest thermal resistance are the best, because they are thinner and retain heat better. In general, thermal conductivity of a material is considered a sum of several units:

display math(11)

where

λT

is the total thermal conductivity;

λs

is the thermal conductivity of the material skeleton through the atomic bonds;

λg

is the thermal conductivity accounting for gas in the material pores;

λr

is the thermal conductivity accounting for the radiation transfer between internal pore surfaces of the material;

λc

is the thermal conductivity accounting for air and moisture convection within the pores;

λl

is the thermal conductivity accounting for thermal leakage transport because of pressure differences;

λcoup

is the thermal conductivity accounting for second order effects between the mentioned thermal conductivities.

However, the unique structure of VIP allows it to achieve negligible levels of the last three components, thus reducing the thermal conductivity equation to [12]:

display math(12)

Therefore, achieving lower conductivity levels for VIPs is possible by reducing these components. Solid conductivity can be defined as a function of the core material structure, density and external pressure [2]:

display math(13)

where

ρ

is the core material density;

a

is a value of foam unity.

From Equation (13), it is clear that materials with lower density will achieve lower solid conductivities.

Gas conductivity for VIP can be measured by using the Knudsen number [2]. [37] estimated gas conductivity for VIPs at temperature 25 °C as follows:

display math(14)

where

λo

is the thermal conductivity of air at atmospheric pressure;

p

is the gas pressure;

Φ

is the pore width of insulation material.

As follows from Equation (14), lower pressures provide a positive influence on reducing the thermal conductivity of gas. Further, the gaseous thermal conductivity decreases with smaller pore size of insulation material [20]. This relationship is shown in Figure 14.

Figure 14.

The gaseous thermal conductivity as a function of gas pressure and pore size of an insulation material [2].

The final component of the VIP thermal conductivity value is radiative conductivity. As was noted previously (Section 2.5), opacifiers are commonly used in VIPs to reduce radiative conductivity of core materials. The positive results of opacifiers' applications were noted by [42], who demonstrated that at a room temperature, opacified precipitated silica has from 0.002 to 0.003 W/m/K lower conductivity than pure silica.

3.2 Ageing and service life

Time has an important effect on VIP's thermal conductivity by decreasing it because of effects of water and gas permeance into the panel. [43] proposed that the function of the total thermal conductivity change over time is as follows:

display math(15)

where

λe(t)

is the thermal conductivity of the evacuated VIP in the beginning of service;

λg(t)

is the thermal conduction caused by GTR over time;

λwv(t)

is the thermal conduction caused by WVTR over time;

λw(t)

is the thermal conduction over time based on adsorbed water within the core material.

Envelopes and panel size play an important part in reducing the effects of time on thermal conductivity of VIPs. Figure 15 shows the total thermal conductivities as functions of time for the VIPs using fumed silica as the core material, aluminium (AF) and metallized (MF) envelopes for panels of two sizes: 50 × 50 × 1 and 100 × 100 × 2 cm. AF, larger size VIPs, shows slower thermal conductivity reduction over time.

Figure 15.

Vacuum insulation panel thermal conductivity as a function of time [20]. Temperature, humidity and porosity are assumed constant. Gas pressure is set at 0 bar. No getters, desiccants or pacifiers are used.

Literature defines two approaches to measuring the useful life of a VIP [33]. The first approach is the one used by VIP manufacturers and incorporated into the American Standard C1484 for VIPs. This approach defines service life as a time span from the panel manufacturing to the time when the effective thermal conductivity of the panel exceeds the established limiting value:

display math(16)

The second approach defines service life as a time span from the panel manufacturing to the time when the time-average thermal conductivity exceeds the established critical value. This approach takes into account the non-linear nature of ageing and can be expressed formulaically as follows:

display math(17)

Both approaches to VIP service life assume it as a function of several factors [2, 3, 20]:

  • size: larger panels have longer life spans,
  • manufacturing quality: higher quality panels have longer life spans,
  • core materials: materials with lower pore sizes have longer life spans,
  • environment and conditions of use: high moisture, humidity and temperature decrease the life span,
  • use of opacifiers, getters and desiccants increases life span,
  • physical handling: scratches and cracks reduce life span of VIPs.

The second approach also takes into account the building construction heat loss through thermal bridges. Thermal bridges are heat transfers where a noninsulating, heat transmitting material is placed between the conditioned space and the exterior environment of a building assembly [44]. For VIPs, bridge transfers occur at the panel level, building components level and facade level [45]. The linear thermal transmittance that represents thermal bridge effect depends on the thickness of a VIP, its length and perimeter area [2]. The effect of the thermal transmittance on the total thermal conductivity of a VIP can be expressed as follows [21, 26]:

display math(18)

where

λcop

is the thermal conductivity of VIP centre;

Ψedge

is the linear thermal transmittance;

dp

is the panel thickness;

Ip

is the perimeter length of VIP;

Sp

is the surface area of VIP.

The effect of thermal bridges on thermal conductivity and, therefore, service life of VIPs was a subject of both numerical [26, 45-48] and analytical [27, 48, 49] investigations. In general, the studies showed that VIPs with aluminium foil envelopes have higher (up to 50 times) thermal transmittance rates than VIPs with multilayer foils or metallized polymer films [50, 51]. Studies also showed high measures of thermal transmittance (0.170 W/mK) for such types of panels even if there was no air space between the panels [51]. For these reasons, [20] did not recommend use of VIPs with laminated aluminium foils (type AF in Figure 10) for buildings if their size is less than 1 m2. Some studies suggested encapsulating AF-VIPs with EPS or extruded polystyrene to reduce the total thermal conductivity [51, 52]. Although the results of those studies showed improvements in thermal conductivity, the obtained measurements were still higher than for the coated multilayer foils. Another proposed solution has been the use of serpentine edges [53] to increase the path for the heat flux. Theoretically, it was shown that the linear thermal conductivity of the panel edge could be significantly reduced [3]; although no practical investigations were conducted to confirm the feasibility of these innovations.

Studies on thermal transmittance of VIPs also showed that VIP envelopes with higher thermal conductivity have stronger thermal bridge effect [27]. This means that the materials with lower thermal conductivity are preferable for VIP envelopes. [2] suggested application of thin films of SiOx and SiNx coated on a polymer substrate for these purposes. It was also found that larger panels are thermally advantageous over smaller ones, although the effect decreases with the size. According to [3], a 2.5 m2 VIP panel provides a nearly 10% thermal efficiency increase over a 1 m2 panel, although further increasing the size to 4 m2 only provided increase by a few percent.

A typical VIP has two protecting facings on each side that are linked with a spacer. Use of EPS for VIPs with aluminium foil and aluminium-coated envelopes was shown to reduce thermal transmittance losses [54, 55]. However, [45, 49] showed that thermoplastic spacers showed lower transmittance rates. Further, it was found that EPS along the encapsulated VIP perimeter provided additional thermal bridge effects [56]. [2] suggested by using a better insulator or minimizing EPS strips to reduce the thermal bridge effects.

4 ECONOMICAL FEASIBILITY OF VACUUM INSULATION PANELS

As was discussed previously, the major advantage of VIPs is its extremely low thermal conductivity rate, which allows to apply thin panels to ensure the same level of thermal resistance as with the traditional insulation materials. Therefore, the benefit of VIPs comes in form of space savings. At the same time, VIPs are rather costly in comparison with the traditional materials. Therefore, applications of VIPs for buildings need to be evaluated also from the economic point of view. [57] showed that the life cycle costs for VIPs and a traditional insulation material, such as mineral wool, are comparable (Figure 16). Their simulation analysis was based on the insulation of a 15 cm brick wall, energy costs of €0.06/m2, floor height of 2.8, land cost of €400/m2 and space renting cost of €200/m2.

Figure 16.

A comparative analysis of life cycle costs of mineral wool (λ = 0.036; p = €100/m3) and vacuum insulation panel (VIP) (λ = 0.008; p = €2000/m3, service life 50 years. Land cost is €400/m2, energy costs are €0.06 kWh, space renting costs €200/(m2 · a), floor height 2.8 m, and capital costs 5% p.a.) [57].

[57] proposed that the economic benefit of using VIP in buildings shows up whenever there is high cost of building rent so that the saved wall thickness compensates for the high cost of VIPs. [18] expressed the same idea with regards to space driven savings achieved with VIPs. Jelle applied economic analysis for replacement of a 35 cm mineral wool insulation (p = €20/m2) with a 6 cm VIP (€200/m2) on a 9 cm timber frame to maintain the required wall thickness for the load-bearing building properties. The simulated interior wall area and floor area were 100 m2 each. Figure 17 shows the expected cost savings using VIPs instead of mineral wool as a function of living area cost. As is seen, the higher the cost of living area, the more savings are provided with VIP applications. At about €2400/m2 living area, installation of VIP represents a break point of costs.

Figure 17.

Vacuum insulation panel (VIP) profit as a function of market value living area [18].

[2] used a payback analysis to determine economic feasibility of VIP applications in buildings. The payback period was calculated as follows:

display math(19)

where

Cins

is the cost of insulation material;

CA,cur

is the annual heating costs for the current thermal conductivity (traditional material);

CA,impr

is the annual heating costs for the improved thermal conductivity (VIP).

Annual costs of heating are calculated as follows:

display math(20)

where

HDD

is the heating degree day;

Cf

is the cost of fuel;

U

is the average U-value;

PWF

is the discounting factor N/(1 + i) where N is the life time in years and i is the inflation rate;

Hv

is the heating value of fuel;

η

is the efficiency of the heating system.

Four scenarios (Table 2) were considered by [2] for a VIP and an EPS panel for comparative purposes. The analysis was conducted on the basis of the values for the UK buildings and, therefore, cannot be generalized onto other countries. However, similarly to [2] and [57], it showed that the economic benefit of VIPs in buildings (this time in the form of shorter payback) significantly improves (Figure 18) the value of space savings is taken into account.

Table 2. Payback period scenarios for vacuum insulation panel and expanded polystyrene in buildings [2].
 ParametersValue
  1. VIP, vacuum insulation panel; EPS, expanded polystyrene; HDD, heating degree day; Cf, cost of fuel; η, efficiency of the heating system; N, panel life time; i, inflation rate.

Scenario 1Average building U-value0.40 W/m2/K
VIPThickness10 mm
 Cost€82.37/m2
EPSThickness48.3 mm
 Cost€2.82/m2
Scenario 2Average building U-value0.31 W/m2/K
VIPThickness25 mm
 Cost€164.74/m2
EPSThickness113 mm
 Cost€7.28/m2
Scenario 3Average building U-value0.27 W/m2/K
VIPThickness40 mm
 Cost€164.74/m2
EPSThickness180 mm
 Cost€9.86/m2
Scenario 4Average building U-value0.24 W/m2/K
VIPThickness60 mm
 Cost€164.74/m2
EPSThickness256 mm
 Cost€12.69/m2
 FuelNatural gas
Other parametersEmission conversion factor0.2
 HDD1931 °C day
 Cf€0.47/m3
 Hv39.5 × 106 J/m3
 η0.9
 N25 years
 i10%
Figure 18.

Payback period of vacuum insulation period (VIP) and expanded polystyrene (EPS) boards based on scenarios from Table 2 (redrawn from [2]). The calculations were performed by using a base rent of €506/m2.

As is seen, the initial payback period of VIPs is higher in comparison with EPS, which is dictated by their high initial costs. Therefore, decreasing the costs of production is the priority for VIP manufacturers in order to make VIPs a cost-effective alternative to the commonly used insulation materials. However, even considering the current costs of production, VIPs could be an attractive alternative for listed buildings where insulation of outer surfaces is impractical. In this case, the economic value of space saved increases with the required degree of insulation. As scenario 4 demonstrates, under high insulation requirements, VIPs could have even lower payback in comparison to EPS.

5 VACUUM INSULATION PANELS IN BUILDINGS

5.1 Recent progress in applications

Research shows that VIPs for buildings are being produced in many parts of the world [3, 20]. VIPs reduce the space required for insulation in most common applications. Installation of VIPs successfully combines high thermal performance and a sleek building structure. The multilayered metallized polymer VIP provides a high barrier material for long-lasting performance in buildings, as it minimizes the thermal bridge effect through edge zones. For wet, alkaline environments at high temperature VIPs are not yet suitable recommended to use in buildings.

The research in VIPs and their applications has also been growing rapidly in several countries including Scandinavian countries, Switzerland, Germany, the UK, Canada, USA and a number of Asian countries [58]. Several types of applications of VIPs in buildings have been studied so far. Internal [47] [59] and external [22, 46, 52] insulations of buildings have been researched because of VIPs' potential to increase available living space. VIPs have also been researched for their applications in window frames and door frames, where reduction of thermal transmittance of up to 50% was reported [27, 60]. Investigations of VIP applications for roof and terrace insulation were conducted, where the expected service life of several decades were shown for VIPs [34, 61]. VIPs were also investigated as a sustainable insulation material for the new buildings [34, 62]. [63] showed 20 different building applications of VIPs being applied on practice.

5.2 Performance of various vacuum insulation panels on the market

5.2.1 Vacuum insulation panels market for building sector

Although the research on VIPs has progressed in the recent years, there are very few prefabricated VIP systems available specifically for building sector today [64]. Types of VIP are generally determined by the type of core materials, envelopes, as well as by the presence of getters, desiccants and opacifiers, and the presence of additional cladding materials.

Table 3 shows the most common types of VIP currently available for building applications. The manufacturers represented are from different parts of the world, including the UK (EnviroHomes), USA (LG Hausys, Dow Corning, ThermalVisions, Microtherm and AmericanAerogel), Germany (va-Q-tec), Iceland (Sokkull) and China (Xiamen Goot and Suzhou). The data for each type of VIP is obtained from the companies' websites and personal communication with the company representatives where possible. Fumed silica and mixed silica VIPs are the most common core materials used by the manufacturers, although some manufacturers prefer fibreglass as a core material. Rarely offered are the panels with PUR foam, aerogels and other materials such as microfleece as cores. It is clear, however, that these panels offer short service lives comparatively with the silica panels. For silica panels, several variations of envelopes are offered, although multilayer aluminium envelopes seem to be the most common material applied.

Table 3. Vacuum insulation panel available on the market.
ProducerTitleCoreBarrierThickness (mm)Initial thermal conductivity (mW/mK)*Actual thermal conductivity (mW/mK)Regular size (cm)Maximum size (cm)Service life (years)
  • PUR, polyurethane; PES, polyester foil.

  • *

    Thermal conductivities, both initial and actual, are given for 20-mm-thick 50 × 50 cm panels.

va-Q-tecva-Q-vip BFumed silicaHigh barrier film10–504.37–850 × 60100 × 6060
 va-Q-purPUR foamAluminum foil laminate10–40-7–950 × 50120 × 10015
 va-Q-micMicrofleeceAluminum foil laminate14–202.8–3.5-50 × 50130 × 1005
LG Hausys FibreglassAluminum foil laminate1–304-50 × 50180 × 70-
EnviroHomesVacupor PSMix (SiO2, SiC, others)Aluminum foil laminate10–3045–1950 × 50100 × 5040–50
 Vacupor RPMix (SiO2, SiC, others)Aluminum foil laminate, rubber10–5045–1960 × 50220 × 10050
 Vacupor NT B2Mix (SiO2, SiC, others)Aluminum foil laminate10–5045–1960 × 50220 × 10040–50
Dow Corning Fumed SilicaAluminum metallized laminate6.3–382.13-60 × 90-60
ThermalVisionsTHRESHHOLD--5–512.88-50 × 50100 × 127-
MicrothermSlimVacFumed silicaMultilayer polymer6–404.2950 × 50140 × 8030–50
American AerogelAerocoreAerogelAluminized PES12–251.9–4.2---10–15
Xiamen GootGOOTFibreglassAluminum foil, multilayer polymer10–354-50 × 50170 × 8010–15
SokkullQasaPyrogenic silicaMetallized high barrier10–5048–1950 × 50125 × 300-
SuzhouHD-002FibreglassMetal foil laminate8–353.5–4-50 × 50-10–15

The numbers demonstrated in Table 3 for each type of panels are provided by manufacturers. In many cases, only information on initial, postproduction thermal conductivities is available. However, studies have been conducted to evaluate the actual performance of various VIP systems, some of which are already commercially produced. The following sections review the performance of various panels based on the independent researches.

5.2.2 Fumed silica vacuum insulation panels for buildings

Fumed silica VIPs are the most popular among the commercially produced panels for building applications and also the most researched ones [20, 65]. Fumed silica is commonly chosen for its low thermal conductivity at normal pressures.

A number of studies have been conducted to determine the optimal envelope material for fumed silica boards. Among the envelopes, variations in metal foils, metallized polymers and laminated materials were investigated. Researchers also noted the difference in silica VIP performance based on the panel sizes and thickness. [66] conducted hot box measurements for fumed silica va-Q-tec VIPs with seven variations based on panel envelopes and thickness: 2 and 4 cm single layer metal foil, single layer 2 cm, double layer 2 cm and double layer 2 cm with staggered joints and two 18 mm VIPs with tapered edges. The results of the analysis demonstrated that the real thermal conductivity values (U-values) were higher than the numerically predicted. At the same time, the performance of 20 mm double layer VIPs was comparable with 40 mm VIPs.

[20] investigated the performance of four different types of fumed silica VIPs for buildings: a metal film panel, a single layer metallized film panel and two multilayer metallized film panels. The authors also conducted measurements for different thickness values of the panels. In general, it was determined that single metal foil VIPs performed worst in terms of thermal conductivity. The VIPs with multilayered aluminium/polymer envelopes showed the best performance, whereas the single layer metallized film VIP had worse measurements of GTR, WVTR and service life. Notably, the study showed that 100 × 100 cm multilayer envelope VIP maintained the thermal conductivity value of 8 mW/mK after 60 years of a lifetime (generally quoted by the manufacturers), whereas 50 × 50 cm VIP of the same kind would have around 10 mW/mK [20]. This indicates the effect of panel size on its lifetime performance.

5.2.3 Other vacuum insulation panel types for buildings

Although silica VIPs remain the most popular type of panels on the market for building applications, research continues for other types of panels as well. However, so far, the results demonstrate the superiority of silica panels. [64] demonstrated that PUR-core VIPs have worst performance because of high outgassing rates. [29] showed that the use of low density polyethylene with polystyrene at some temperatures may provide thermal conductivities as low as 6.5 mW/mK, which, however, is still higher than those demonstrated by silica VIPs. [32] proposed by using double envelopes and three-sided sealing envelopes with getters with PUR foams as cores for VIPs. However, despite the relatively low thermal conductivities achieved, the service life of the panels was only about 20 years. [67] investigated the application of steel-clad VIP systems and found them to be much better suited for building applications because of high durability rates. However, in terms of thermal conductivity, the sheet steel cladding VIPs demonstrated values of 14.7–20.0 mW/mK [67].

6 CONCLUSIONS AND OUTLOOK

In 2011, the European Council acknowledged that the Union's target for energy efficiency and greenhouse emissions is not on track [68]. One of the major actions recommended by the council to attain the required energy efficiencies is achieving higher energy savings in buildings, which represent 40% of the Union's final energy consumption. The Energy Efficiency Directive 2012 binds the member states to establish a long-term strategy for residential and commercial building innovations to improve their energy performance. These new requirements establish the need for new, highly efficient insulation methods and materials. VIPs represent one of the most promising technological innovations in this regard. These state-of-the-art insulation systems provide a lower degree of thermal conductivity than the traditional insulation materials. They help achieve the desired energy standards while at the same time saving living space because of thinner building walls.

So far, however, these materials are not in the widespread use in buildings because of their high cost, susceptibility to perforation and the effects that worsen their performance (GTR, WVTR and thermal bridging). The currently available on the market VIPs are not numerous in variations. VIPs with silica cores and multilayered envelopes are the most popular types of VIP systems, although other variations in terms of both are present. Silica based VIPs with multilayered envelope structure provide low thermal conductivity rates (4–10 mW/mK) and have relatively long service lifetime of about 60 years for standard 50 × 50 cm panels with 20 mm thickness. Larger and thicker panels have even better analytically and practically measured performances. Moreover, some manufacturers, such as va-Q-tec and EnviroHomes, are already providing customized VIPs in terms of size and shape. This allows to eliminate one of the disadvantages of VIPs based on the inability to cut the material without losing its thermal conductivity performance.

Yet, many unresolved issues remain. Fumed silica, while having the best thermal performance, is also one of the most expensive materials, which drives VIP cost to about €2000/m3. Other core materials, such as fibreglass or PUR foams, are cheaper, but they provide worse thermal conductivity and have shorter service lifetime. The analysis performed in this paper showed that current VIPs can be economically feasible only in cases of rental costs starting at around €2400/m2 considering that the saved space contributes to the rental savings. In order to spur the widespread application of VIPs in buildings, it is of utmost importance to reduce the cost of their production without sacrificing the high level of thermal performance. In this regard, research considering both core materials and envelopes is needed to answer the question whether more efficient solutions are possible than the currently existing on the market.

The European Union member states are obligated to implement the major Energy Efficiency Directive 2012 points by June 2014. This means that the importance of energy efficiency research, including thermal insulation initiatives, is higher than ever. This review demonstrated that under certain conditions VIPs could be economically feasible and provide short payback periods after their installation. This means that these insulation materials deserve further consideration for sustainable building construction. It is expected that with more manufacturers entering the market and with improvements in the manufacturing process, the market price on VIPs will decline, making them more attractive for wider applications in buildings.

NOMENCLATURE

Abbreviations
AF

= metal film

EPS

= expanded polystyrene

GFP

= gas filled panels

MF1

= a single layer metallized film

MF2, MF3

= three layer metallized films

PE

= polyethylene

PET

= polyethylene terephthalate

PUR

= polyurethane

VIP

= vacuum insulation panel

XPS

= extruded polystyrene

Formulae variables
A

= total area of the panes, including front and back sides (m2)

C

= cost (€/m3)

Cacur

= annual heating costs for the current thermal conductivity (traditional material)

Caimpr

= annual heating costs for the improved thermal conductivity (VIP)

Cf

= cost of fuel

Cins

= cost of insulation material

Ea

= activation energy (J/mol)

H

= heating value (J/m3)

Hv

= heating value of fuel

K

= coefficient representing the degree of slope

L

= panel circumference (m)

N

= panel life time (years)

Q

= permeance (m3(STP)/(m2/day/Pa))

Qtotg

= laminate permeance

Qwtot

= total water permeance

R

= the gas constant (J/(mol K))

S

= surface area of VIP (m2)

T

= temperature (K)

U

= average thermal conductivity (W/m2/K)

V

= pore volume (m3)

Veff

= effective pore volume in the panel

Xw

= water content (% mass)

Ρ

= core material density

p

= pressure (Pa)

pwout

= water vapour pressure outside of VIP

pwin

= water vapour pressures inside of VIP

pw

= partial water vapour pressure

∆pg

= pressure difference across laminate barrier.

∆pw

= water vapour pressure across the film

a

= a value of foam unity

d

= panel thickness (m)

i

= inflation rate (%)

l

= perimeter length of VIP (m)

m

= mass (kg)

md

= the mass of VIP dry

mv

= the mass of VIP panel

GTR

= gas transmission rate, (m3/(m2 day))

GTRA

= GTR of the surface of the laminate cover per panel area

GTRL

= length-related GTR along the panel's circumference

GTRt

= total gas transmission rate of material

HDD

= the heating degree day (°C day)

PWF

= discounting factor (N/(1 + i))

WVTR

= water vapour transmission rate (g/(m2 day))

ϕ

= relative humidity (%)

ϕ(Xw)

= water vapour saturation pressure based on relative humidity level

ϕout

= relative humidity outside and inside of the panel

ϕin

= relative humidity inside of the panel

λ

= thermal conductivity (W/m/K)

λT

= total thermal conductivity

λs

= thermal conductivity of the material skeleton through the atomic bonds

λg

= thermal conductivity accounting for gas in the material pores

λr

= thermal conductivity accounting for the radiation transfer between internal pore surfaces of the material

λc

= thermal conductivity accounting for air and moisture convection within the pores

λl

= thermal conductivity accounting for thermal leakage transport because of pressure differences

λcoup

= thermal conductivity accounting for second order effects between the mentioned thermal conductivities

λ0

= thermal conductivity of air at atmospheric pressure

λe(t)

= thermal conductivity of the evacuated VIP in the beginning of service

λg(t)

= thermal conduction caused by GTR over time

λwv(t)

= thermal conduction caused by WVTR over time

λw(t)

= thermal conduction over time based on adsorbed water within the core material

λcop

= thermal conductivity of VIP centre

Φ

= the pore width of insulation material (m)

Ψ

= the linear thermal transmittance (m/K)

ρ

= the core material density (kg/m3)

η

= the efficiency of the heating system

Subscripts
A

= annual

atm

= atmospheric pressure

c

= convection

cop

= centre of panel

crit

= critical value

cur

= current

e

= evacuated VIP

edge

= edge

eff

= effective

g

= gas

impr

= annual heating costs for the improved thermal conductivity

ins

= insulation

lim

= limiting value

p

= panel

r

= radiation

s

= the material skeleton through the atomic bonds

sat

= saturation

tot

= total

totg

= total gas

w

= water

wtot

= total water

wv

= water vapour

Superscripts
Ea

= activation energy (J/mol)

R

= the gas constant (J/(mol/K))

a

= a value of foam unity

  • *

    All prices in the paper are presented in euros. Whenever cited sources used other currencies, they were converted to euros based on the currency rates established on 24 June 2013 (£1 = €1.18; $1 = €0.76)

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