A Comparison between Aerogel and Vacuum Insulation Panels

1.0 Abstract

The purpose of this document is to look at the comparison between aerogel and VIP (Vacuum Insulation Panels), two of the cutting edge insulation materials today. Topics such as pricing of current products from different companies, thermal performance of products and their durability and life expectancy will be addressed, in order to determine which material is better suited to be used in the new development on Shakespeare Street.

2.0 Introduction

Foamed silicates, also known as aerogels are super insulators with very high thermal resistance (John Fernandez, 2006, p. 225). The structure of aerogel consists of 2-5nm diameter particles that cluster together to form a highly porous three dimensional structure with 1-100 nm pores (5-70nm for silica aerogels, depending on the purity) and it is the lightest solid material known at the moment (Ruben Baetens et al, 2011, p. 764). The silica based aerogels are effectively transparent, though some light scattering has been known to occur. The heat transfer through the aerogel is comprised of the heat conduction of the gas in the pores, the conduction within the chains of particles that form the matrix of the material and infra-red radiation (Philip C Eames, 2009, p. 17-18). Aerogels can be produced as either opaque, translucent or transparent, allowing it to be used in a wide range of building applications (Bjørn Petter Jelle, 2011, p. 2552).

Vacuum Insulation Panels (VIP) were originally developed for use in the manufacturing of fridges in the 1980s. They are factory assembled, gas tight enclosures with rigid fibre, powder or foam cores, from which the air has been evacuated. The cores hold the vacuum and prevent the enclosing membrane from collapsing due to negative pressure (Felix Mara, 2010, p. 35). This product combines high thermal resistance with a relatively thin panel (20-40mm thick) and provides insulation values that are three to seven times better than that of an equivalent thickness of rigid foam boards, foam beads, or fibre blankets. (Xiaoxin Wang et al, 2007, p. 145) 

 Left) Peter Tsou from NASA with a translucent aerogel sample developed for space missions, (middle) matches on top of aerogel are protected from the flame underneath and (right) an example of aerogel as a high performance thermal insulation material.

(Left) typical VIP structure showing the main components and (right) a comparison of equivalent thermal resistance thickness of traditional thermal insulation and VIP.

3.0 Thermal Performance

Two techniques are currently used and accepted for the determination of the thermal performance of insulation materials: the guarded hot plate method, BS EN 12664:2001, BS EN 12667:2001, ASTM C177 and the calibrated and guarded hot box BS EN ISO 8990:1996, ASTM C1363 – 05. (Philip C Eames, 2009, p. 9)

Aerogels and VIP have demonstrated lower levels of thermal conductivity than standard insulation materials when tested using standard test apparatus. (Philip C Eames, 2009, p. 17)

3.1 Aerogel Thermal Performance

As the average pore size (1-100nm) is less than the mean free path of air molecules (70nm at normal pressure and temperature), silica based aerogels can reach a lower thermal conductivity than that of still air. However, an opacifier is required to be introduced during the manufacturing stage, in order to reduce long-wave radiative heat transfer.

Silica aerogels have been incorporated into commercially available insulation blankets that are suitable for building applications.

A thermal conductivity for silica based aerogels of as low as 0.005W/mK has been achieved (thermal conductivity of still air is 0.026W/mK), however, the thermal conductivity of commercially available aerogel insulation panels has been reported to be 0.012W/mK as a minimum.  (Philip C Eames, 2009, p. 18)

In ‘Thermal conductivity and compressive strain of aerogel insulation blankets under applied hydrostatic pressure’, Erik R. Bardy et al (2007, p.232-233) wrote that the thermal conductivity of aerogel goes as low as 0.013W/mK. They also show the thermal conductivity of other commercially available aerogel blankets from different suppliers (from most effective to least effective):

-          Two Spaceloft blankets with the thermal conductivities of 0.0141-0.0135 W/mK (composed of silica gel, Capolyolefin Poly – ethylene terephthalate) and 0.013 W/mK (composed of silica gel, carbon black Poly - ethylene terephthalate).

-          Two Pyrogel blankets with thermal conductivities of 0.0147 W/mK (composed of Silicon dioxide, Zircon silica gel – trimethylsilyated, Amophous silica fiber felt) and 0.0157 W/mK (composed of silica gel, Oxidized polyacrylonitrile).

-          Nanogel blanket with a thermal conductivity of 0.020 W/mK (composed of Treated precipitated silica/silica gel, trimethylsilylated blend with copolyolefin bicomponent fiber with nylon cover.

Its thermal properties are affected by density, pore size, distribution, opacification, and form. Operating pressure, temperature and filler gas also affect performance. The optimum product is therefore dependent on the insulation operating conditions.

They employed modelling to achieve the thermal performance of an aerogel monolith while employing aerogel granules. An experiment was made, which concluded in improving the thermal and mechanical properties of the aerogels. This was achieved with the application of a 1 bar load when producing opacified granules with the optimum density.  (D.M. Smith et al, 1998, p. 258)

3.2 VIP Thermal Performance

The panel centres in VIPs reach U-values lower than 0.005 W/m²K and are slowly approaching 0.001 W/m²W. Panels may need to be lined in order to control reverberation noise. Wall ties, which could be fibre-reinforced plastic, need to be integrated with panels and avoid cold bridging (Felix Mara, 2010, p. 35-36).

The core materials serves to provide physical support to the barrier film envelope so it does not collapse on itself when the vacuum is applied. Additionally, it has a role in the thermal performance of the overall material, as it interrupts the flow of the gas molecules that remain in the evacuated space, reducing their ability to transfer heat between the walls of the VIPs. The skin also has to offer high resistance to heat transfer through the barrier material, so plastics are chosen over metals due to this.

The size and shape of the panel is a factor that influences the material’s overall thermal performance as well. If the panel is small, there is an increased amount of heat flow around the edge and it can undo all the heat resistance offered by the evacuated area (Xiaoxin Wang et al, 2007, p. 146).

Xiaoxin Wang et al (2007) also wrote that granular silica aerogels can also be used as a core material for VIPs, due to their properties.   

Below is a list of vacuum insulated panels with their thermal conductivity values:

-          Kevothermal VIP thermal conductivity value – 0.0036 W/mK (aged value – 0.007 W/mK)

Available at: http://www.greenbuildingstore.co.uk/page--kevothermal-vacuum-insulated-panels-vip.html

-          va-Q-tec vacuum insulation panels:

·         va-Q-plus thermal conductivity value - < 0.0035 W/mK

·         va-Q-mic thermal conductivity value – 0.0028 to 0.0035 W/mK

·         va-Q-vip thermal conductivity value - < 0.005 W/mK

·         va-Q-pur thermal conductivity value - 0.007 to 0.009 W/mK

Available at: http://www.va-q-tec.com/en/appliances/products/ 

4.0 Supplier prices

Both aerogels and VIPs are still expensive compared to other insulation materials (John Fernandez, 2006, p.225; Felix Mara, 2010, p. 36; Bjørn Petter Jelle, 2011), but it has been demonstrated that VIPs may be cost effective. Also, “aerogels in their transparent or translucent state offers application areas where one may be willing to accept higher costs“(Bjørn Petter Jelle, 2011, p. 2557).

Below is a list of price examples for different aerogel insulation blankets:

^-           Spaceloft 5mm blanket - £18.96 per m²

-          Spaceloft 10mm blanket - £27.92 per m²

-          Pyrogel XT-E 5mm blanket - £20.72 per m²

-          Pyrogel XT-E 10mm blanket - £28.54 per m²

Note: all prices have been taken from www.buy-aerogels.eu and converted from euros to pounds.

Kevothermal Vacuum Insulated Panels price table as an example (maximum panel size of 900mm x 600mm):

 Table available at: http://www.greenbuildingstore.co.uk/media/page_content/Insulation/Pricelist/GBS%20Revised%20Combined%20Insulation%20-%20%20Sept%202015.pdf

Note: As an example for a brief comparison with a traditional insulation material, the prices for rock wool insulation range between ~£2 and ~£8 per m², depending on thickness and brand, however its thermal performance is significantly weaker than aerogel and VIP.

Rock wool insulation price range taken from: http://www.insulationshop.co/glass_and_mineral_wool_insulation/rock_wool_insulation.html

5.0 Product life and physical degradation possibility

Insulation performance is the key selection criterion, however the material’s long term performance is determined by the durability of the insulation, which can be affected by factors such as settlement, physical degradation, vapour permeability and air moment (Raymond Ogden et al, 2012, p. 309).

The thermal conductivity of future thermal insulation materials should not increase substantially over a lifetime of 100 years or more. Additionally, they should maintain their thermal performance even if they are perforated by external objects, thermal conductivity increase due to thermal bridging is an exception, however it must be avoided. Another major requirement is the possibility to adjust and cut the material on site, if needed, without it losing its thermal performance. Other properties that should be taken into consideration include mechanical strength (compression and tensile strength), fire protection issues and fume emissions during fire (preferably no toxic gases should be released) (Bjørn Petter Jelle, 2011, p. 2558).

5.1 Aerogel

The aerogel blankets have little resistance to compression and experience a residual strain effect upon exposure to elevated pressures, even though they are flexible. However, Aspen Aerogels Inc. have created prototype aerogel blankets with increased resistance to compression and they have also minimised the residual strain from the exposure to elevated pressures. (Erik R. Bardy et al, 2007, p.232)

Samples of normal product-line aerogel blankets and of prototype blankets have been tested separately for compressive strain at incremental pressure stops of up to 1.2 MPa. (Erik R. Bardy et al, 2007, p.232)

The strain of the prototype blanket reached a level of 0.25mm/mm compared to the product-line blanket that reached a level of 0.48mm/mm. The thermal conductivity of the prototype was slightly higher than the product-line blanket, before the compression. During the compression, the thermal conductivity of the product-line blanket increased by 46%, while the prototype’s thermal conductivity only increased 13%. (Erik R. Bardy et al, 2007, p.232)

The total thermal resistance of the product-line blanket decreased by 64% and maintained that value after the decompression to atmospheric pressure. The prototype blanket lost only 33% of its thermal conductivity, however it returned to within 1% of its initial value after the decompression.

It was concluded that the prototype had approximately twice as much resistance to compression, compared to its product-line counterpart, and it also almost completely recovers to its original state upon decompression. (Erik R. Bardy et al, 2007, p.232)

In opposition to the above, Bjørn Petter Jelle (2011, p. 2552) wrote that aerogel has high compression strength, but it is fragile because of its low tensile strength (also Gaosheng Wei et al, 2011, p. 2355). Due to the fact that its greatest disadvantage is being brittle, some researchers are trying to composite aerogel with other materials with higher toughness and are already making progress (Xonolite-aerogel for example) (Gaosheng Wei et al, 2011, p. 2355).

The thermal conductivity is not considered to be increasing substantially with aging, and perforating the material does not represent a problem. (Bjørn Petter Jelle, 2011, p. 2557)

5.2 VIP

The lifetime of vacuum insulation panels are a concern at the moment. The impossibility to completely prevent air from filling the vacuums in VIPs prevent it from being used in cavity walls, as their maximum life is currently 30 years, as opposed to the required 100 years (Felix Mara, 2010, p. 36; Bjørn Petter Jelle, 2011, p. 2552).  In addition, the thermal conductivity of the material can double after 25 years of use due to water vapour and air diffusion through its envelope and into its core. Depending on its envelope the aged thermal conductivity after 50 and 100 years will be substantially higher (Bjørn Petter Jelle, 2011, p. 2552).

Quality is difficult to control. Core materials must be carefully handled and the internal gas pressure is critical. According to va-Q-tec, a manufacturer of VIPs, 90% of VIP failures are due to invisible micro-leakages or perforations (Felix Mara, 2010, p. 36; Xiaoxin Wang et al, 2007, p. 145; Bjørn Petter Jelle, 2011, p. 2553, 2560). The risk of damaging the panels is higher before the installation and during the transportation, however, unless well protected, they are also vulnerable after the installation (Xiaoxin Wang et al, 2007, p. 145).

Bjørn Petter Jelle (2011, p. 2560) reported that even if VIPs are handled with great care, they may sometimes suddenly lose their vacuum for no evident reason (event that happened in their laboratory during their experiments with VIP from different manufacturers), so there is a chance that this can happen after they are installed in the building envelope.

  

The size or VIPs cannot be adjusted on site without losing a large part of their thermal performance. (Bjørn Petter Jelle, 2011, p. 2553).

Additionally, even though the core material can reach a class 0 fire rating, the membrane enclosing the core can still be a fire risk (Felix Mara, 2010, p. 36).

6.0 Conclusion

Aerogel and VIP are the insulation materials leading the market in thermal performance. The vacuum insulated panels are more efficient in terms of thermal performance, however come with significant risks and weaknesses which could affect the performance of the building after the installation. The VIP’s loss of performance if punctured and the reported cases of loss of vacuum for no reason (Bjørn Petter Jelle, 2011, p. 2560), which could happen even a short while after construction has been completed and the insulation is in the building envelope, put the VIP in a disadvantage next to the aerogels. The only disadvantages of aerogel seems to be its low tensile strength, which shouldn’t pose a problem if handled with care, and the thermal conductivity issue after compression, reported by Erik R. Bardy et al (2007, p.232). In regards to the latter, steps seem to have been taken towards fixing it.

Additionally, there are no reports of aerogels losing their thermal performance due to aging, as opposed to VIPs.

Another aspect, which is quite important, is the price. Both materials are more expensive than traditional insulation materials, but the VIPs cost around three to four times more per square metre than the aerogels, in spite of their flaws.

Furthermore, the fact that aerogel can be produced as either opaque, translucent or transparent, would be useful in the new development.

Overall, both products are at the top in regards to thermal performance. VIPs, however, seem to be in need of more research and testing until they minimise the disadvantages.

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