Aerogel insulation for buildings

[edit] Introduction

Aerogels are synthetic low-density materials with unique physical properties. They are formed by removing the liquid from a gel under special drying conditions, bypassing the shrinkage and cracking experienced during ambient evaporation. This creates a solid three-dimensional nanoporous structure containing 80-99% air.

Due to their high porosity, aerogels exhibit the lowest thermal conductivity of any solid, whilst being transparent to light and solar radiation. Aerogels are often cited as a promising material for translucent insulation applications. They can made from practically any material, although the most common form is silica aerogel which can be produced as granules or in solid (monolithic) tiles.

Commercial products for the building sector include:

Cavity insulation.
Glazing units and cladding systems containing granular aerogel.
Translucent and opaque insulation boards, blankets and tensile roof membranes embedded with aerogel particles.

Transparent monolithic silica aerogel has been cited as the ‘holy grail’ of future glazing technology, with the potential to achieve U-values as low as 0.1 W/m2.K. However, research and development into monolithic glazing is limited due to the high cost of production, long processing time and the difficulty of creating large uniform samples with complete transparency.

[edit] How it works

The total thermal conductivity of porous insulation depends on the heat transfer through convection in the pores, conduction through the solid and pores, and radiation. Typically, pores within conventional insulation are over 1mm wide, allowing gas molecules to move freely and transfer thermal energy by convection. By comparison, pores within aerogel can be as small as 20-40nm (even smaller than the ‘mean free path’ of air at 60-100nm). As a result, individual air molecules within the pores have no space to transfer thermal energy by convection.

Conduction through the solid structure and air molecules within aerogel is also minimal. With little space for convection, air molecules constantly collide with the walls of the pores, suppressing gas conduction. Furthermore, as aerogel only contains 0.1-5% silica and the thermal conductivity of air is very low, heat transfer is minimal and conduction in the gas will diminish with any decreases in pressure. A vacuum inside the pores results in the best insulating properties, with thermal conductivity of 0.004 W/m.K (ten times better than conventional insulation).

The amount of radiative heat transfer through aerogel is dependant on the intensity and wavelength of the thermal radiation, the optical properties of the material, the size and shape of its pores and its overall thickness. At ambient temperature, the nanosized pores and particles provide effective attenuation of infrared thermal radiation due to high levels of absorption and reflection.

The optical and infrared properties of silica aerogels have been well studied. Silica aerogels can be considered transparent insulation materials that effectively transmit solar light, but block thermal infrared radiation.

The material exhibits high translucency, often accompanied by a slight bluish haze. This can be attributed to ‘Rayleigh scattering’, an optical phenomenon that occurs when light scatters off particles smaller than the wavelength of light, where shorter wavelengths in the blue spectrum are most easily scattered.

[edit] Environmental impact

To make aerogel involves three key steps: gel preparation, ageing and drying. The aim is to create a gel, strengthen and purify it, then remove all liquid from the pores without collapsing the solid structure (achieved through super-critical or sub-critical drying techniques). These processes typically involve mixing reasonably toxic chemicals and undertaking complex diffusion controlled processes that consume a lot of solvent. Furthermore, the final step is often accompanied by intensive drying processes which may consume large amounts of energy and CO2.

Despite this, In 2008, the two major manufactures of silica aerogel both received ‘Silver’ cradle-to-cradle environmental awards from McDonough Braungart Design Chemistry (MBDC) for their aerogel production. MBDC claim to evaluate a products complete formulation, energy use, water use and recycling potential when assessing environmental impacts. However, the data from these studies is confidential, making it difficult to assess the rigour and validity of the results.

To investigate these issues further, Mark Dowson conducted a first hand life cycle assessment (LCA) of silica aerogel following ISO standards. Working in collaboration with the University of Bath (who manufacture aerogel as part of an optics research programme) Mark made some aerogel himself, and measured the raw material and electricity use across the production line.

The findings from the study demonstrated how the energy and CO2 required to manufacture high performance silica aerogel insulation can be recovered within 0-2 years (when comparing results to the in-use savings arising from retrofitting to single glazing). Key factors influencing the environmental payback were the efficiency of production and re-use / recycling of materials such as liquid CO2 and solvents.

[edit] Potential developments

Within Buro Happold, Mark Dowson undertook an Engineering Doctorate (EngD), in collaboration with Brunel University, focusing on developing and testing new building fabric technologies incorporating translucent granular aerogel encapsulated inside clear polycarbonate sheets to improve the thermal performance of new and existing buildings. Three technologies have been developed, with promising initial results: