Aerogel: future of glazing?

Note: units are presented in International Units (SI), non-SI units are presented in italicized blue between brackets.

1 ft²·°F h/Btu ≈ 0.1761 K m²/W,
or 1 K m²/W ≈ 5.6745 ft²·°F h/Btu

A paper published in the journal of Solar Energy Materials & Solar Cells (volume 89, pages 275-285) describes the results of a thermal analysis of monolithic silica aerogel based glazing. Current window manufacturing industry has provided us with single-glazed or multi-glazed windows with either high or low insulation-to-optical transmittance ratios. So if a highly insulated window is desired, one could opt for a triple-glazing, low-e window with an R-value between 0.88 and 1.0 K°m2/W [5 – 6 F°ft²h /Btu]. Such choice would result in a window having visible transmittance (VT) reduced to 50% and a solar heat gain coefficient (SHGC) of 0.40. Conversely, if high solar heat gain is desired, a window with an R-value of 0.17 K°m2/W [less than 1 F°ft²h /Btu] would be needed providing a VT of 75% and a SHGC of 0.76. These options are not well suited for poleward climates where both a high SHGC and high R-value are desired. Aerogel windows may finally help overcome this limitation.

Silica aerogel was first "extracted" in the 1930s at the College of the Pacific (California, USA) from hydrogel by removing the water component by use of alcohol substitution. The developer of the aerogel, Steven S. Kistler, was later hired by Monsanto Corporation where the aerogel was produced and marketed. Aerogel did not benefit from a huge market until the 80's where modified versions of the product evolved in Europe then later in the USA. One of the niches being carved out by aerogels is in the fenestration industry. Silica aerogel is a very low-density silica-based solid (2-3 mg/cm3) composed of 99.8% air. It has high compressive strength, very low thermal conductivity (0.01 W/mK°), and under recent manufacturing techniques transmits 76% of the visible light (for a 15 mm thick slab of aerogel).

In the journal of Solar Energy Materials & Solar Cells, J.M. Schultz et al. test the thermal characteristics of a window built around a layer of aerogel 15 mm ( 0.59 inches) thick. Because of aerogel's poor tensile strength (i.e. it is brittle and will readily fragment when bent), the authors of the study sandwich the sheet of aerogel with 2 thin layers of low-iron glass glazes. The low-iron content of the glass optimizes the daylight/solar transmittance of the window assembly. The air is evacuated from the assembly down to a pressure of 5 hPa, and the rim is sealed with a laminated plastic foil (Mylar 250 RSBL300) and butyl sealant providing low thermal heat transfer across the rim of the window assembly. R-value at the center of the window was measured to be 1.51 K°m2/W [8.57 F°ft²h /Btu] and 1.388 K°m2/W [7.88 ft² F°ft²h /Btu] for a 120x120 cm window assembly with 4 aerogel glazing (55x55 cm each) joined in a frame whose material is not defined in the article. The authors state that such windows installed on a typical Danish home (i.e. one meeting modern energy efficient standards) would provide energy savings of 16% over similar homes using argon-filled triple glazed windows.

This is promising technology with many positive potential for the PACCS community. Future manufacturing technology should help improve solar heat gain (higher VT) allowing for thicker aerogel glazing, hence higher R-values. Could aerogel someday be incorporated into SIP systems where plywood would be substituted with a transparent plastic allowing for translucent walls?
At the time of this writing, it's not known when such windows will become readily available to the general public... hopefully soon ;-)

A simulation comparing the thermal response between a triple-glazed and an aerogel equipped structure follows. The structure is modeled with low internal reflectance (i.e. high solar radiation absorption) and highly insulated walls and roof. Typical Mean Year ((TMY2) meteorological data for Portland, Maine (USA) is used. Internal temperatures for the structure using triple-glazed windows (blue line), aerogel windows (red line) and external temperatures (green line) are plotted for 2 dates: January 14th (a clear day) and February 1st (a partly cloudy day). Y-axis represents temperatures in degrees Fahrenheit and the x-axis represents military time. The structure has no internal source of heat, solar radiation is the sole source of heat.

Disclaimer: these simulations are provided for educational purposes only. The results presented have not been validated.

Model used for simulation. All three openings are glazed and facing the equator.

Air temperature profiles within and outside the structure using TMY2 meteorological data for Portland, Maine (US) for January 14. Y-axis represents temperature in °F. X-axis represents military time in hours.

These examples show that for a clear sunny day with an outside air temperature below 0 °F for most of the day, the aerogel windows increase the internal temperature of the structure by an additional 10 degrees over the triple-glazed windows equipped structure at solar noon. When the sky is partly cloudy, the solar heat gain is a bit more modest as expected

Air temperature profiles within and outside the structure using TMY2 meteorological data for Portland, Maine (US) for February 1. Y-axis represents temperature in °F. X-axis represents military time in hours.


Last Modified on Friday, December 06, 2013