Silica aerogel is a porous insulating material that isstransparent to solar radiation. To understand itssinsulating performance in a window system, it issnecessary to first study component heat transferspaths. Aerogels absorption coefficient, a measure ofsthe attenuation of radiation heat transfer, was determinedsover the spectral range 1-200 pm. Althoughsradiation heat transfer is negligible over much of thissregion, there is a transmission window between 3-6spm. At ambient temperatures, for aerogel thicknessessof 0.5-5.0 cm, radiation heat transfer through ansunmodified aerogel window is less than 15% of thestotal heat flux. For evacuated or high-temperaturesfurnace windows, this contribution can be over 50%.sThermal radiative transfer can be somewhatsdecreased by allowing the aerogel to absorb moisture,sbut solar transmission and optical clarity aressacrificed. Absorption of water vapor over timescauses irreversible structural changes that increasesscattering in the solar spectrum. Aerogels thermalsperformance can be improved by replacing the poresgas with one of lower conductivity or by evacuatingsthe aerogel to pressures below 0.1 atm. A hypotheticalsevacuated aerogel window has a calculated UValuesof 0.5 = W/m2-K for a gap spacing of 12.5 mm,swhich is four times better than currently availableslow-emissivity gas-filled units of similar size.