How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
site-built, mobile/manufactured and modular homes
What is low-E window glass?
Tuesday, July 10, 2018
When sunlight strikes an object, part of the energy is reflected and the rest of it is absorbed/emitted. Asphalt paving is an example of a material with low reflectance and high emittance. At the other extreme, aluminum foil has extremely high reflectance and virtually no emittance. In other words, the heat bounces off of aluminum foil instead of being absorbed and emitted.
You would think that regular window glass would have high reflectance and low emittance, but its thermal emissivity level is almost identical to asphalt and concrete. A high level of heat is absorbed and passes through uncoated glass, so heat moves readily from the outside to the inside during the summer and vice-versa in winter months.
Low-E glass has an invisible surface coating tuned to reflect the infrared/heat wavelengths of thermal energy, while letting visible light pass through. This enables the glass to reduce the heat entering the home during the summer and leaving it in winter. The emissivity can be further adjusted, by changing which glass surface the coating is applied to and tweaking the wavelength range that is reflected, to make low-E windows optimized for cold or warm climates.
Also, see our blog post Where is safety/tempered glass required for the windows of a house?
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