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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To learn more about doors and windows, see these other blog posts:
• What causes sweating (condensation) on the inside of windows in the winter?
• Is every exterior door of a house required to have a landing outside?
• What are the small slots at the bottom of the outside of my window?
• Why does condensation form on the outside of some windows and not others in the morning?
• Why is the garage door track a white tube?
• What is the raised metal plate on the floor under the garage door?
• Why do I have to hold down the button to close the garage door?
• How can I tell if a window or glass door is safety glass?
• What are the code requirements for safety tempered glass for doors?•
• Should a front door swing in or out?
• How many exit doors are required for a house?
• How many exit doors are required for a mobile/manufactured home?
• Are openable windows required to have window screens? Will windows with no screens pass a home inspection?
• Can a bedroom door open into the garage?
• What are the building code requirements for a door from the garage to the house?
• What is "low-E" window glass?
• What does ANSI 297.1 on glass mean?
• Why is a double cylinder deadbolt lock on an exterior door a safety hazard?
• How can I check my garage door to make sure it is safe?
• What is an egress window?
• Does a home inspector test all the windows and doors in a home?
• How difficult is it to change a window to french doors or a sliding glass door?
• How do you determine if a door is left-handed or right-handed?
• Why are window security bars dangerous?
• What are the common problems you find inspecting windows?
• What is causing a foggy haze on my windows?
• What do those numbers on the manufacturer's stickers in new windows mean?
• What does a home inspector check on an electric garage door?
• What is the tempered label on glass at windows and sliding glass doors called?
• Why is pressure washing double pane windows an expensive mistake?
• Do I need to have two exterior exit doors in my house?
• When is safety glass required for windows at stairs and stair landings?
Visit our DOORS AND WINDOWS page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.