Gas Fills

Windows are more efficient when they block air and heat from passing through them. In cold climates, they keep heat inside. In hot climates, they keep it out. In both settings, air conditioners and heating systems won’t have to work as hard to keep temperatures stable.  

Filling the space between the panes of a double- or triple-glazed window with gas, instead of air, boosts efficiency by reducing heat loss. Manufacturers use argon and krypton, which block more heat transfer because they are denser than air. Both are safe to use because they are non-toxic, clear, and odorless, and will stay that way even if leaked or mixed with other elements. The Window Selection Tool identifies options for gas-filled windows best suited to your home. 

Krypton offers the biggest efficiency gain but is more expensive. Argon is the popular and affordable alternative, and manufacturers also offer windows with a mix of both gases, to balance premium performance with affordability. Some gas can leak from a window over time, but this is not a safety or health concern because the gases are non-toxic. However, this leakage will lead to a small drop in efficiency. The leakage rate for an efficient window is about 0.5% of gas fill per year, so even after 20 years a window with gas leaks would still perform at 90% of its original capacity.* 


*For more information on NFRC’s sources and statistics, the Partners page of this site details the groups with which NFRC’s experts collaborate and share data. 


About Us

provides unbiased information and education to the public on the energy efficiency, technical, and human considerations that influence window selection and use. The EWC collaborates with allied organizations, educational institutions, and government agencies that promote energy efficiency in residential windows. We do not sell, service, install, or recommend products, or endorse manufacturers. Descriptions and specifications of the products in the Window Selection Tool are provided by the manufacturer. The EWC is not liable for any manufacturing defects or inaccurate product descriptions and specifications and does not guarantee the performance of NFRC-certified windows, doors, and skylights.

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