This time of year we get a lot of questions about condensation. Your windows* are not the source of condensation – “condensation forms on a surface when the temperature of the surface is below the dew point of the air. During the winter, when the coldest surface in a room is often the window, it’s fairly common to see water droplets or ice on window glass – especially in a room with elevated indoor humidity.”
We found a great article on GreenBuildingAdvisor.com about Rating Windows for Condensation Resistance!
The American Architectural Manufacturers Association (AAMA) created a rating called the Condensation Resistance Factor (CRF) ranging from 30 to 80; the higher the number, the better the window is at resisting condensation. They’ve actually created an online calculator to estimate the CRF based on indoor relative humidity levels and outdoor temperatures.
The National Fenestration Rating Council (NFRC) also developed a method for rating the condensation resistance of windows called condensation resistance (CR). Their scale ranges from 1 to 100; the higher the number, the better the window is at resisting condensation.
Although these methods seem to be effective, it is very hard for a window shopper to evaluate the window’s performance in the field without including all the factors of the home.
The article recommends addressing any problems that could be directly connected to the condensation problem (ex. a wet basement, multiple houseplants, or a hobby that introduces a lot of water vapor into the air). If your home is very tight, and you have lots of condensation on your windows, you probably need to install a mechanical ventilation system. Also, remember that curtains or shades keep windows cool at night, thereby increasing the chance of condensation. If one of your windows is regularly wet, try opening up the curtains before you go to bed.
*Condensation occurs on the outside of the panes and can be wiped away. If moisture or fog are occurring between the panes of glass, that is a potential seal failure in the window and should be inspected by a window contractor.