Interior window condensation is caused by higher levels of humidity inside your home, with drier humidity levels outside.  Poor performing windows can allow the interior surface temperature of the glass to cool enough to turn the humidity into condensation.  Condensation can cause damage to your home by rotting wood and damaging plaster around the windows.  You’ll need to pay attention to the contributing factors such as house plants, or a lack of ventilation in the kitchen or bathrooms.  Perhaps you’ve noticed other issues with your windows, or know that the glass package isn’t the most efficient.  Upgrading your windows can help with interior condensation depending on the glass package you ultimately select.

Condensation is more likely to form on cold surfaces than warm surfaces. That’s why it’s more common to see condensation on a single-glazed window than a double-glazed window. Double-pane glass systems perform more efficiently than single pane by creating a warmer interior glass surface than the single pane, though not all double pane systems are able to handle the low dry temperatures.  The addition of a low-E coating and argon gas fill further reduce condensation potential.  Triple pane systems can offer such a warm interior surface that condensation on any interior surfaces may be eliminated if humidity levels are maintained at reasonable levels.

The NFRC tests windows to give a consumer metrics to compare windows and their efficiency.  The NFRC measures as how well a window resists the formation of condensation on the inside surface. CR is expressed as a number between 1 and 100. The rating value is based on interior surface temperatures at 30%, 50%, and 70% indoor relative humidity for a given outside air temperature of 0° Fahrenheit under 15 mph wind conditions. The higher the number, the better a product is able to resist condensation. CR is meant to compare products and their potential for condensation formation.  The MSHI recommends it’s readers in Minnesota to look for a window with a NFRC CR above 50.  The double-pane systems we’re recommending here in Maryland have a NFRC of 60, and we can values of around NFRC 70 with triple-pane systems.

Most find a humidity level between 40-50% to be comfortable.  Maintaining a relative humidity below 50% prevents dust mites, mold and mildew, and inhibits bacteria growth.  When the weather is cold and dry it may be necessary to lower the humidity in your home to avoid condensation if you’re not ready to replace your windows.

If you see condensation developing on individual windows rather than the entirety, it could be that some environmental causes in your home are be to blame.  If you have an air register blowing directly on the window the hot air can condense as it reaches to cold surfaces of your window.  Opening your curtains or blinds can help by distributing the air.  Turning on ceiling fans can help as well.   If you see condensation developing on all of your windows it could be that the glass package isn’t very efficient.   Eliminating condensation takes looking at a lot of factors- but depending on the glass package, new windows can help a lot.