In cold weather you might find condensation forming on the interior of windows, sometimes resulting in ice build-up. This happens for two reasons:
- The interior pane of glass is very cold,
- There is excessive humidity in the house air.
When moisture in the ambient air comes into contact with a cold enough surface, the moisture will condense from a gas to a liquid, forming droplets of water on that surface. If the window is cold enough those droplets will freeze. This is more common to find on single-pane windows, or double-pane windows with a broken seal, which greatly reduces the R-value associated with having two panes of glass. The condensation itself can cause rot damage to the window frame, which can be expensive to repair. Also, the condensation pointing toward excessive humidity poses a concern on its own, as humidity and moisture can create issues with mold growth.
To prevent rot damage to the window frame, you can install plastic over the window - but it needs to be dry when you do this (you may need to spend some time with a blow-dryer), or fungal growth will occur. The plastic will act as a "vapor barrier", which prevents moisture in the air from contacting the cold glass in the first place.
To address the humidity issue and prevent mold growth, you want to address the sources of humidity as much as possible, and use ventilation. Here are a few things you can do:
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- If you have an earth-floor crawlspace, install plastic over it as soon as possible (moisture from the soil entering the house air can amount to gallons of water every day).
- If you have a sump pit in the basement, make sure it has a sealed cover.
- Use the bathroom fan when you have a shower, and leave it running for at least 30 minutes afterward. Install a fan if you do not currently have one (arrange it to vent outside if it does not - sometimes they vent into the attic space, which is likely to cause fungal growth).
- Use the hood vent when cooking (arrange it to vent outside if it does not - often they re-circulate air back into the kitchen).
- Do not hang-dry laundry in the house.
- If issues persist, purchase a stand-alone dehumidifier.