Frost and Condensation on Sliding Windows

And What You Can Do About It

What’s causing the frost and condensation on my windows?

Sometimes we see a lot of condensation or frost on the windows in the homes we inspect. Often they’re even frozen shut. Sliders and single-hung windows are very hard to seal well. It is very challenging to make a seal that can also slide. As a result, even the most well-built windows of these styles will leak some air or let in cold air. That is part of what is going on.

Air can only hold a given amount of water for a given temperature. The lower the temperature, the less moisture it can hold. If the temperature of a surface is below the water-bearing temperature of the air, water will condense out. This is what is happening at the windows. Often at doors and windows, the temperature is lower than the air moisture carrying capacity so that you can get condensation. In extreme cases of moisture overload, you can even get condensation on the walls. This can happen particularly in closets or beside beds where air circulation is poor. A little condensation is usually OK. Vinyl windows can bear some moisture and even some frost without damage, while wood-framed windows are more susceptible to moisture damage. Condensation left for a considerable time can result in moulds, algae and other growths if the temperatures warm up. Monitor any areas with condensation and do what you can to mitigate.

Frosted window

So what can I do?

The good news is that typically, vinyl windows are not adversely affected by some condensation. Even some frosting will not typically damage the frame, as might happen with wood-framed windows.

Solving the condensation issue is a matter of controlling the contributing factors. Having your furnace blower fan, ventilation fan or heat recovery ventilator (HRV) on is a great first line of defence as it is serving two purposes:

First, it’s exchanging moist air from the inside with much drier air (now that the temperature has dropped so much) from the outside.

Secondly, it’s blowing air around, which will help eliminate dead zones, particularly near windows. Poor air circulation allows temperatures to go down, increasing the possibility of condensation.

Make sure that your humidifier is set properly for the outside air temperature so that it is not adding unwanted moisture to the air. This is especially true given a sudden drop in temperature. You might consider opening the door and the heat registers in any unused rooms, at least during really cold spells.

Typical ideal humidistat settings

On the other hand, unless the condensation and frost buildup is extreme, and you monitor it once it warms up, no damage to vinyl windows should occur.

There is not a lot you can do about the leaky seals on the windows, especially when they are facing the wind. I know there is removable caulking designed to be applied in the fall and then removed in the spring. That is more appropriate to wood windows and might get caught up in the fibre seals. I have seen people use removable tape to seal just the joint at windows as well. I use the plastic sheeting that you apply over the windows using double-sided tape at the edges and then tighten with a hairdryer for my leaky sliders. It is very effective at reducing condensation, icing and drafts.

Sudden drops in temperature can be very challenging for homes to adapt to, especially for well-sealed homes. It will take a little bit of time for the house to get back into balance, even with the air exchange fan working. There is a huge moisture load in the typical home. From the flooring to the drywall, the moisture content of the materials in the home has become balanced to a much higher temperature than there is after an extreme temperature drop. As the air inside the home dries out, the moisture on the windows will evaporate and eventually should mostly disappear. Monitor the windows for any dripping that may run down the walls from extreme build-ups.

We’re always happy to help!

If you have any questions about anything you read here, or about any other issues happening in your home, please don’t hesitate to reach out and contact us! We’re always happy to help.