Condensation or ice built-up on the windows

On the very cold days like we have  in Toronto this winter many people experience significant condensation or even an ice built-up at the bottom of the windows. There are few reasons for this but one of the common misconceptions is that there is something wrong with the windows. I had few requests during these days from people who live in brand new homes to conduct energy evaluations to determine what is wrong with their windows. As I expected nothing is wrong with the windows. But there are other issues that contribute to this problem and need to be properly addressed.

First of all let's see why the condensation is forming on the windows. Windows (and here it's indeed depend on their type) is the coldest area in your entire building envelope. Of course, if you have old, single pane windows their R-value is close to zero. But for the majority of the homes, especially new ones, the windows are double glazed, with insulated spacer, argon filled and low-E coating. These windows were designed and built to keep the heat inside. Nevertheless their R-value is much less than the surrounding wall, and they are still the coldest surface in the house.

Moisture that is contained in the air in the form of vapour reaches its dew point and condenses on the cold window surface. The dew point depends largely on the humidity level in the house and outside temperature. The lower the temperature outside the faster air moisture transforms from its vapour state into the liquid or even ice. When we talk about moisture in the air we mean relative humidity, which is the ratio of the existing amount of moisture and the total amount of moisture that air can hold in the form of vapour. The higher the temperature of the air the more moisture it can hold. So when the temperature in the house is lower the relative humidity is higher. That is why the relative humidity of the air near the window will be much higher than in the middle of the room. As a result we will see condensation on the window surface.

The problem is even more obvious in the newer homes that tend to be very air tight. These houses require additional mechanical ventilation, such as HRV. Lack of ventilation contributes to significant moisture accumulation and higher level of relative humidity. Existing bathroom or kitchen exhaust fans do not provide adequate ventilation in tighter houses. Only properly installed and balanced HRV or ERV can help substantially with the issue of moisture evacuation.

And of course the last but not the least is the obstruction to surface ventilation created by window coverings. In some homes especially in the bedrooms people have window coverings or blinds closed all the time. This prevents window surface from proper ventilation  and contributes to moisture accumulation and mold growth.

So to summarize, if you live in the newer tighter house install HRV or ERV for proper and adequate ventilation. Keep relative humidity in your home in the recommended 30-40% range (control it with hygrometer). Always open your windows coverings during the day for proper ventilation.