I recently received an email from a homeowner having problems with her windows. The issue is condensation—in other words, wet, “leaky” or “weeping” windows. That’s when condensation builds up on the inside surface of the windowpane, which can be pretty common during winter. Windows get all fogged up or sometimes it’s so bad water pools on the windowsill, or on the floor below.

We get condensation for a few reasons. Sometimes there’s too much moisture in the home, or not enough ventilation. If you have a humidifier it might be set too high. Drop it down about ten percent and see if the condensation persists.

Another problem might be bad windows, or windows that don’t insulate properly. These windows are usually single-pane.

If you have bad windows you will have warm air meeting cold, which creates condensation. During winter warm air inside the house hits the cold surface of a single-pane window. Or the opposite happens in the summer—cool air inside the house hits the glass, which is warm thanks to the warmer temperatures outside. In both cases we get condensation.  

Newer windows are double or triple-paned. In between the glass they have air or gas that helps block outside temperatures. This provides an airtight seal. If that seal is broken you will get condensation in between the glass panes. If that’s the case the window must be replaced.

If you don’t know why you have condensation on your windows bring in a professional, like a certified home inspector. The homeowner who emailed me hired an inspector to do a home audit, which was smart. They told her that the problem wasn’t her windows. Her home is too airtight.

Most homes nowadays are built to be very airtight. It’s great for energy efficiency but not for condensation. Homes have to breathe.

If they don’t let in some air they also don’t let moisture out, and that moisture gets inside the walls and can eat away at structure, leading to rot and mould. One of the first signs is condensation on windows and bubbling paint.

We do all kinds of things inside our homes that create moisture and condensation—cooking, taking a shower, even breathing. If you want your home to be healthy you must get rid of this moisture.

During summer time, running the air conditioner or dehumidifier can help lower humidity, but it won’t solve the problem. Plus, what do you do in the winter?

Some homeowners will get a dehumidifier and think that will solve the problem. It usually doesn’t work because a dehumidifier will only get rid of the moisture in the area, not from the entire house. Instead I recommend getting an HRV, or a heat recovery ventilator.  

According to today’s building code you must have an HRV if your home is built to be airtight—usually if you have spray foam insulation. You could open the windows to let in fresh air, but then you would be losing energy, which isn’t good for the environment or your energy bills. That’s why an HRV is smart.

An HRV brings in fresh air, conditions it to the temperature inside the house and then feeds it throughout the home, so there is this constant supply of fresh air. And because it’s wired to the furnace fan it will also pull stale air out.

Some HRVs are set on a timer, so it kicks in every so often. Others have a humidistat, which should be installed in a centralized spot in the house, just like the thermostat. It’s usually set at around 35-40 percent. When the humidistat detects there’s too much moisture in the air it will start up the HRV.

HRVs work, but they aren’t cheap. With installation it can cost between $2,200 and $2,500 to get one. That might seem like a lot but if you think about the potential damage that too much moisture in your home can cause, not to mention the health risks, an HRV is a no-brainer. It’s code for a reason.