Winter Window Condensation and What It Means

On a cold Calgary morning, it is common to wake up and find the bottom of the windows filmed with water or edged with frost. Many people see this and blame the windows, but most of the time the glass is just the messenger. It is showing a symptom of what is going on with the air through the whole house. Knowing why this happens helps you manage it, protect the house from damage, and keep indoor air in a reasonable range through the long winter.

Where the Water Comes From

The science behind window condensation is straightforward. Warm, moist air meets a cold surface. The air inside a Calgary house in winter is much warmer than the air outside, and it carries moisture. That moisture comes from everyday living: breathing, showers and baths, cooking on the stovetop, and even from houseplants. All of these add water vapour to the air.

When this warm, humid air moves through the house and touches the cold window glass, the air cools quickly. Cold air cannot hold as much moisture as warm air, so it sheds the extra water onto the glass as condensation. The colder it is outside, the colder the glass surface will be, and the more condensation you tend to see. Modern homes are built fairly tight against the winter cold, so this moisture often has nowhere to go and builds up indoors.

When Condensation Is a Problem

A light fog at the very bottom of the glass on a minus thirty morning is not something to lose sleep over. When the condensation is heavy enough that it forms droplets, runs down the glass, and pools on the sill, it is time to pay attention. That steady presence of water can cause real damage if it goes on season after season.

I once walked into a bedroom where the owner had kept the blinds down all winter, thinking it would help insulate. The sill was soft with rot and there was a dark patch of mold growing right up the drywall beside the frame. All of that damage started from a little bit of condensation that was never allowed to dry out.

Constant moisture will soak into wooden sills and frames and lead to rot and decay. It can peel paint and stain clear finishes. If it reaches the drywall around the window, it can weaken the wall surface and create a spot where mold can grow. This is not just a cosmetic problem; mold can affect your family's health.

Checking Your Windows First

Before you start worrying about humidity through the whole house, it is worth taking a close look at the windows themselves. Old single-pane windows will almost always collect condensation in a Calgary winter. They offer very little insulation, so the inside surface gets very cold. In that case, upgrading the windows is really the only long-term fix.

With modern double or triple-pane windows, the situation is different. These units have a sealed space between the panes, often filled with an inert gas like argon for better insulation. If you see fog or moisture trapped between the panes of glass, the seal on that unit has failed. Once the seal breaks, moist air gets in and condenses on the inside surfaces. This cannot be repaired. The sealed glass unit has to be replaced by a glazier, though the existing window frame often stays.

Managing Humidity in Your Home

If the windows are in good shape, the next step is to cut down the moisture in the indoor air. The simplest tool is ventilation. Use the exhaust fans you already have. Run the bathroom fan during every shower and for a while after to pull the steam outside. Use the kitchen range hood fan whenever you cook, especially when boiling water or simmering something for a long time. Make sure these fans really vent outdoors and are not dumping air into the attic, which creates a different problem in a different part of the house.

Check the humidifier on the furnace. Many homeowners set it too high. Very dry air can cause dry skin and static shocks, but air that is too humid will show up on the windows. The trick is to find a workable middle ground. A small, inexpensive hygrometer will tell you the relative humidity in the house so you can adjust with some confidence. As a general rule, the colder it is outside, the lower your indoor humidity should be. It also helps to open blinds and curtains in the morning. This lets air move against the glass, warms it a bit, and lets any moisture dry off.

A Healthy Exchange of Air

A modern energy-efficient home is usually quite airtight. This saves on heating but can trap stale, humid air. The house still needs some fresh air. Some newer homes have a Heat Recovery Ventilator, or HRV. This system trades indoor air for outdoor air and keeps some of the heat from the outgoing air. If there is an HRV in the house, it should be running, and its filters need regular cleaning.

It is also worth checking other simple sources of moisture. The clothes dryer should vent directly outdoors through a clean, unobstructed duct. The house envelope includes every part, including an attached garage. A cold, drafty garage can influence the main house, so making sure you can keep your garage door working in the deep freeze is part of managing the home as a whole.

Window condensation is not a defect so much as a signal. The house is giving you information about its indoor conditions. By watching what happens and making small, practical changes to ventilation and humidity, you can find a balance that protects windows and walls and keeps the home more comfortable through a Calgary winter.