Why do I have frost and ice buildup on windows and what to do about it?

Why do I have frost and ice buildup on windows and what to do about it?

Frost and ice buildup on windows may be common in winter months, but that does not mean it should be shrugged off. Moisture buildup can cause damage if it is not dealt with.

Sometimes condensation will disappear almost as quickly as it appeared, especially in rooms such as your kitchen or bathroom.  However, if it seems like ice on the inside of windows in your home is a persistent problem, there are some things you should consider.

How does condensation form?

Condensation on interior windows occurs naturally when moist and warm indoor air meets a cool surface, like a window.

Air is only able to hold a specific amount of moisture before it reaches saturation and is then forced to condense and form dew. This could appear as fog or water droplets on your windows.

Interior condensation

In winter months, this will likely show up as an accumulation of frost or ice along the bottoms, in the corners, or between tracks of windows.

Frost on windows

Condensation typically appears on windows before any other surface because windows generally have the least insulation value on an exterior wall and react the quickest to changes in outdoor temperature.

Chronic frost and ice buildup on windows can cause physical damage to your home. If you have wood window frames, for example, prolonged exposure to moisture can cause the wood to begin to rot. This also increases the risk of mold and mildew growth.

Molding windows

For windows, doors and skylights, condensation can form:

  • on the glass
  • on the frames and sashes
  • near the weather seals
  • on the walls around a skylight
  • between the glass panes if the seal is broken

How can I reduce condensation and ice buildup on windows?

There are simple steps you can take to avoid or reduce condensation.

Let us begin with some theory.

One of the most important things to understand when it comes to preventing condensation on the inside of windows is Relative Humidity.

In the simplest of terms, Relative Humidity refers to the level of moisture in the air at a given temperature in relation to the maximum level of moisture that air can hold at that same temperature.

There are tools, such a dehumidifiers and hygrometers, that can help you determine what the Relative Humidity level is in your home.

Next, you will want to determine what the ideal Relative Humidity should be. Based on outdoor temperature, your ideal home humidity levels will fluctuate:

Ideal Home Humidity Levels Based on Outdoor Temperature

Once you have determined what the ideal humidity level should be in your home, then you can begin work on managing it.

Practical guidance

1. Create better air circulation/increase ventilation in your home.

The movement of air helps moisture to evaporate. Increasing air circulation and establishing adequate ventilation is an essential step in reducing the formation of window condensation in your home.

2. Make sure your HRV is on.


If your home is equipped with HRV (Heat Recovery Ventilation), make sure to turn it on and set it to run for about 20 minutes/hour. If the frost and ice build up on windows are particularly bad, try running the HRV continuously, until the issue is resolved.

3. No HRV? Turn Ventilation switch on instead.

Ventilation switch

If you have a ventilation switch instead of an HRV, turn it on (day and night) during cold, winter months. This will help maintain air circulation in your home. (Having trouble finding the ventilation switch? It should be located next to your thermostat and will likely be labelled.)

4. Useful tips for new homes!

When moving into a new home, it is also suggested that the HRV or Ventilation Switch remain on for at least 2-4 hours per day for the entire first year. Additionally, you should run your ventilation systems for at least 20 minutes after showering and 10 minutes after cooking.

5. Make sure all your windows are locked (not just cranked).

Locking mechanism

If you leave your window lock open, the frost and ice buildup that may appear on your windows will also appear inside the locking mechanism. You might remember from elementary science class – water expands when frozen. The additional pressure from the ice may damage and potentially ruin the locking mechanism for your windows.

6. Check and clean various filters.

HVAC Filter

Checking and cleaning furnace filters, air ducts and your HRV system is also good practice to ensure that your home’s winter performance is energy efficient and effective. You should also check and clean your humidifier’s filter if you have one.

7. Check and clean exterior vents.

Home exterior vent

It is also recommended that you check exterior vents on your home and be sure to remove and snow or ice buildup.

Some other routine or daily practices you may want to implement include:

  • Shortening the time spent in the hot shower,
  • Running your bathroom exhaust fan during and after showering for a minimum of 30 minutes,
  • Covering boiling pots of water,
  • Running the range hood fan while cooking and for an additional 10-20 minutes after
  • Opting to use the dryer or hanging clothing outside (when weather allows) instead of hanging clothing to dry indoors,
  • Opening blinds or window coverings to allow airflow,
  • Moving furniture so it is not touching outside walls,
  • Removing items from window sills,
  • Leaving bedroom and other room doors open to allow better circulation, and
  • Storing firewood outdoors

Additional Resources from Government of Canada


Condensation is a natural process that inevitably occurs in many homes. It does not have to be a troubling issue, if the source is addressed, and preventative measures are put in place.  However, if you have tried all the suggestions we have listed and you are still dealing with excess frost on windows, it is time to contact a professional to help you assess the situation and determine the next steps.

Our friendly and knowledgeable staff at Window Mart are always happy to help.

Contact our Alberta, Saskatchewan or Ontario team for your free evaluation today.