Controlling Excessive Condensation on Windows

CCI-Windsor-Essex, London and Area, November 2015

The beautiful months of winter are upon us! Of course, with all that fluffy white snow comes colder outdoor temperatures, warmer beds, and more comfortable couches. Just because we’ll all be inside for a few months doesn’t mean we shouldn’t be able to look out our windows though! Yes, it’s time to talk about ways of controlling excessive condensation on windows.

What is Condensation?

Condensation is the process where moisture that is present in the air turns into water when it contacts a cold surface. Since window glass and frames are typically the coldest surfaces in your home, this is where condensation forms. If the glass is cold enough, this water can freeze. While it is normal for some condensation to form on windows during cold weather, excessive moisture can result in damage to paint and plaster at the window sill and jambs. Further, an excessive amount of condensation can result in mould growth, damage to the exterior walls and windows or, for those living in large buildings, the condensation can even appear as a leak and cause damage to suites below.

When condensation occurs, water should be mopped up as soon as possible to minimize the damage.
Relative humidity (RH) is a measure of the moisture in the air. The higher the RH, the more moisture in the air and the greater likelihood that condensation will form on the windows. As a resident, you should try to keep the RH at levels which will not cause excessive condensation - this may require some changes to lifestyle. Let’s take a look at a few ways to minimize risk of condensation forming.

What Can Be Done to Minimize Condensation?

The following measures can help minimize condensation:

  1. Use of humidifiers should be restricted to provide comfort while minimizing condensation on windows. The maximum humidity in the winter should be about 35%. This should be reduced as the exterior temperature decreases (see table below).

  2. Exhaust fans should be operated continuously during and after cooking or bathing. It can take several hours of running an exhaust fan after showering to reduce humidity levels to levels that existed before showering.

  3. If condensation starts to form on the windows, exhaust fans should be operated until the condensation clears.

  4. Exhaust fans and the exhaust system for dryers should be maintained regularly. This maintenance includes removing lint from fan blades, grilles and ductwork traps. The suite occupants should check with Property Management if they are unsure of how to perform appropriate maintenance.

  5. Suite entrance doors should not be weather-stripped. Gaps around these doors are provided to allow fresh air that is supplied to the corridors to enter your suite. In the winter, this fresh air is dry and helps remove moisture from the air in the suite.

  6. Excessive numbers of plants contribute significantly to moisture in the air. Avoid placing plants on window sills because they increase the humidity immediately in front of windows.

  7. Where possible, turn air grilles to direct warm air flow downwards and across the window surfaces. This helps to keep the windows warm and to evaporate condensation that does form (in the summer, reverse this flow to point upwards to distribute cool air throughout the room).

  8. Blinds or curtains should not be installed tight to windows or kept continuously closed, and furniture should not be placed tight to the windows. These conditions prevent circulation of air over the window surfaces making the window colder and increasing condensation problems. Coverings should be kept fully opened during the day to allow condensation that formed at night to evaporate.

  9. Condensation forming on the inner surface of exterior sliding windows is a normal occurrence. Drain holes are provided at the bottom of these windows to remove the water.

  10. For horizontal sliding windows, make sure both interior and exterior sliders are fully closed – to function as intended and reduce risk of condensation forming.

The following table provides a guideline for maximum humidity levels to prevent condensation for typical residential aluminum framed windows (present in most highrise buildings):

Exterior Temperature Interior Humidity Required to Prevent Condensation
0°C ~35%
-10°C ~25% to 35%
-20°C ~10% to 20%
-30°C Less than ~10%

*Assumes 20°C interior temperature. For higher interior temperatures, the maximum humidity levels become lower.

This means that on those cold winter days, your interior humidity levels will need to be almost uncomfortably low in order to prevent condensation from forming. Condensation then becomes an issue to control and manage, rather than one to try to prevent outright.

The Cold, Hard (and Wet!) Truth

The fact is, condensation isn’t entirely the fault of the windows and can even form on brand new windows if the conditions allow. By following the guidelines above your corporation can improve the comfort level of your owners, reduce complaints, and ultimately extend the life of your windows.

When it does come time to repair or replace your windows, be sure to hire an experienced professional to assist in decision making with regard to air leakage resistance ratings, condensation resistance ratings, types of weather-stripping, quality of coatings, types and locations of low-e coatings, and a number of other factors to ensure that the windows you purchase align with the objectives and financial constraints of your corporation. New windows and sliding doors are a considerable investment, and not all new windows and doors are created equally. An experienced professional can help your Board navigate this important process. In the meantime, follow our guidelines above and enjoy the great indoors!