Condensation Explained

One of the common phone calls regarding condensation we’ll typically get in our office in the Spring and Fall months goes something like this:

“Why do my windows have condensation on the inside?”

Or, like this:

“Why do my windows have condensation on the outside?”

It is important for homeowners to understand that condensation is a normal occurrence and can occur on any type of window; high efficient or not. 

The first step in understanding condensation on windows is to understand what condensation is. 

From: North Star Windows

What is Condensation?

Condensation is the passing of a substance from a lighter to a denser physical state.  In this instance, water passes from a gaseous (vapour) state to the denser liquid one.  Because it is caused by too much humidity, condensation is also the symptom of a more serious problem – excessive moisture at work in your home’s atmosphere.

In seeking cooler, drier outside air, water vapour exerts pressure and forces its way through most building materials.  Wood, brick, and drywall are too porous to stop moisture from migrating to the outside.  But because window glass is denser and its surface colder than the surrounding walls, vapour is stopped by glass and condenses on it.  While glass – the coldest, least porous (and most visible) material in a building – may show condensation first, it may also be noticed on exposed nail heads and other metal surfaces.

Why does Condensation Occur?

Problems arise because air can hold only a limited amount of water vapour, varying with the temperature.  Condensation is related to two conditions always present in the atmosphere inside your home.  It is likely to form whenever there is an improper balance between:

Relative Humidity:
A ratio between the water vapour in the air compared to the maximum amount that air can hold at a given temperature.  For example, 50% means the air is carrying ½ of the total water vapour it is capable of holding at that temperature.

A  particular temperature where the relative humidity becomes 100%, called the ‘dew point’ (see chart).  At the dew point, the air is saturated with moisture and begins to lose it in the form of condensation.

Relative HumidityDew Point (Celsius)

Note: a new home has significant quantities of moisture because of the water used in various areas of construction (700 gallons of water are used in plastering alone for a six room house).  This will be eliminated after a period of time.

Where does Condensation Occur?

In practice, condensation will occur first over the lower part of the window because glass surface temperatures are not uniform, being lower at the bottom than at the top.  Windows with a metal sash usually have more severe problems at the base and sides.  Occasionally, it may occur on cold spots, such as nail heads and in corners of outside walls and closets where insulation value is reduced.

Sources of Moisture

The humidity level in a house during the winter will depend on both the moisture added to the air through family living habits (see chart) and the rate at which this moisture is removed by ventilation or condensation.

Quantity of Moisture Added to the Air through Normal Household Activities

Activity (for a family of four)Moisture in Liters (each week)
Cooking (3 meals per day for 1 week)6.3
Dish washing (3 times per day for 1 week)3.2
Bathing (.2 liters per shower, .05 liters per bath)2.4
Clothes washing1.8
Clothes drying indoors, or using an non-vented dryer10
Floor mopping (per 9.3m)1.3
Total Moisture Production per Week63 Liters

In addition to the above sources of humidity, such things as gas appliances, dryers vented inside, plants (which put out almost as much water as they receive), pets, humidifiers, damp basements, etc., all increase the humidity level in a home.

Solutions and Facts about Humidity


– Turn off all humidifiers, particularly in homes with forced-air heating
– Move plants away from windows (where water vapour will be released directly onto glass)
– Vent the clothes dryer and gas appliances outside
– Do not dry firewood indoors
– Ensure kitchens and bathrooms are well ventilated by windows or exhaust fans
– Ensure that basement walls are kept as dry as possible since they will act as humidifiers when wet
– A dehumidifier may have to be installed (note: a dehumidifier alone cannot eliminate the problem)
– Do no cover windows with heavy curtains since this will restrict air flow over glass
– Produce less moisture inside the house


– Unless indoor humidity is kept below 10%, it is impossible to avoid some condensation
– Lower outside temperatures require lower inside humidity levels (see chart)
– Electrically heated homes are difficult to rectify because there is very little movement of air
– Ventilation is generally the most effective means available to reduce humidity and can be done by opening windows, operating exhaust fans and/or installing a ventilator from outside into the cold air return plenum of a forced air heating system

Sources for Information

Ontario Ministry of Municipal Affairs and Housing
Canadian General Standards Board
National Research Council (Division of Building Research)
Rolscreen Company (Pella Windows)

Condensation on the Exterior Window Surface

According to Cardinal Glass Industries, condensation on the outdoor glass surface of an insulating unit is not an indication that the glass or insulating unit is defective.  Under the right set of atmospheric conditions it is possible to get condensation on the outdoor surface of an insulating glass unit.  Specifically, these conditions are as follows:

– Glass temperature is below the dew point temperature
– Clear night sky
– Still air
– High relative humidity
– Coated glass products (i.e. Low E coated)

Exposed to the above conditions, the outdoor glass surface can radiate heat away to the night sky, and with good insulating glass products the outdoor glass temperature can fall below the dew point of the ambient air.  When this occurs, moisture from the air can condense on the outdoor glass surface.  Similarities of this condensation on glass products can be drawn to moisture or dew on lawns or on automobiles when exposed to the above mentioned natural occurring phenomena. 

Sources for Information

Cardinal Glass Industries –

CLICK HERE to download a copy of our Moisture Assessment Checklist to see how you can lessen or help eliminate condensation in your home!

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