Can condensation appear on windows all year round?

Most people believe that buying energy-efficient windows will alleviate the problem of condensation, however, even the most efficient windows on the market do not create a 100% insulating barrier between the inside and the outside of a home. In fact, windows are still very inefficient (R4) compared to walls (R13).

More on R-value

Your window panes will generally be the coldest or warmest surfaces (depending on the time of year) in any given room because the glass will have direct contact with the outside air. As a result, they are prone to condensation, especially as outside and inside temperature differences are extreme.

Condensation Variables

To understand exactly what’s going on, we need to comprehend the nature of a few important variables and how they are related:

Vapour (V)

• All air contains water vapour of varying quantities. The lower the air temperature, the smaller the maximum possible capacity for vapour.
• Humidity is water vapour or moisture in the air.
• Cold air does not retain moisture as much as hot air does.

Vapour Saturation (VS)

Vapour saturation is a state in which temperature can hold a maximum amount of water vapour (in a gaseous form). The higher the temperature the more water it can hold.

Absolute humidity (AH)

Absolute humidity is the measure of water vapour (moisture) in the air, REGARDLESS of temperature.

Relative humidity (RH)

• Relative humidity is the measure of water vapour in the air, but RELATIVE to the temperature of the air.
• As relative humidity increases, so does the dew point.
• The temperature must increase to increase relative humidity.

Dew Point (DP)

• Dew Point is the temperature that air has to be cooled to in order to reach vapour saturation. The higher the Dew point, the higher the water content in the air. The dew point is calculated using air temperature and relative humidity.
• When air is cooled, relative humidity increases until it reaches a dew point (air becomes saturated). Further cooling below the dew point will induce condensation.
• The temperature of the dew point will depend upon the absolute content of water vapour, that is the absolute humidity (AH), measured in g/m3 (grams per cubic metre).
• The dew point of humid air will be higher than the dew point of dry air.

Temperature (T)

• When temperatures are high (hot), the air in the atmosphere can handle more water vapour than when the temperature is low (cold).
• As the temperature increases, so does the dew point.
• When an object is cooler than the air around it, the water molecules in the air come together and stick to its surface, forming a thin layer of water droplets.
• Both air temperature and absolute humidity will determine what type of condensation will occur when the air is cooled.

Condensation (C)

• Condensation occurs when water vapour in the air is returned to its original liquid state.
• Condensation is not a matter of one particular temperature but of a difference between two.
• Condensation of water vapour occurs when the temperature of the air is lowered to its dew point.
• Condensed water is called dew when it forms on a solid surface, or frost if it freezes.
• Window condensation is a simple occurrence that takes place under the right conditions of temperature and humidity.

Differing environments

Another important factor when looking at window condensation is the difference in temperatures between the inside and outside of a home, there will always be a transference of hot or cold on the window glass.

Other considerations

Extreme hot and cold temperature changes in a short period of time inside or outside the home can result in condensation on the window glass.

Seasonal condensation scenarios

If you bring a very cold metal object from outside into a warm environment, the air close to the surface of that object is cooled below the inside environment's dew point causing the air around it to condense leaving water on and around it.

The differing temperatures between the outside and the inside, the window and the moisture in the air all have an effect on causing condensation any time of the year (see some examples below).

Spring Scenario

Fog on the exterior of the window

• The outside overnight temperature -10°C
• Inside temperature +20
• Inside relative humidity 40%
• Outside humidity 40%

Result

The quick cool overnight left lots of dew in the outside air, which was transferred to the exterior of the window. If the following day is sunny and warm, the exterior fogginess on the window will disappear quickly. Energy-efficient windows in this scenario may not make much of a difference, condensation in the form of dew will be everywhere.

Summer Scenario

Fog on the exterior of the window

• The outside temperature is +30°C
• The inside temperature is +20°C
• Inside relative humidity 10%

Result

Typically, this will result in very mild fogging if any, on the exterior of the window. Again, the more efficient the insulating properties of the window, the lesser the amount of fogging on the outside of the window.

Fall Scenario

Fog on the exterior of the window

• Outside overnight temperature -10°C
• Inside temperature +20
• Inside relative humidity 15%
• Outside humidity 40% (rained all day the day before)

Result

The quick overnight cooling temperatures left lots of dew in the outside air, which was transferred to the exterior of the window. If the following day is sunny and warm, the exterior fogginess on the window will disappear quickly. Energy-efficient windows in this scenario may not make much of a difference, condensation in the form of dew will be everywhere.

Outside condensation means your windows are not allowing any heat transfer.

Winter Scenario

Water drops and ice build up on the interior of the window)

• The outside temperature is -20°C
• Inside temperature +30°C
• Inside relative humidity 60%

Result

There will be a fair amount of condensation on the inside of the window. The more efficient the insulating properties of a window the lesser the amount of ice build up on the inside of the window.

While interior condensation is usually the result of shower steam or stove pots, it can indicate high levels of humidity levels in your home. 60% humidity, as in our example above is an extremely high humidity to have in a house at any time of the year.

Excessive moisture in a house can be very damaging to your home and to your family’s health. 

Condensation problems

• Condensation on only a few windows in the home may be an indication of a leaky window frame or a draft.
• Condensation between the panes indicates a broken seal.