What Causes Condensation on Windows?

Why is there condensation on my windows and why does it occur? This is a question that people in the window and door industry get asked on a regular basis.

Condensation as defined in Wikipedia…”is the change of the physical state of matter from a gas phase into a liquid phase. Condensation commonly occurs when a vapor is cooled and/or compressed to its saturation limit when the molecular density in the gas phase reaches its maximum threshold”.

In our northern climate that threshold reduces as the temperature drops which is why you see condensation on the inside of windows in colder weather. When the humidity is too high, it will collect on windows, walls, doors, toilet tanks, electrical outlets or any cool surface. Typically, condensation is not a big problem unless it has been occurring for a long time and is promoting the growth of mold or damaging your walls or wood window jambs.


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If you are experiencing condensation on your windows, one way to reduce it is by adjusting your humidifier setting on your furnace as the outside temperature drops. By keeping the temperature of the inner surface of the glass on your windows warmer than the saturation limit or due point condensation will not collect on your windows. High efficient triple pane glass will keep the temperature of the inner surface of the sealed unit warmer than dual pane glass. Dual pane glass with Low E and argon will keep it warmer than dual pane glass without them. The table below gives you a guide to go by if your furnace doesn’t have it listed on it.

Relative Humidiy

Another common cause of condensation collecting on windows is tight fitting window dressings. Curtains or blinds that inhibit air flow will limit warm air to come in contact with the windows glass causing it to cool down to a point where condensation starts forming. The solution in this case is simple, by opening the curtain a bit or lifting the blinds should reduce the amount of condensation.

A bay or a bow window is a window that protrudes out from a wall that in most cases has no foundation below it or heating vents built into the structure. Unless there are heating vents near it that move air onto the window surface  the window glass may be cooler than other windows in your home. If condensation is a problem, make sure your blinds or curtains are open. If condensation still forms use a fan to blow air directly onto the windows glass.

If you have recently renovated an older home by replacing the windows and doors or added insulation and siding, chances are you’ve made your home more efficient but you may have also sealed many of the places warm humid air used to be able to escape. Although you’ve increased the efficiency you may have also created a situation where you need to vent old stale humid air. Installing exhaust fans in bathrooms and your kitchen may help. When you move air out new fresh air must come in so make sure you have sufficient fresh air suppling your heating system.

For more information about read the City of Edmonton CO2RE document Condensation Concerns.

Outdoor Condensation on Windows

In summer sometimes condensation will form on the exterior surface of your window. This isn’t an indication of a problem with your windows, it’s an indication that they are performing the way they are supposed to.

High efficiency glass is designed to keep the heat inside in cold weather. As the glass keeps more of the heat inside, the outside surface of the glass stays cooler. When the temperature drops below the dew point condensation may collect on the outside of your windows. This can happen during spring and summer in the evening and early morning but as soon as the sun comes out and the temperature starts to rise the condensation disappears.

Condensation Inside the Glass

When condensation forms inside the window, the seal the spacer provides between each piece of glass whether its a dual pane or triple pane has failed. When this happens ambient air along with the moisture, dust or whatevers in the air in enters through the failed seal and gets trapped inside.

brokensealWhen the temperature drops condensation forms on the cool glass surface and as the tempurature of the glass warms up it disappears. But unlike the exterior surfaces of the glass where you can wipe off the condensation its residue , the inside surfaces are not accessable.

Although there are companys that re-seal sealed units that have failed cleaning the interior surfaces is impossible. The best solution is to replace the sealed unit. Always check your warrany to see if the window is covered for seal failures.