condensation and humidity in windows

Condensation in windows can reduce natural light and, most of all, be a cause of moisture, mould and related damages.

Very occasional condensation on the exterior glass surface of the windows is usually just a small annoyance. But if it happens with frequency, you should check the issue.

Window condensation damages

Condensation in windows, if left unchecked, can damage the window frames, the sills, the surrounding paint, plasterboards, etc., and in more extreme situation the moisture can seep into walls, damaging their insulation and their framing.

Condensation is mostly a cold-climate-winter problem, but it also happens in hot-humid climates, where moisture can condense on the outside of poorly insulated windows of air-conditioned rooms.

Energy efficient windows can stop condensation

New energy-efficient windows may solve many condensation problems. Special glasses and good quality frames and spacers makes the temperature of their inside glass surface higher and more constant than that of other windows, making condensation much more unlikely to happen.

Top window manufacturers have new materials and models to increase the condensation resistance of their products. New types of glass (with inert gas between the panes) and new and better spacers and frames make qualified windows much more moisture-resistant.

In living in a climate where window condensation is a recurrent problem, choose a window with a very high Condensation Resistance coefficient (more than 80 in a 1-100 scale). See: Best Windows Rating

Weather-resistant barriers

Pay attention to the weather-resistant barrier installed between the frames of the windows and the surrounding walls. This barrier, often made of perforated asphalt felt paper or woven waterproof polyethylene, is particularly important, mainly in wood-frame homes, to avoid condensation problems around the window frames.

That barrier should be sealed at the edges and also overlapped at the seems. Without it and a highly water-tight installation around the window perimeter, the moisture will penetrate through the siding around it, causing problems.

Humidity and the window Rough Opening

The gap between the windows frame and the rough opening may also be a cause of leaks and condensation. A poor air-sealing of this gap can have very negative impacts.

Window installers should fill the whole gap carefully, using several layers of expandable polyurethane foam, in order to get a fully airtight window.

Improper installed windows

Window installation is a particularly sensitive job, mainly in cold and rainy climates. Mold, rot and paint damages around window perimeters are often associated with weather barriers and improperly installed windows.

Be careful. Choose a good installer, and pay special attention to the replacement of old windows. When the weather-resistant barrier associated to the windows is disturbed, the risk of moisture is great.

Other causes of condensation in windows

Windows (and glazed doors and skylights) are commonly seen as the source of condensation and moisture problems that occur on them. But that doesn't mean that they are the ultimate sources.

In most cases, condensation occurs firstly on windows just because they have lower thermal resistance than floors, walls and other surfaces in the house, but not because of them...

In other words: since the inner surfaces of the windows are the coolest surfaces in the house, there are weather conditions in which condensation occurs on windows just because they are the coolest surface (and not because of them).

Window Condensation & Ventilation

Extreme outdoor temperatures combined with high interior water vapor, can cause condensation in windows, even if they are efficient enough. That's common in cold climates...

Do not underestimate excessive humidity due to internal sources of condensation. People, plants, cooking, baths or laundry are important sources of warm air which may condensate on windows...

To solve the problem, increase your home's ventilation system by installing a good mechanical ventilation system (a simple set of exhaust fans - a kitchen exhaust fan and a bathroom exhaust fan - can help but is often insufficient). See: Moisture control.