What Causes Roof Condensation?

Caitlin Kenney

Roof condensation occurs when moisture becomes trapped between the roof and the insulation. Usually, this problem stems from either poor workmanship in installing the roof or insulation, insufficient ventilation, or excess moisture in the building. If this problem is not caught early, it can be a very expensive and possibly dangerous problem to fix. Left alone, roof condensation can cause shingles to buckle, tin roofs to rust, and, in severe cases, lead to mold and algae growths that can pose serious health concerns, especially for children and pregnant women.

Roof condensation can cause shingles to buckle.
Roof condensation can cause shingles to buckle.

A normal household produces moisture from cooking, showers, laundry, and various other activities. During the day, especially in the warmer months, the heat causes this moisture to evaporate and rise. If the ceiling or attic is not properly sealed, that vapor will travel up past the ceiling and become trapped in the space between the insulation and the roof, where it can be as much as 50 to 70°F (10-21°C) warmer than the rest of the house. If the insulation is not the right insulation or the attic is not properly ventilated, the moisture will not completely dry. Then, as the cavity cools during the night or winter, the water condenses, or turns from a vapor back into a liquid.

Moisture can enter an attic via poorly sealed windows.
Moisture can enter an attic via poorly sealed windows.

Roof condensation is a thorny but fairly common problem. It is typically caused by excess moisture, inadequate ventilation, shoddy workmanship, or some combination of these factors. Moisture can be increased by the occupants if they are, for example, running a laundry or doing some other activity that produces more than average moisture in the air. Living in a particularly humid area can also increase the risk of roof condensation. Leaks in the roof and installing insulation before the roofing wood has sufficiently dried can also contribute to the excess moisture problem.

The surest sign of roof condensation is a stain, either on the purlin hangers or on the ceiling. A purlin is a wooden or metal horizontal structure that helps to support the roof and the hanger is the metal piece that connects the end of the purlin to the wall or the rafter. On a purlin, the stain will usually look like dark streaks down the wood around and under the hanger. On a ceiling, there are several circular stains that are light-colored in the middle. This stain is one way to identify whether or not the problem is roof condensation or a leak in the roof, as a leak stain usually leaves just one large stain of concentric circles that are darkest on the inside.

Having the roof inspected and maintained by people experienced in ventilation and condensation is the best way to deal with roof condensation. If damage has already begun, there are several ways of dealing with it, depending on what the root problem is. Ventilation systems, such as in sky windows or in the roof, should be checked to make sure they haven’t been sealed. If these are insufficient, more ventilation may need to be added. If there is a foil barrier over the insulation, this may also need to be cut back along the purlins to allow better breathability.

The household may need to reduce its moisture by installing dehumidifiers, keeping the fans running, or keeping the windows open. The attic or ceiling may also benefit from being resealed with a vapor retardant paint or barrier, so that the water doesn’t reach the roof. Generally, this is not considered a do-it-yourself job and is best done with the help of experts in roof condensation and ventilation.

If roof condensation is not addressed early, a new roof and extensive repairs may be necessary.
If roof condensation is not addressed early, a new roof and extensive repairs may be necessary.

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Discussion Comments


This is a problem here in the hot, humid South. The problem is often a symptom of terrible roof insulation. There was a time when it was in fashion to use an electronically-activated ventilator instead of the trusty, time tested "whirleybird" that vents both heat and humidity from a roof (and you know what those look like -- they appear to be globular fans that sit on top of large tubes sticking out of a roof). The old whirleybirds, it seems, work better and cost less. Who knew?

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