What causes roof condensation? (with photos)

Roof condensation can cause shingles to buckle.

Roof condensation occurs when moisture is trapped between the roof and insulation. Typically, this problem stems from a poor finish in the roof installation or insulation, insufficient ventilation, or excess moisture in the building. If this problem is not detected early on, it can be very expensive and possibly dangerous to fix. Left alone, roof condensation can cause shingles to bend, tin roofs to rust, and in severe cases, can lead to mold and algae growth that can cause serious health problems, especially for children and pregnant women.

Moisture can enter the attic through poorly sealed windows.

A normal house produces moisture in the kitchen, bath, laundry room and many 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, steam will rise beyond 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) hotter than the rest of the house. If the insulation is not the correct insulation or the attic is not well ventilated, the moisture will not dry out completely. Then, as the cavity cools overnight or in winter, the water condenses or changes from a vapor back to a liquid.

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

Roof condensation is a thorny but common problem. It is usually caused by excess moisture, inadequate ventilation, poor quality workmanship, or some combination of these factors. Humidity can be increased by occupants if they are, for example, doing laundry or doing some other activity that produces above average air humidity. Living in a particularly humid area can also increase the risk of roof condensation. Roof leaks and installing insulation before the roof wood is sufficiently dry can also contribute to the problem of excess moisture.

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The surest sign of condensation on the roof is a stain, whether on the plinth hooks or the ceiling. A plinth is a horizontal wooden or metal structure that helps support the roof, and a hanger is the piece of metal that connects the end of the plinth to the wall or rafter. On a purlin, the stain will usually look like dark streaks on the wood around and under the hanger. On the ceiling, there are several light colored circular spots in the middle. This stain is one way to identify if the problem is condensation or a leak on the roof, as a leak stain usually leaves only a large stain of concentric circles that are darker on the inside.

Roof inspection and maintenance by people experienced in ventilation and condensation is the best way to deal with roof condensation. If the damage has already started, there are several ways to deal with it, depending on the root of the problem. Ventilation systems such as panoramic or roof windows should be checked to ensure they have not been sealed. If they are insufficient, it may be necessary to add more ventilation. If there is a foil barrier over the insulation, it may also be necessary to cut along the purlins to allow for better breathability.

The home may need to reduce its humidity by installing dehumidifiers, keeping fans running, or keeping windows open. The attic or ceiling can also benefit from being resealed with a vapor-retarding or barrier paint so that water does not reach the roof. This is generally not considered do-it-yourself work and is best done with the help of roof condensation and ventilation specialists.

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