Condensation in Roof and Wall Systems

New England winters are the perfect time of year for many things: skiing or snowboarding, maple syruping, snowshoeing, and sitting inside by a fireplace perplexed by a “roof leak” when the roof is clear and it hasn’t rained or snowed in days.

For anybody not from the northeast, winters here are cold and can create the perfect conditions for condensation to occur within a building’s roof or walls, as we recently observed within a vented cathedral ceiling/roof assembly. First, let’s take a step back. Warm air can hold more water than cold. So, when warm humid air within a roof assembly contacts a significantly colder surface, such as the underside of a frozen roof deck, condensation may occur causing deterioration to concealed materials, staining at interior finishes, or leakage to the building’s interior.

There were two main problems here: lack of a reliable air barrier—allowing the air to carry moisture to the cold underside of the roof deck, and inadequate venting—allowing roof system moisture to accumulate. We considered two general approaches to fix this: adding insulation above the roof deck to warm the sheathing and reduce condensation risk, or adding air-tight insulation below the sheathing to prevent moisture-laden air from reaching a condensing surface.

While this example is a roof assembly, understanding the conditions contributing to condensation within both roof and wall systems is vital to prevent such an issue. If not adequately controlled, trapped condensation and moisture can cause serious damage to building envelope components and should therefore always be considered as part of building envelope projects.

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