Resolving Tongue & Groove Moisture Problem

Many people who have tongue and groove ceilings or attic slopes in Maine have a common problem: at certain times of the year it "rains inside."

Because tongue and groove (T&G) is not air tight, warm and moist interior air can seep constantly through the seams. Unless there is a good air barrier, under certain conditions, this moist air will condense when it meets the roof deck, which leads to mildew or drops of water seeping through the small cracks between the boards.

Tongue and groove can be a nice, attractive finished surface, but our climate requires special considerations when installing it if the home will be occupied through the winter.

Below is a picture from a recent renovation we worked on. The top arrow points to mildew, the bottom arrow to the original fiberglass insulation. While this is certainly not the worst case we have seen, it demonstrates the cause of the problem really well: there is no air barrier. The issue here is that behind the T&G, fiberglass was used as insulation. Because air easily passes through fiberglass, this installation is inappropriate.

Our solution was to dense pack cellulose behind the remaining T&G that was not removed for a near air tight barrier. To insulate behind the removed T&G, we used 2-part spray foam. Spray foam provides both an air and thermal barrier; and has a high R-value per inch of material for great insulation. Our goal was to build an assembly that prohibited warm air from touching a cold surface.

Once properly insulated and air sealed, the T&G was replaced, keeping the original rustic charm.