In this article, I’m going to discuss a building science mystery: namely, summertime condensation near the peak of cathedral ceilings. I will propose a mechanism to explain these problems, in spite of the fact that my proposed explanation is somewhat unsatisfying.
I’m hoping that this article will prompt GBA readers to share more examples of the phenomenon — and perhaps to share more data and a better explanation for what’s going on.
As an introduction to this topic, let’s hear from six GBA readers.
Candi wrote, “Why is there condensation on my cathedral ceiling? It’s warm outside (80 degrees) and 75 degrees on the inside thermometer. The roof is in full sun all day, so it’s hot up there. There is nothing except insulation between the ceiling and roof and the condensation only forms along the beam at the peak of the roof inside. … It doesn’t happen when it’s under 70 degrees or raining. It just started when the weather got warm. We haven’t had much rain in the last three weeks, but had a lot in March and April and there wasn’t ever water [then]. The water also forms on the underside of the beams — not the actual roof side of the beams but the side facing down to the floor. The [forced air] registers are all in the floor. They run through the crawl space under the living room and between the basement ceiling and kitchen floor.”
Marty wrote, “I have cathedral ceilings on my second floor with closed-cell spray foam insulation on the bottom of my roof. There is only about a 1 to 2 inch gap between my ceiling and my hot roof. I had the roof done 4 years ago and it is completely covered with Ice & Water…