Insulating a Cathedral Ceiling
LESLIE: David in Michigan has some concerns in the attic. Tell us what’s going on.
DAVID: Hi, Tom, Leslie. Like your show.
TOM: (overlapping voices) Thank you very much.
LESLIE: (overlapping voices) Thanks so much.
DAVID: We have a ranch that was built in ’58 and we wanted to ask about improving the insulation in our ceiling but we don’t have an attic. The ceiling follows the same angle as the exterior roof, with the distance between them really just being the height of the joists.
TOM: Yeah, you have a cathedral ceiling. You said the house was built in the 50s?
DAVID: Right, ’58.
TOM: Yeah. Very, very common. Not really possible to easily improve the energy efficiency of that roof structure because there are only two places you can do it: on top of the roof or under the roof. (chuckles) You can’t – you know, on top of the roof involves adding sort of a sandwich roof structure, which changes the whole line of the roof. And underneath means you have to give up part of the cathedral by basically lowering – you can create something that’s called a scissor truss, where you have – you know, one angle is the roof and the other angle is the ceiling and there’s space in between and fill that up with insulation; but short of doing something like that, I don’t see how you’ll be able to get additional insulation into that space.
I mean I guess another thing that you could do is cover the underside of the ceiling with, say, something like isocyanurate foam boards or something of that nature, which would add a little bit of r to it; but then you’d have to drywall over that. But it’s a lot of work no matter how you look at it.
The main concern with that type of ceiling, by the way, is condensation. Because, typically, if you have like a 2×12 rafter, you only want to use 10 inches of insulation because you need to leave space for air to move above it. So make sure you keep an eye on that roof for any signs of decay or delamination in the roof sheathing as a result of inadequate airflow. That happens, you’re going to end up taking that ceiling down anyway and you can just do it over at that point.
DAVID: OK. Thanks for letting me know for sure. Now I know.
TOM: Alright. Well, good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.