How to Insulate a Sloped or Cathedral Ceiling

LESLIE: David in Georgia is looking to properly insulate a bonus room. What’s going on at your money pit?

DAVID: Got a question about some insulation on these rafter systems on a bonus room upstairs.

TOM: OK.

DAVID: What do you guys prefer to insulate that with? Is it just a paper-backed insulation or do you go with a hardboard or …

TOM: Is this is a cathedral ceiling, David?

DAVID: I’d like to leave it that way, yes.

TOM: OK. So, this is a sloped ceiling in like a great room kind of a thing, right?

DAVID: Up above our family room. It’s just a bonus room upstairs that we had.

TOM: Alright. OK. So the question is how do you insulate a cathedral ceiling where you don’t have a horizontal flat ceiling and then an attic above it. In other words, your ceiling goes straight up. When we’re looking up in that bonus room we’re basically looking at the underside of the roof rafters, correct?

DAVID: Yes, sir. Yes, sir.

TOM: Alright. So in a situation like that let’s assume, for the purpose of this call, that your roof rafter is an eight-inch beam; like a 2×8. What you would do is you would install six inches of insulation into that. You would not fill up the entire eight inches deep because you want to leave that extra two-inch space between the end of the insulation and the underside of the roof sheathing so that you can flush air through that.

So I would use a foil-faced, six-inch insulation in this example. I would put normal drywall on it but I would also make sure that I had a ridge vent at the top and then a soffit vent at the base and, this way, air will get into that cavity that’s left and it’ll dry out that insulation.

Because remember, you’re going to get a lot of moisture that’s going to collect in there. If you try to seal the whole thing up, it’s not going to vent and a couple of things will happen: if the insulation gets really damp – if you add just two percent moisture to it, it loses up to a third of its ability to insulate and secondly – and more importantly – your roof sheathing will rot. It will decay and it will rot over time, so you need to have ventilation in that space.

So make sure you don’t fill it up all the way. You leave an extra couple of inches and you vent it properly and that room will both be warm and dry and safe at the same time.

DAVID: OK, super. And you said a foil-faced insulation.

TOM: Yeah, it could be foil-faced or it could be paper-faced or it could even, frankly, be unfaced.

DAVID: OK.

TOM: But make sure you leave the space. I would prefer foil-faced if it’s available to you in your area.

DAVID: Foil-faced towards the …

TOM: Yeah, with the foil …

DAVID: … drywall or towards the roofing?

TOM: The foil – no, foil goes toward the heating space – heated space.

DAVID: Towards the heating space. OK.

TOM: Yeah, vapor barrier always goes toward the living space.

DAVID: That’s – OK. That’s the way I had the rest of it. I just wanted to double check on the roof and make sure that was right.

TOM: Alright.