Tips to Insulate a Cathedral Ceiling
LESLIE: Now I’ve got Andy in New Jersey on the line with an insulation question. How can we help you today?
ANDY: I added on an addition – an enclosed porch – to the back of my rancher. It’s a 12×24 addition. And so, first, it was just a porch. And now, we’re closing it and trying to make it part of the house. So, the question I have was about insulating the ceiling. Because what it is – it has a gambreled (ph) roof on it. And it comes out of the house 14 feet to the back door and it’s 24 feet wide.
And then there’s an A-frame, OK, that goes on top. So I call it a “great gambreled (ph) roof.” I don’t know if I’m using the right terminology but – so the insulation in the ceiling on the two sides, OK, it’s like a vaulted ceiling, I guess you might say. See, the rafters are 2×8 and then they drop into the eaves. So, I’m not sure about the ventilation of the roof.
TOM: So that’s what we call a “cathedral ceiling.”
ANDY: Right. But it only comes up that far for about 8 feet.
TOM: Right. It’s like a partial cathedral, so part of it’s flat and part of it is cathedral. Is that correct?
ANDY: Yes. It comes up – yeah, it comes up right along the rafters of the ceiling for about 8 feet and then it cuts right across.
TOM: OK. So let me give you some suggestions.
So, first of all, unrelated to your question, you just mentioned that you built this addition on a porch. Does the porch have a proper foundation?
ANDY: Well, no, I’m sorry, we built the whole porch there as a porch.
TOM: Oh, it was all part of it. OK, fine. Because a lot of times, we see folks that take old porches and try to turn them into additions and they don’t have the right foundations. Because before we put money in this, we want to make sure you had a good foundation.
Now, in terms of insulating the cathedral section, the way you do that is if you have a 2×8 cathedral, roof-rafter kind of span, you can only put 5½ inches of insulation in that. You need to leave the balance of the space for ventilation, as you’ve mentioned. And you are going to need to make sure that you have ventilation at the peak and also towards the bottom of that.
Now, depending on how it’s configured and how it intersects with the lower slope or the flatter section, you need to figure out a way for air to move above that insulation and get up underneath between the insulation and the rafter and out.
Now, another way to do this is to not use fiberglass at all. What you could do is use spray-foam insulation – Icynene spray-foam insulation. I did this in my house. In fact, I just got an assessment of how well the home was insulated compared – or how energy-efficient the home was compared to my neighborhood. And it went up to being in the top 19 percent of the neighborhood for insulation, which I thought was quite an accomplishment because my house was built 125 years ago. It’s not like we started with a house that was built in the year 2000. This is a 125-year-old house. It’s in the top 20 percent of the most insulated homes in the area because I used Icynene spray-foam insulation.
And if you use the spray-foam insulation, you don’t need to ventilate. Basically, you’re changing that area from an unconditioned space to a conditioned space. You can spray up right against the underside of the roof sheathing and case the whole thing in foam and it’ll be far more insulated than you could ever get with the fiberglass. Because let’s face it: we like to see R-30, R-40 in terms of insulation ability. But all you can get is R-19 because you can only get 5½ inches of insulation in there.
ANDY: Alright. Thank you very much for your help.