How Much Attic Insulation Should I Add?
LESLIE: Our next victim listens to The Money Pit on WTKS in Georgia.
Welcome, Mark. How can we help?
MARK: I have a question about insulation in my attic. Got about 6 inches, what’s not matted down from when they originally built the house in ’87, and I want to add more to increase my r value so my electric bill’s not so high and the rising cost of electricity. How much should I add?
TOM: That’s a great question. You know, in your part of the country, you probably want to have 10 to 12 inches of insulation. So we would recommend another 6 inches …
LESLIE: Is it 3 inches equals an r?
TOM: It’s 3 r per inch.
LESLIE: Ah, thank you. I had it backwards.
TOM: (laughs) I was thinking you had me going there for a second. (Leslie chuckles) No, it’s 3 r per inch. So if you have 10 inches, that’s 30 r; 12 inches is 36 r. And if you’re wondering what we “r” talking about (chuckles), it’s – the r is the – r factor stands for resistance to heat loss. And so that’s how the efficiency of insulation is measured.
But what you really want to do is add another 6 inches of unfaced fiberglass batts to that, Mark. And the way to do that is to put them perpendicular to the existing attic rafters.
MARK: Can I blow it in instead?
TOM: You can. You can but remember, if you blow in insulation – I don’t know if this is an attic you want to store in – it makes it really, really hard to do any storage in there if you do use blown-in.
MARK: These are non-storable attics.
TOM: Alright, well then you can blow it in. The other thing to do is to make sure that you have plenty of ventilation at the soffit area. A 1987 house probably has a vented soffit.
MARK: Actually, I don’t have any vented soffits. They’re all gable vents.
TOM: Oh, OK. Well, a vented soffit is going to help make the insulation more effective. You know why?
MARK: Force of the air through there?
TOM: Well, it dries out the insulation. Because if insulation gets slightly damp, it loses its r value. In fact, if you add two-percent moisture to insulation, the r factor goes down by a third.
MARK: Well, we have 100-percent humidity down here almost all the time.
TOM: Yeah. So more ventilation is also important. The best ventilation is where you have continuous soffit vents and a ridge vent at the peak.
MARK: OK. So I should do that first before I insulate.
TOM: I think you definitely should. And if you use the blown-in, you’re going to need something called an insulation baffle that basically goes under the roof sheathing at the soffit edge and ensures that you have an air channel there. An insulation baffle is very, very cheap – it’s usually made out of Styrofoam – but it plays an important role in keeping that air flow channel open. Because if you blow it in, it could fly into the soffits and then it’ll block the vents and that defeats the purpose.
MARK: So what kind of r value should I be shooting at down here in Georgia?
TOM: Thirty to thirty-six.
MARK: Thirty to thirty-six.
MARK: And then how much – if I only have – I guess it’s r19 I have up there now.
TOM: Right. If you add another six inches, then you’ll be done.
MARK: So how much will that save my electric bill? Any ideas?
TOM: Well, there’s a lot of factors there but I can tell you that it’s a great investment. Of all of the energy-efficient investments, adding insulation is one of the best ones you can do. Gives you the surest return on investment.
MARK: OK. That’s what I’m going to do.
TOM: Alright, Mark. Thanks so much for calling us at 1-888-MONEY-PIT.