Spray Foam Insulation for Sill Plates: Open or Closed Cell?

LESLIE: Now we’ve got Mark in Iowa on the line who’s got an insulation question. How can we help you today?

MARK: Live here in the Midwest so, obviously, we do get the temperatures getting below zero. So I’ve been kind of doing some research on the spray foam. And the one question that I’m not for certain of is when it comes to the walls and then the sill plate.

TOM: OK.

MARK: The best thing I can determine is it looks like when it comes to the walls, it’s probably the closed cell. But then getting up into the sill plate, I can’t tell – I’ve seen it two ways: one says it’d be OK with the closed cell and others, eh, just fill the whole thing with open cell. Don’t know what’s the best way to go.

TOM: OK. Have you taken a look at the Icynene products?

MARK: Yes, I have.

TOM: Because we’ve had some very positive experiences with Icynene. And we also have been in homes where Icynene has been applied and in particular, they use them on a number of This Old House properties and had very good success. So I’m very comfortable with that product.

Now, in terms of open cell versus closed cell, don’t necessarily have a preference. But the key with the spray-foam insulations is to make sure that A) you have a good-quality product and B) that you have very trained installers. Because the installation can – you can really make it or break it when it comes to the installation quality. If you don’t have installers that are really experienced with the products, they can leave areas that are underinsulated. They can actually apply too much insulation and cause problems as a result of that.

So I would focus on the product and the installers that are going to put it together first.

MARK: OK. Alright. I guess my biggest concern is I haven’t seen any indication to worry about water/moisture issues. But it’s the wind barrier – the air barrier …

TOM: Well, there’s two benefits to using spray-foam insulation over a fiberglass insulation. The first is, of course, the insulating ability but the second is the air-barrier ability. Because spray-foam insulation both seals out drafts and insulates at the same time. So that’s the benefit of that product over, say, a batt product like fiberglass or frankly, even cellulose because you don’t get the air-sealing capabilities.

Now, is this a new home that you’re constructing? Where is the insulation going to be used?

MARK: Yeah, it was an unfinished basement when we moved in. Built in about 2005, 2006.

TOM: How’s the rest of the house insulated?

MARK: Up in the ceiling, it is kind of like, oh, the real fluffy type of cotton spray.

TOM: It sounds like blown-in fiberglass.

MARK: Yeah. It’s not rolled or anything. It is loose, so it could be raked around and everything but it doesn’t itch to the touch. For the most part, it’s well-insulated. It’s just the basement was poured concrete. It looks like the brick look and I’m finishing the basement. And if I’m going to spend a little money, I’d rather do it right and that’s why I’ve been trying to bypass the fiberglass and looking at the spray foam.

TOM: Yep. Well, also, you’re going to find that there’s a lot of drafts that get into that band-joist area and that’s going to make the first floor a lot warmer.

MARK: Yep.

TOM: You know, finally, to kind of address your question about open versus closed, what we hear from the marketplace is that many people really prefer closed over open, because it reduces the chance of moisture getting into the product.

MARK: Yep. And that’s why the walls, I’ve kind of leaned more towards that but the sill-plate area, there’s some areas where it might be hard for them to spray in there. And that’s where one of the quotes I got back was recommending to going just straight open cell and just fill it.

TOM: It really depends on whether – what you need to do to get 100-percent coverage. And if the tools – and that may be the truth, because the tools have to get up in there and they may not be flexible enough for some of those nooks and crannies in that particular scenario. So, yeah, if that’s what feedback – I wouldn’t necessarily disagree with that.

OK. Well, good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.