Spray Foam Insulation at the Edge - A Forensic Analysis
An attic that's brought inside the building enclosure is a beautiful thing. This is most often done with spray polyurethane foam (SPF) insulation, either open-cell or closed-cell. Here in the southeastern US, open-cell is most often the spray foam of choice. But how well does this process of encapsulating the attic actually work? Well, as any good building scientist will tell you, that depends.
In my case, I just bought a house that had spray foam installed in the attic in 2012. I've discovered many problems with what the contractor did and I'll have more about that in future articles and videos. Today, though, I want to focus on just one aspect of spraying foam in an attic: what happens when the installer sprays the foam into the eaves.
Shooting the eaves
In the lead photo above, the foam appears to be filling the eaves pretty well. But that's the attic view. What would it look like if you could see it from the outside? Well, I've had that opportunity this week because we're having all the soffits, fascias, and gutters replaced. After the crew removed the old boards, I got in there to take a look.
This one looks good:
The spray foam made it out to the edge of the cavity, covering the top plate and providing what looks like a pretty good seal.
But not all of the cavities look so good. Here's another one:
This one is really interesting. You can see the spray foam back in the cavity. You can also see that some of it shot out past the cavity because there's some on the roof deck in the soffit area, along with a spray foam shadow on the roof deck where that ceiling joist in the cavity blocked the spray foam.
It's possible that the foam did make a good air barrier in that location, preventing air from moving between the soffit and the encapsulated attic. But even if that's true, there's still a deficiency here: There's little insulation over the top plate of the exterior wall. It's bad enough that the low-slope roof means that we can't get enough insulation over that exterior wall, but when spray foam doesn't cover that area much at all, we have extra heat loss in winter and heat gain in summer in that area.
Another interesting aspect of what happens when the installer sprays the foam at the eaves is the overshoot. In the photo above, you can see it on the roof deck over the soffit. The photo below shows that it doesn't stop there. It shoots all the way across the soffit and hits the back side of the fascia.
How to get the edges insulated and sealed
To prevent this problem, the spray foam installer needs to do a little extra prep work. To keep the spray foam from shooting all the way across the soffit and leaving the cavity above the exterior wall empty, there needs to be some kind of material to stop it. This could be a piece of fiberglass batt or cardboard or foam board. There's just got to be something to stop the foam and keep it where it's supposed to be.
In existing homes, doing this can be difficult with a low-slope roof, but that doesn't make it less important. If the house is undergoing additional work and the soffits happen to be open, that extra prep work can be done from the outside. In new homes, it's a matter of planning for this as you go. Install the baffles or whatever blocking you're going to use during the framing.
I like spray foam insulation; we got more installed in our attic last month, and now it's much better than it was. (More about that coming soon!) Now that we're having the soffits and fascias replaced, I've been able to address the problem with the eaves I described above...and that's the subject of my next article.
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