Spray foam insulation in existing construction--yea or nay?

February 17, 2015 12:32 PM

As a way of remediating our ongoing ice dam/roof leak/no bathroom ceiling woes, we're looking at getting spray foam insulation in the roof. I know a lot of people love it, but my contractor (who is not my spray foam guy) hates it--and thinks its a good way to get your roof to rot. What's the straight dope?

We would get closed cell in the accessible areas but would have to get open cell in some closed spaces (so that the expansion doesn't blow the drywall off the rafters--our bedrooms are right under the roof). It's chilly in the winter and warm in the summer, so something has to be done.

As it stands, our insulation is pretty iffy--it's doing something, but not a lot, and we're getting ice dams. Either way, I'd like to get the insulation upgraded--batts or cellulose versus spray foam.

I like my (regular, not spray) contractor, and his concern is that water gets between the roof and the foam, and ultimately does more damage than, I dunno, an isolated ice dam. I've known a number of people who have had the foam installed in the past few years and they have all been happy--but the installs are recent enough that they would not necessarily have had any harm to their roof.

And, it goes without saying that I don't want my roof to leak in the first place; that's a bad place to be--but I guess the concern is that your roof might leak eventually (or maybe even get another ice dam, since our retrofit install would not be 100%) and you don't know and then your house explodes.

Some of you must have weighed these concerns--what's the scoop?

posted by Admiral Haddock to Home & Garden (5 answers total)

We built a studio for my wife in 2007 and used regular insulation. Last year we added 900 sq ft to it and used closed cell. According to the installer nothing is perfect, and even closed cell can leak if there are voids. The problem with foam, is that if it does leak, you won't notice it as quickly and it can cause rot. We went with it because the insulating factor is great. Snowzilla has created a huge ice dam on the old studio roof ( shed style). The new roof is fine, but I am concerned with the old one. We are considering metal roofing for that area because it doesn't get much sun, and metal sheds the snow very well and almost eliminates damming. If you can climb up there, take a bag of calcium cloride and make 3" high dunes that run from beyond the dam to the edge. If you can't get up there, try those roof salt discs. They are shaped like urinal cakes and you can toss them up on the roof and let them do their magic. Back to your main question; I wouldn't use foam if you cannot get 100% coverage with closed cell, ... open cell will let water permeate. Also: The actual spraying is a nasty job, and closed cell leaves an odor that is impossible to live with, and lasts for days. I mean REALLY nasty.
posted by lobstah at 1:24 PM on February 17, 2015

No experience but I've also read that it provides perfect cover for termites (thought that's more of a basement thing.
posted by bonobothegreat at 1:53 PM on February 17, 2015

Between the insulation and the exterior wall there is a difference in temperature. In the right conditions (relative humidity), you can have condensation between these two temperature zones. The more you can attenuate this temperature change zone the better your chance of avoiding moisture buildup. And, because of the relatively loose, air-circulating nature of most insulations (including leaving a small air passage between roof and insulation) things have a chance to dry out. With (I think all) spray foams, the foam sticks to the roof itself, there's no air-gap to dry things out if moisture builds up.
And yes, what's more, termites love moisture.

So, if you like your contractor and he would rather use something else I would trust him.
posted by From Bklyn at 1:39 AM on February 18, 2015

As with all roofs and all insulation materials, proper ventilation is essential. Spray foam, directly to the underside of the roof is not the proper application.
posted by humboldt32 at 1:56 AM on February 18, 2015

Yes, the issue with open-cell is not that water might leak through it, but that moisture from inside the house will transfer through it and then condense in your roof sheathing (causing mold and potentially structural failure, especially if the roof sheathing is OSB). You need to install a vapor barrier with open cell foam and you can't if the area isn't accessible.

Insulation in your roof is very important, but ventilation is more important when it comes to ice dams (unless you are going to use a vapor impermeable insulation with no air space). Can you build the roof higher (only make sense if you need as new roof soon anyways) to allow for air space and proper insulation? Can you drop the ceiling to give more space? These are expensive options, for sure, but they allow you to use much cheaper materials than spray foam.

Also make sure you have enough ventilation at the eaves and enough at the ridge. Consider a membrane underneath a new roof to prevent leakage.

This is a tough problem. Pretty much the only solutions are to make your roof cold by ventilating or insulating in specific ways or to make it highly waterproof.

Don't trust the spray foam guy; he will claim spray foam cures all ills.

Here is a lengthy primer on open cell foam used in a unventilated roof.
posted by ssg at 1:59 AM on February 18, 2015