Spray Foam Insulation Against Old Roof Decking?

Old 10-05-09, 07:14 PM
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Spray Foam Insulation Against Old Roof Decking?

Okay, so Iíve been doing some research on spray foam insulation. Iíve waded through the forums and articles, observed the duel to the death between Open Cell foam guys and Closed Cell foam guys. Iíve heard about the escalating tensions between the Always Vent the Roof camp and the Spray it to the Deck guys. Iíve read the chest beating over properly installed fiberglass and blown in cellulose. But thus far, I havenít found much information on my particular concern...

So hereís the situation. I just purchased a 1.5 story home in upstate New York. The house was built in 1910. It still has the original windows, and the traditional amount of insulation (i.e. almost none.) The roof is a hip roof, and the rafters are rough hewn 2x6 (so they actually MEASURE 2Ē x 6Ē.) My plan is to open the ceiling on the second floor up a bit and make it a vaulted ceiling.

Iím planning to insulation the entire roofline, as well as the rim joists in the basement and a wall or two I have open. Based on the estimates Iíve received so far, Iím planning to use open cell foam. I figure 6Ē of open cell should do the job well enough, and itís a bit more affordable too. (Though perhaps closed cell would be better in the rim joists, since itís a vapor barrier all by itselfÖ)

In any case, now we come to my issue. The roof rafters are, as I said, 2x6s. Atop the rafters is Ship Lath (Or so people have told me.) It seems to be 1Ē thick boards running along the roofline perpendicular to the rafters. On top of THAT it seems is the original cedar shingles, which have since been covered with a new layer of plywood and then modern asphalt shingles. This configuration does not fill me with confidence, but it seems to be holding nicely. Thereís no evidence of water damage or leaks. The previous owners did each portion of the hip roof at a different time, so one roofline is only a few years old, and the others have some wear on them, but still seem to be in good shape.

My main concern is that I donít want the insulation to adhere directly to the underside of the decking / cedar shingles. When I have the roof redone (hopefully not for some time!) I plan to have it stripped down to the rafters. Iím afraid that if the foam adhered to the underside of the shingles) or any decking really) then when the roof is pulled off, my expensive spray foam insulation will go with it! Iím also not sure if the ship lath is likely to be left when the roof is redone. I see no reason why it couldnít stay and new plywood be placed atop it, but Iím hardly a roofing guyÖ

Anyway, Iíve heard of a few different ways to keep the insulation from sticking to the decking. Iíve seen people staple cardboard to the underside of the roof on TV. One installer said he usually staples landscaping fabric to the underside of the decking for these situations. Another guy said something about using 1Ē closed cell foam boards, and then spraying open cell foam atop it. The closed cell foam guy I used next pooh-poohed that idea though, saying that the high temperature of the foam when sprayed on might melt the board. (But maybe thatís just true for closed cell foam? Does open cell foam get sprayed in at a lower temp?)

In any case, this post is long and rambling enough as is. Anyone have any advice for me?
Old 10-05-09, 07:54 PM
Join Date: Oct 2008
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Unfortunately I don't think the test of time has given us the final picture as to how a "hot roof" is going to stand up. Will those boards be in good shape or rot through in 20 years, I don't know. I've probably read much of the same information as you have, but I'm probably 20 or 30 years older, so I still prefer the vented (cold) roof, when possible. But, there are times when everything just adds up to where spraying the cavity full of insulation is the only realistic solution.

As for trying to keep it from sticking to the roof deck, that I have never tried. If your attic area is completely open, have you considered a combination of rigid foam fitted between the rafters and a layer over them, covered with drywall. The layer over the rafters helps to eliminate the thermal bridging through the wood. I'll add a link related to capes, not exactly your issue, but maybe some pointers. Welcome To Home Energy Magazine Online

Old 10-07-09, 11:58 AM
Join Date: Oct 2009
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Just an idea: Use ridgid foam board instead og spray in. Paint the underside of your decking, before the ridgid foam board install, with Fosters 4020. When you reroof, use a layer of ridgid foam board there too. If you search on it you'll get what I am talking about.

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