Spray Foaming Under Rafter Baffles
Hi all,
With my current building project (detailed in this thread here), it came to light that the wrong kind of roof underlayment was installed (synthetic instead of asphalt felt), which doesn’t breathe enough to allow moisture to evaporate to the outside and therefore requires a vented roof assembly.
So now I’m left trying to figure out how best to insulate the roof given that it must be vented. I’m wondering if I could install rafter vent baffles like you would with fiber insulation in a vented roof assembly, but instead of installing fiber under the baffles, install spray foam. I’d have to protect the soffit and ridge vents and separate them from the foam to maintain venting, but that’s doable.
As far as I can tell, that would get me the best of both worlds: the roof would stay vented for the purposes of removing moisture, but it would be air sealed to the conditioned space below and with, say, five inches of foam plus another six inches of fiber, I’d have enough R-value.
I know that I’ll have to create a backer for the spray foam along the eaves and ridge to provide a seal below while allowing air to pass between the soffit vents and ridge vent, but if I can successfully do this, would this idea work?
Thank you for your advice!
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Replies
Upon further research, I found Joseph Lstiburek's article in Fine Homebuilding, "A Crash Course in Roof Venting,", in which he discusses the exact arrangement I discussed above, venting the roof deck with baffles then spray foaming under the baffles.
Chris,
Just to be clear: unless you installed your roofing so that it could dry to the exterior, what type of underlayment you used doesn't really matter. As Martin wrote in your other thread:
"If you have installed standing seam roofing without purlins, then you won't get any drying to the exterior. In that case, your synthetic roofing underlayment will do no harm."
Hi Malcolm,
Right, and it was not built to dry to the exterior (for a number of different reasons: in addition to the synthetic underlayment, there are no purlins; the screw-down corrugated metal panels were screwed directly to the roof deck).
My impression is that since it's not going to dry to the exterior, it needs to be vented on the interior.
With only 10 1/4" to work with (2x12 nominal = 11 1/4 actual, minus 1" for the baffle) the best I can get with fiberglass is R-38. If I instead spray 5" of foam then supplement with 6" of fiberglass, I can get to R-49.
As always though, I'm new at this, so if others have wisdom to share, I would greatly appreciate it!
Chris,
With your proposed stack-up both types of insulation are in the cavities leaving each rafter as a thermal bridge. If you insulated the cavities below the ventilation channels with your choice of blown or batt insulation, you could use foam sheets on the underside of the rafters and get a much better whole roof R value - and save some money too.
Have you thought of insulating above the ceiling instead? That would allow you to have the cold attic.
Chris,
By now, I guess that you have figured out that your plan will work. I discussed that approach in my article, Creating a Conditioned Attic: "Even if you insulate between your rafters with an air-impermeable insulation like spray polyurethane foam, you may want to provide a ventilation channel under your roof sheathing. The main function of such a ventilation channel is to separate the roof sheathing from the foam; this facilitates future repairs of sheathing rot."
Hi Martin and CConti,
I realize that this is an old thread, but I am in a similar boat and could use your advice. I have a standing seam roof installed with no venting between it and the roof sheathing. My plan is to vent below the sheathing with prefab styrofoam venting baffles (as mentioned in the OP and in Joe Lstiburek's article). It is a century home and a pain to get it all fitted, but now that I have brought in a large experience spray foam company to review the location that they will be spraying, they are concerned about applying to the baffles as the heat generated by the foam's expansion and curing process could distort and even melt the baffles entirely. This is the first I have heard that this could even be a possibility even combing through the vast web of forum posts. Could someone please weigh-in here and even offer some suggestions that do not result in me having to rip out all of my hard work and use a different product? Thank you in advance!!!
You could just build baffles with 1x2 furring strips and 1/4" waferboard. Use a finish nailer to tack 1x2s onto the inside edge of each rafter, with one edge tight against the sheathing and the wide side of the 1x2 facing towards the opposite rafter. Do this on both sides of every rafter. Now use your finish nailer to tack 1/4" waferboard panels up to those 1x2s. I find it's easiest to nail at an angle and go through the waferboard, a little of the 1x2, and the rafter. It's not super critical though since none of this is carrying any real load.
The 1x2 furring strips create a 1.5" vent gap, the 1/4" waferboard closes off the gap from the interior and gives something for the spray foam crew to spray against. No worries about melting or deformation this way, and the materials are readily available and relatively inexpensive.
I'm not so sure the spray foam would melt the foam baffles, but I wouldn't be surprised if it caused them to deform, maybe significantly so. The heat of the spray foam certainly will soften the baffles at the lease, and the expansion will then have an easier time deforming them. I have never tried and install like this though, so I can't say for sure, but I think the spray foam crew's concerns are probably justified here.
Bill
Hi I know this is an old thread... would this approach work even if the roof had hip roofs, dormers etc? I know its not great for venting but if you have the tight air seal below with closed cell you don't have to worry about moisture condensing right? thanks!
BPortnoy,
There are several un-vented roof assemblies that work well, and not needing venting means that having hips and dormers doesn't matter.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Thanks Malcolm. I’ve read this. Great article. For some reason I still like the idea of venting if you. I know it’s hard with hips and dormers etc but if you have closed cell in proper amounts as an air barrier underneath hypothetically there won’t be moisture to dry out and having perfect venting won’t be as important right? Or is my logic all wrong.
BPortnoy,
Leaving code requirements aside, there isn't much point in adding vent channels if they won't be effective. For that they need an inlet and an outlet. You can still vent hips and dormers by cross strapping, which allows some air-movement, but not of course as effective as a straight gable would. That's the minimum I would do, even if you want to rely primarily on your air-sealing and vapour-barrier.
Depending on what roofing you are using, there are also proprietary hip vents for shingles, and you can use the same detail you would at a vented ridge on metal roof hips.
Thanks so much