Possible to spray foam onto (under) rafter vent baffles to maintain a ventilated roof?
Hi all,
With my current building project (detailed in this threadhere), 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.
现在我试图找出如何最好地离开我nsulate 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!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
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."