Combining open-cell foam with rigid foam
Hi All,
I live on Long Island, NY (zone 4a) and have a cape with 2×6 rafter bays that are filled with open-cell foam. My heating/cooling system is geothermal, with duct work throughout the knee walls. I would like to address thermal bridging of the rafters, which was quite evident during our last frost. My eventual goal is to use rigid foam on top of the roof deck, but a relatively new roof makes that impractical for the near (10 years minimum) future. However, the attic is still exposed, leading me to wonder if I could frame some offset or perpendicular 2×4 to rafters to be filled with some sort of air-permeable bat insulation (e.g. fiberglass or rockwool). My questions are:
1. Will the bat, open-cell and eventual rigid foam be compatible? It seems to me that I am still allowing any vapor diffusion problems to dry to the inside, making this a non issue.
2. What material would you recommend for the bats, if this option is indeed viable?
3. Should I allow an air gap between the bats and open-cell?
4. Lastly, if you do not recommend using bats and just waiting for the roof deck to be replaced, is it alright to have a gap between the open-cell and drywall? I need to provide nailers in the corners for the drywall, thus creating this gap. I suppose I could chisel out some of the foam for the nailers, but this seems pretty work intensive and diminishes the performance of the spray foam.
I appreciate any insight!
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Replies
1: Yes, it's compatible with an eventual foam-above-the-roof deck solution, but you need a class-II vapor retarder NOW to avoid excessive moisture accumulation in the o.c. foam over the winter, since 5" of o.c. foam is usually barely a class-III vapor retarder.
2: High density R15s- rock wool preferred for it's phenomenally good fire resistance, but R15 fiberglass still a decent product. R15s are significantly more air-retardent than R13s or R11s, comparable to 3lb density cellulose.
3: No- the fiber should be tight to the foam, no gaps or voids if you can help it.
4: Yes it's fine to have an air gap between the foam and the drywall.
4a: But you need to either use vapor barrier latex (about 0.5 perms) on the drywall, or install a smart vapor retarder (preferred) between the foam & drywall. (Intello Plus, or Certainteed MemBrain), since the 5-6" of o.c. foam is too vapor open on it's own.
4b: If you add the batts, you'll still need a class-II vapor retarder or smart vapor retarder.
4c: The IRC prescriptive R15 above the roof deck for zone 4 presumes an R49 total, but it's the RATIO of exterior R to total R that determines the temperature of the roof deck, which has to average above the winter time indoor dew point. That ratio is R15/R49, or about 30%. If you have R20 o.c. foam + R15 batts, you'll just make that ratio with R15 above the roof deck. But if you go higher than R35 total t on the interior, you need to bump the exterior R as well. The Class-II vapor retarder allows you to cheat that (a lot), but it's still a far more resilient assembly if you follow (or exceed) the prescriptive ratio, keeping the roof deck temp warm enough in winter.
Thanks Dana, I really appreciate your response.
My understanding of an insulated cathedral ceiling was that any potential moisture problems should be allowed to dry to the inside. I have both a supply and return in the room, thus further reducing some of the widely-discussed issues problems with open cell between the rafter bays. In fact my biggest complaint in the winter is that the house is too dry; sometimes the humidistat (although located on the first floor) reads within the teens. With this in mind, would you still recommend a vapor barrier? If so, would faced batt insulation suffice, or would you go with un faced fiberglass or rock wool followed by the vapor barrier?
I also worry about a potential foam sandwich with the future roof deck membrane and rigid insulation on top and vapor barrier underneath.
If your humidity is in the teens it's a symptom of excessive infiltration or excessive (active or passive) ventilation rates.
Your mention of a humidistat implies that you are using an active humidification system, which is usually only necessary in a zone 4 location if you have VERY high ventilation rates.
Even if you actively dehumidify to Atacama Desert the vapor retarder is still required, both for meeting code, and overall moisture resilience. Active humidification is a common cause of indoor air quality problems due to excessive moisture and mold in walls & cathedralized ceilings.
My personal preference is for rock wool due to it's extreme fire retardency characteristics.