Standing Seam + Hot Roof. What to do under eaves?
We have a standing seam metal roof, and are planning on doing a hot roof by doing 3 inches of closed cell spray foam followed by a layer of batt insulation.
We plan to cover the eaves in T&G wood siding in some places and stucco in others. It’s my understanding in this installation method, we dont have any soffit vents under the eaves. However, I’m wondering if we also need to fill the void between the roof deck and the sofit cover with insulation, and if so what should we use? I’m worried if we leave it as an airgap that will lead to potential sweating under the eaves. But at the same time I dont want to pay for extra insulation if I don’t have to.
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If your standing seam roof is on furring strips or has any sort of air gap between it and the sheathing you would benefit from doing a vented roof, even if the attic is unvented. Matt Risinger did this on his personal build and there are YouTube videos on it. You need a ridge vent opening at the top and you use traditional perforated soffits at the bottom but they are not connected to the attic space, only the air gap under the metal roofing. A strip of perforated metal would also work if you don’t want the entire soffit to be perforated, such as the stucco finish you described.
You don't need to do anything at the eaves. Warm air rising from sun hitting the wall cladding may melt snow if you're in a place that snows, but that's more of an issue with vented roofs and soffit vents placed close to the wall, which leads to melting higher on the roof which causes ice dams.
What is your climate zone and how much fluffy insulation are you planning to add? 3" of foam is skimpy in colder climate zones for a flash-and-batt system to be safe from risk of condensation on the face of the foam. Despite sales claims, all closed-cell foam will eventually be around R-5.6/in so you will have R-17 in the foam layer.
Be sure you use a foam (spray or board) that is insect repellent treated. Some brands claim their products are treated - usually with borates/boric acid products. Check GBA/internet for ants in foam - huge problem - including for me! I used polyiso boards covered with spray polyurethane in interior rafter bays under plywood roof deck covered with asphalt shingles (interior cathedral ceiling). The polyiso boards were furred down from deck to leave 1 1/2” vent space from soffit vent to ridge vent. Roof structure was entirely dry- no moisture incursion at all. But Carpenter ants chewed into and nested in the foam. No wood structural damage that I have found because wood was dry, but ………? I now have another regular (every couple months) of maintenance of putting out carpenter ant poison (boric acid) into the soffit vents!
We are in California Climeate Zone 3. We are also in the high fire zone (WUI), which is why we are going for a non-vented roof. Also the house is almost mostly cathedral ceilings.
Our specs call for R-38 in ceilings, so was assuming R-17 foam, and then R-23 6" Rockwool batt. Goal is to leave space for recessed lighting, and get increased noise mitigation (we are on a airport flight path). If others have suggestions of best insulation make-up to achieve the best results I'm open to suggestions.
But given these details, should we just airseal under the eaves and then do a aesthetic/fire proof covering like siding or stucco and leave the cavity empty?
That's IECC climate zone 3 marine. IRC table 806.5 shows the requirements for hybrid foam/fluffy roof insulation. In your case, it's R-5. So 1" of foam would meet code requirements. https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR806.5
That assumes code-minimum overall insulation, which in the 2021 IRC is R-49. But earlier versions of the code say R-38 and also have R-5 as the foam requirement. So you're easily safe from condensation.
https://codes.iccsafe.org/content/IRC2021P1/chapter-11-re-energy-efficiency#IRC2021P1_Pt04_Ch11_SecN1102.1
You have a good reason to go with an unvented roof. My approach is to use as little foam as reasonably possible, mainly due to its climate impact. Mineral wool also has a climate impact but it's lower than foam's, and it's much better at sound attenuation, so I would use more mineral wool and less foam. For ceiling lights I like the low-profile LEDs, which even with a gimbal to allow swiveling don't take up much ceiling depth if you use their remodel-style option.
I see no reason to put anything in your eave cavity. The air inside will be the same temperature or higher than the outdoor air so no condensation will occur, the only reason you might consider insulating them.