Insulating Vented Roof with Closed-Cell Spray Foam and Rigid Foam Board
alexza | Posted in General Questions on
I’m turning my attic into a conditioned space by spray foaming the roof underside. Some background:
location – south Florida
roof – cedar shake on tar paper w batons and no plywood, no gables all hips, 2×4 trusses
roof venting – excessive soffit vents but no ridge vent
existing floor insulation – cellulose
Since my shake roof needs to dry from both sides, I’ll leave the roof vented. I’m weighing two options:
Option 1 – Cut and cobble .75″ Rmax Polyiso between the rafters, reflective side out, and leave a 2″ channel for air movement. This channel would extend up and over to the other side of the roof to ensure the free flow of air. Then spray 2.5″ of closed-cell, giving me ~ R22. The spray foam would attach to .75″ of the exposed rafter (3.5″ rafter – 2″ air gap – .75″ foam = .75″).
Option 2 – It’s much less work running the foam board across the rafters, leaving a 3.5″ air cavity. I have the headroom, so I would still spray the 2.5″ onto the board. In this case, I’d run strapping 2.5″ off the board to provide an additional surface to adhere against and provide a reference depth for the sprayer. Same R-value.
Question – closed-cell provides the additional benefit of structural rigidity. I’d expect Option 1 to provide more rigidity than Option 2, but is it enough for the extra effort? For example, would it be enough to help in a hurricane?
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I don't see the spray foam contributing any structural rigidity if it's not on the sheathing.
Have you thought about using just board foam for the whole depth? I don't see this as a good application for spray foam. It needs to have a fire-resistant coating, either drywall or intumescent paint. You can get polyiso foam board that is fire-rated. If you ever want to finish the space the spray foam will be a nightmare, whereas with board foam you can just run furring strips over it and hang drywall. Polyiso is about half the cost of spray foam, and about a third the cost if you go with more environmentally friendly HFO. If you do two layers and stagger and tape the seams the polyiso should be plenty airtight.
Also check out this FHB tip: https://www.finehomebuilding.com/project-guides/roofing/rigid-foam-vent-baffles
I agree with DC -- spray foam isn't really of any structural benefit here. I would also consider using all rigid foam and no spray foam too. There is no reason to not install the rigid foam under the rafters, and you'd probably find this would require much less labor to do. Use thicker rigid foam, and install several layers with seams staggered to get up to the final R value you need.
Thanks for the replies. I did consider all board and no foam. Although I'll be selling in a few years and not sure how an inspector would react to seeing all foam board - ie too DIY vs spray foam.
As long as you meet all the requirements for thermal and/or ignition barriers, you should be fine. Spray foam has similar issues when left exposed in accessible spaces. Rigid foam board shouldn't be an issue for inspectors -- it's used all the time commercially, not just by DIY people. I'd use polyiso here, and consider Dow Thermax (or similar) for the innermost layer of the assembly if you need an ignition barrier.
I'd consider well-detailed foam board more professional that spray.
Rigid foam will certainly have a cleaner finished appearance -- especially if you use the white Dow Thermax and tape it with their matching white tape.