Skylights with foam insulation over roof
So I am in the process of putting about 8 inches of foam on our roof. The house has skylights and with this we are Replacing the skylights with more energy efficient Fakro skylights. The skylights are located over cathedral ceiling made of 2x12s. Since 8 inches of foam will now be above the original sheathing along with 2×4 strapping to hold the foam/create a cold roof I am wondering what people have done in the past for placing skylights in this situation. The two options I have thought of are to box out the extra 9.5 inches with 2x4s this I think would create stronger support but will create more thermal bridging through the 2x4s. The other option would be attaching the skylight to 2×4 strapping which would have long screws attaching to the rafters/headers below. This option to me seems not as strong but would have better thermal performance. I am curious what others have done or typically do in this situation?
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I haven't done that thick of an insulation, but for thinner rigid I just go with deeper curbs. Think of it as an outie window.
The one thing with thicker roofs, it is good to angle the opening at the top and bottom, it makes the skylight feel much bigger on the inside.
I haven't done this myself (yet), but I've been thinking about it since I have a roof with 8 large skylights in it that I may add exterior foam to in the future.
You can't bring the roof up above the level of the skylight since you'll end up with water pooling around the skylight that can't drain. You need to extend some structure up through the foam so that the skylight can mount above the roof where it can be flashed correctly for proper drainage. I have thought about recessing part of the skylight frame into the top layer of rigid foam a bit with just maybe 1/2" or so of the skylight above the level of the foam, and then using a flat-roof style sealant to seal the perimeter. This would help to insulate the frame of the skylight better, but I'm not sure how well the seal would hold up over time.
The skylights are always going to be a weak spot in the roof, so you have to prioritize the weather sealing around the skylights over the insulating value of the rigid foam.
Bill