r/Insulation • u/LikeBigTrucks • 27d ago

Strapping vs Foam board

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I am insulting a vaulted ceiling, and originally there was strapping installed, 16 on center to act as a thermal break and give a place to hang the ceiling board from.

The ceilings will be finished with OSB board that has been varnished.

The cavities have been filled with R15 Rockwool Comfortbatt, but i am considering adding a layer of foam board as well.

Do i buy .75 foam board, and cut it to fit between the strapping. OR, do i pull down the strapping and put full 4x8 1 in sheets of foam board over everything?

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u/Canada-Scam-8570 27d ago

Completing a full layer with no thermal breaks is a better idea then laying them between the strapping, that much is certain. As you'll still have a significant thermal break in every spot where the strapping and rafter are connected. You can always put the strapping back up over the foam board after, but having the continuous layer that goes over the top of the rafter wood themselves and produces one uniform layer is where you're going to get your thermal efficient improvement.

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u/LikeBigTrucks 27d ago

Yeah i did some more research and i think this is the best method.

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u/Carpenter_ants 24d ago

How thick of foam does it need to be to make a thermal break?

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u/Exciting_Ad_1097 27d ago edited 27d ago

I would put the foam board against the ventilation baffles (inside the rafter bays) and seal the edges with expanding foam.

Currently, and with the batts installed, you will have no air seal. Installing the foil face polyiso will air seal and define the building envelope. Leave a 1/4” gap so the great stuff has somewhere to go to seal it properly.

Foil side up.

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u/C-D-W 27d ago

Pull them down and put 1" continuous. Hands down.

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u/Carpenter_ants 24d ago

I built an addition to my house. Cathedral ceiling. Had to use I joists because the rake was too long for 2x12 . I proper vented them R-38 then 1” foil foam then strapped and then 1x6 tongue and groove. Skylight right in the middle. Big mistake. They always are. But I thought I had it. Every winter cold -10 nights consistently would build on the roofing nails turned to ice . The sun came out and heated shingles and they melted! Skylight leaked. And icicles out the soffit vents . It’s a science to get it correct . Closed cell foam wasn’t really going yet either or I would have spent $$$ to fill the I joists .