Wet sheathing/wood between metal roof and exterior rigid foam.
Hi all,
I’m having a cedar shake roof replaced with metal standing seam and am concerned about design and moisture issues.
I am in zone 6, east of the cascades so snowy winters and hot dry summers. The house has a 12/12 cathedral roof with exposed beams with the roof being 4x8s on 48, car decking, 3.5in rigid foam, then skip sheathing and the current shake shingles.
We are getting rain and soon snow regularly and I am concerned wet sheathing will not be able to dry in either direction with peel and stick underlayment on the outside and the rigid foam to the inside.
Additionally, I had wanted to add another 3.5-4in rigid foam to the exterior to get closer to code but my contractor is unsure how to do this without putting nails through the car decking. Weve discussed removing all the current foam, laying over_2x4 on the outside of the decking to drive nails into to secure the sheathing outside 7in rigid foam but I’m worried all the current foam will be damaged in doing that.
Alternately we thought of putting a layer of plywood directly on the skip sheathing, then adding rigid foam, then the final exterior sheathing, but we have the same issue of wood sandwiched between impermeable layers.
Any advice is greatly appreciated.
Robie
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Replies
car decking I assume you are talking about T&G 2x6?
If the decking continues outside[like my house] look at sealing every joint as it passes over the wall.
Secure foam with the correct length screws, right into the decking, but not through. Never nails.
I would remove the skip sheathing as any air leakage would negate additional insulation value above.
I can see two ways of doing this.
You can fill in the gaps of the skip sheathing around the edges, install the new rigid and tape the edges together to air seal. As long as outside air can't flow through the skip sheathing gaps, it won't do anything. Having it sandwiched between layers of rigid doesn't matter.
You would still have to air seal your deck as Gusfhb above said. This is generally a much bigger source of heat loss, all those little gaps times many dozens of boards adds up to a big hole. It also increases ice dam issues. It won't fully air seal the deck as you can have a lot of air flow up through the T&G and than along the seams.
The other option is pull the rigid/skip. Air seal the deck from the top by drilling holes over your top plate and injecting canned foam between the boards. Install a peel and stick membrane over the whole roof to form your main air barrier. This peel and stick should be either butyl or acrylic based (VP100 is decent budget option although not meant for roofs) as some of the regular ice and water stuff is modified Bitumen (aka fancy tar) and can smell. The mod bit can also react with sap in the T&G and ooze.
Over this install the new layers of rigid. With continuous exterior rigid, most codes allow U factor based compliance which tends to be R39 or R42, so not as bad as R60. There is also not all that much heating loss once you go above R30 for a roof, so the cost of the extra rigid has very long ROI. You should be able to reuse most of your existing rigid.
For the top roof deck, get screws that don't poke through the T&G. Roofing insulation deck screws (used for flat roofs, available from most commercial roofing supplies) are a good option as they can be had in many lengths. You might have to adjust the thickness of the rigid a bit to match a standard screw length.
The 2nd option is more labor but gets you a solid air tight roof. I've attached a picture how the T&G should be sealed at the wall plates.
Akos has good suggestions. A couple of other points. Vp100 is definitely not waterproof as a roof underlayment but works well in the location suggested, just understand if you get rain in meantime it may permeate through. Pro clima self adhering may be a better option but is a little more expensive (not a huge amount per sf).
Sip screws are often a good option for larger foam build ups
A vapor diffusion port on exterior sheathing could be good insurance against your fears of wet sheathing. BSC has use this to remediate rotten sips with success.
Try to hit your 4x8 with longer fasteners if using a top sheathing
You may be able to fasten your metal without a second deck above, some standing seam has clips that you can use directly on foam.