Venting on top of ridgid foam
DCC
| Posted in General Questions on
I’m planning on putting ridgid foam over roof sheathing and I’m wonder if I could screw down 2×4 vertically on trusses over foam and then nail on my roof deck sheathing to the 2×4 for venting for steel roof. Most such roof have sheathing on top of foam but is it really necessary is what im wondering…
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What is your rafter spacing? Most steel roof manufacturers require 16" o.c. If you have 24" o.c. you will either have to do two layers of strapping, or lay the strapping at a diagonal angle. See some of Matt Risinger's metal roofing videos where he does exactly this. He also has done a full secondary layer of sheathing as well.
24" spacing with 5/8 plywood deck for standing seam.
It doesn't hurt to add a vent channel, but isn't usually necassary, either. What type of attic are you going to have "under" that rigid foam? I assume some type of enclosed attic that is inside the building envelope? Will you be using spray foam in the assembly? If you use closed cell spray foam in the assembly immediately under the rigid foam, then I would absolutely skip the vent channels on top of the foam. If you have something permeable, then I think having some venting helps you. If the entire roof assembly is just a sandwich of overlapping sheets of rigid foam (be sure to overlap the seams), then venting isn't really necassary.
Bill
The roof will be a rigid foam sandwich with conditioned attic. I realize that it's not required but don't you think there are benefits in venting the roof? Is there no issue with sandwiching the exterior sheathing with 2 impermeable layers? I figured if I would add vent channel under exterior sheathing with 2x4's it would keep condensation from building up and it would also allow it to dry out if it would ever condensate. I'm just outside of zone 6 and I feel it would also help with ice damming. Most over foam venting I've studied have another layer of sheathing under the strapping, my question is why do I need it? Can't I just screw down strapping on top of EPS foam and then my Plywood deck for metal roof on top of strapping?
I don't see it as a problem here, because your rigid foam layer can't really take on any significant amount of moisture. This is a fairly common assembly commercially too, where you have the roof deck that has polyiso piled on top of it, then some type of membrane roof (EPM or TPO, usually) over that. I'm not aware of that type of assembly being an issue in any climate zones. Basically moisture can't really get into your assembly, and has no where to accumulate, so you don't end up having condensation problems down the road.
That said, I don't see a problem adding the vent channel, it's just an extra construction step and some additional materials. I see no issue with a stackup that is foam/2x4 + vent channels/exterior sheathing, either -- i.e. no sheathing/2x4+vent/sheathing. One layer under the roofing material should be fine. Either way, you'll end up having to get long fasteners through all that foam into the structural rafters underneath.
Bill
Thanks for info. One more question. I'm having a hard time finding what tape I can use EPS seams. any recommendations?
I would use one of the poly seaming tapes, that stuff should stick pretty well. Give it a test to be safe: you want a tape that takes out chunks of the EPS when you try to pull it off. Flashing tape is another option that should work.
Bill
DCC,
Is the strapping under the sheathing necessary? No.
Does it yield a better performing and more resilient roof? Yes.
Deleted
Pretty "cool" product ...
Cool-Vent is a venting composite insulation board that consists of a 4′ X 8′ panel of closed cell rigid polyisocyanurate, a layer of solid wood spacers, creating a standard 1″ air space and a top layer of APA/TECO rated OSB or plywood
https://www.hunterpanels.com/products/roof-products/cool-vent/
Here is how I did it. Horizontal purlins intermixed with foam were fastened to the trusses. Then the second layer is a thicker piece of foam. 2x4's on edge were placed on top of the second layer and fastened to the purlins. THis saves you from having to hit a truss from above. Two layers of foam saves you from moisture issues that Joe Lstiburek highlighted with SIPS. The purlins should also allow you to do 2x's on edge every 16 inches. The 2x4's on edge yield a terrific ventilation channel for decking under the metal roofing.
Rudy,
That's looks like a good, forgiving system.