Unvented roof assembly
Martin wrote an article in 2016 and gave guidelines for installing rigid foam on the exterior of the roof sheathing and combing that with “fluffy” material under the sheathing. His table illustrates that 61% of the insulation should come from rigid foam and that increasing the fluffy stuff beyond the recommended 39% is risky business. I have access to 2″ recycled polyiso and the use of it does not concern me. The cost of this material is not an issue. What does concern me is finding fasteners that will need to go thru a 2×4 on the flat, sheathing, 8″ of polyiso, more sheathing and finally into the truss. I spoke with a well respected fastener company here in Michigan and he said the availability of long fasteners is very limited. BTW, I’m in climate zone 7.
So my questions is: Would it be acceptable to put 6″ of the polyiso above the sheathing and another 4″ below the sheathing that is connected to the trusses and approximately 8″ of fluffy stuff below that? Or would it be acceptable to put a 2″ layer of polyiso under the drywall? I would prefer putting it against the sheathing.
Thank you in advance for your advice.
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Replies
John,
That's a pretty difficult roof assembly to build. For new construction is there a reason you are going that route, rather than a more conventional vented assembly?
Your proposed alternate is even more complex involving a mixture of exterior and cut and cobble rigid insulation. By moving part of the rigid insulation to below the sheathing it also defeats the purpose of it which is to keep the sheathing dry. If you are set on an unvented assembly I'd stick with your first design.
Part of what makes it difficult is the depth of the rigid insulation you are proposing. A code compliant R-49 roof should only need between 5 and 6 inches of Polyiso.
- Is a second layer of sheathing under your 2"x4"s necessary? It add a huge level of complexity to building the roof.
- Are you sure it isn't just that fastener company that is experiencing shortages? My lumberyard has the full range of GRK RSS structural screws in stock as it appears does Amazon.
Thank you for your reply. I realize this is a complex roofing system and we only got to this point due to a couple of things. First, the site where I'm building had a house that I tore down piece by piece. The bones of the house were useless for reuse as it was hemlock. However, due to extensive remodeling, it also had an ample amount of 2x4 and 2x6 dimensional lumber. Some suitable for walls and all of it suitable for rain screen materials for both walls and roof venting. Secondly, my 2" polyiso is in good shape and I'm only paying $11 per sheet. The house has a small footprint, 24' x 36' with a simple roof lines and we wanted a vaulted ceiling. We plan on energy heal scissor trusses with a 6/12 exterior and 3 or 4/12 interior. I had read how potentially difficult this is to vent properly so I thought this might be the way to go. But I do want to get things right.
Thank you for your suggestions and advice on fasteners. I will consider not using the top layer of OSB which is what I was looking into initially, but with the prices coming back down to earth I'm back to considering a second layer of OSB. Most illustrations of the unvented roofs have the second layer which is was dreading when prices were through the roof.
Keep it simple.
Your interior ceiling slope is low enough to loose fill the entire attic space. The only thing you need to take care with is installing baffles on the low side of the attic to keep the insulation clear of the the vent space.
Even at $11/sheet, the plyiso roof will be more expensive than a simple loose fill roof and won't perform any better. It also is way more complicated to build. There is no reason to go down that road with your build.
Use that cheap polyiso and wrap your walls with it.
Thanks, I am staggering two layers on the walls of the 2 x 6 framed walls and one layer over the above grade portion of the ICF walls, which is why I wanted a high r value roof system. But I’ve received enough warning about the complexity of the roof design that I’ve changed my mind and will keep it simple.
I very much appreciate the feedback and all that GBA provides.
John,
Fair en0ugh. If you are g0ing with that assembly I'd still suggest leaving off the second layer of sheathing which really adds nothing while making installing the roof a lot more difficult.
Another area that needs a lot of thought with that much above sheathing insulation is the eaves, which have no solid backing for facias and gutters. The thickness also poses aesthetic challenges at the overhangs, which are usually a lot shallower.
Thanks Malcom, please read my other reply noting that I am heeding the warnings about the complexity of the the roof system and I will simplify. Your input was much appreciated.
A few more questions: Could I install two layers of the polyiso with staggered joints on the interior ceiling? Then put 2 x 4’s on the flat perpendicular to the trusses and use that space for an electrical chase and for compact recessed lighting. Then apply the drywall. I would attempt to make the polyiso as air tight as possible.
I have a lot of polyiso and framing lumber that I would like to put to good use and I think creating a chase would be worthwhile. Also the work could be done inside. I know they use taped OSB for this purpose. My polyiso is covered with a fiberglass material, not foil.
Yes, you can use the polyiso on the interior side with a vented assembly (it can be risky with an unvented assembly). Putting the polyiso on the interior helps with thermal bridging too. You need a thermal barrier over the polyiso in this application though (which usually means 1/2" drywall).
Bill
The R value above the sheathing should have the right ratio to total R value.