Continuous Rigid Insulation Between Roof Sheathing and Structural Support
I have been pondering why continuous insulation between roof sheathing and rafters is not often studied, if ever used. I know it isn’t specified in code.
I’m trying to understand the technical reasons why this is so. For the sake of discussion, assume a layer of 1″ XPS with 25 psi ratings. I can understand why very thick layers would be a bad idea.
In terms of uplift, even high wind speeds only generate pressure differentials of 0.5 psi or so, or that’s what the literature I’ve found states. With ring shank fasteners or structural screws, that can easily be secured.
Assuming the fasteners aren’t under or overdriven, the shear loads seem that they can be achieved even with the foam. I’m envisioning the wide flanges of I joists provide adequate backing, and even provide for improved fastening schedules compared to 2X lumber.
In terms of insulation, it provides a thermal break between the sheathing and rafters, without requiring the additional cost of a second layer of plywood or OSB, and can serve as a vapor barrier if taped. It could also be used for backing for open cell foam in many zones, further reducing costs. It also reduceces the need for (structurally) oversized rafters, just because space for insulation is needed.
In short, is it just not practical to construct and therefore avoided, or is there a physical reason that I’m not considering that would make this unsuitable for high wind loads? Is it for fire safety? Pest control? Just trying to understand more.
Thanks,
Kyle
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Replies
With the insulation on the exterior of the sheathing, the sheathing stays warmer, and warmer generally means dryer. That's the big plus I can think of for keeping it on the exterior. You also don't have to worry about shear or uplift forces as you mentioned, because you're not deviating from the tables when building with the insulation on the exterior.
I wouldn't expect fire safety to be a concern since there will usually be something else between the rafters anyway that would provide an extra barrier, but I suppose it could be considered an increased risk in some cases with the insulation under the sheathing.
Bill
Thanks for the input Bill.
This line of though is limited to shingles and somewhat metal, but don't we always eventually get to a layer of sheathing that's outside the insulation? Even if it's the furring strips for standing seam / cedar tiles / slate. They have to be nailed to something, right? As far as I know, no shingle manufacturer recommends nailing them through insulation.
So what exactly is the purpose of the first layer of sheathing that's directly on the rafters/trusses, if we're going to have another that could serve as well as the structural layer?
Sheathing applied directly to the framing members is a structural diaphragm ( typically modeled as a deep beam). The diaphragm is used to tie all the walls together and spread the forces around to the inplane shearwalls, then the foundation. Inserting a foam layer between the framing and sheathing will likely introduce excessive deflection in the diaphragm (due to nail bending). Excessive deflection will result in damage to the building in high winds or earthquakes. Zip has a code report on how this works in shearwalls ( lots of nails 3"oc to get the strength of 6"oc nailing for a normal wall)
In shearwalls the code tables and tested designs have a category of walls that have a gypsum layer between the framing and sheathing , they have a reduced load capacity. I have not seen this for diaphragms.
A SIP panel can be used for a roof diaphragm.
What you are asking for would need to be testes for load capacity and deflection for design values. Deflection likely the main problem.
Tim,
I agree. I am certain the loads would need derating compared to the structural diaphram without the insulation, as you say. I guess my question is, by how much? and is it still good enough to be useful? Not asking you directly of course, just thinking out loud. Thanks for mentioning the Zip report. I recall reading it at one point, but I'll have to brush back up on it.