Unvented Low-Slope Roof Assembly Insulation Gap
Hello,
I’m trying to work through the details of an unvented roof assembly in climate zone 4C (Seattle). Please see that attached detail. This roof will be over a new addition to an existing mid-century modern home (no attic space). The assembly has 2×8 rafters at 24″ o.c. There are two layers of R-36 Rigid Insulation amounting to 6 inches. This leaves a 1 1/4″ gap between the roof sheathing and the top of insulation due to the depth of the rafters. Since this is unvented, what should fill the gap between joists on top of the rigid insulation?
I’ve tried looking at a few articles and understand that the sheathing is more likely to get cold on a roof assembly and can cause moisture issues, so it seems it should be some sort of air-impermeable roof insulation. I was looking in particular at “Combining Exterior Rigid Foam with Fluffy Insulation” and wondering if I just fill the gap with spray foam; however, I’m not sure if there is a more cost effective solution. Since the rigid insulation will meet code (R-36), does it matter what infill material is used? Would something more fibrous, such as rockwool still be a risk?
Thank you!
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Replies
Probably others will give you better answers to your specific questions. But my first thought is, why not just put the rigid insulation up tight to the roof deck? It would be easy enough to do. You can get wire "IS Insulation Supports" from Simpson Strongtie and probably find off the shelf stuff at a good big box store. This would hold insulation in place well enough. It would save you additional unnecessary steps.
IRC2021 R806.5.5.1.1 does seem to require that the insulation be installed up tight to the underside of the roof deck. So it's probably advisable to do that. I don't know what vapor/moisture risks would be associated with leaving a gap. Hopefully somebody more knowledgeable will chime in about that.
MidModHome,
The insulation has to be tight to the sheathing. I would do as JGSG suggests and move the foam up. It needs to be sealed at the perimeter of each sheet with spray foam or caulking to prevent moist air reaching the sheathing.
You then have the choice of using a second layer as you show, or filling the rest of the cavity with batts. If you use batts, the thickness of the foam must be at least R-15 for your climate zone.
I would hold the rigid foam against the sheathing, not the drywall, to avoid potential moisture issues. Or have EPS or GPS custom cut to 1 1/4" thickness. Or use 1 1/2" foam and add 1/4" furring to the tops of the rafters.
I would not leave it open or fill it with fluffy insulation. Any spray foam would need to be cut down after it expands, making a mess and wasting material.
I might be ok with 1" rigid foam, preferably with the gap between foam layers, not at the sheathing.
You are showing a cut and cobble roof setup. These are prone to failing as any moisture that makes its way past the rigid has no drying path. Over time the roof sheathing can be come saturated and your roof can fail.
Unfortunately in heating climates you only have two options for unvented roofs. Closed cell spray foam bellow the deck or rigid insulation above it. With either option I would go a combination of foam/fluffy to get your code required R value:
https://buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies
The rigid above is probably your lowest cost option as it is often part of membrane roof installs. If you want your overhangs to match existing thickness, what you can do is trim the bottom of the rafter overhangs the thickness of rigid. For example you can trim your 2x8 to 2x6 height and install ~2" of rigid. The 2" of rigid is good for condensation control of about ~R25 batts (so R36 assembly). You can trim the rafter tails more and install more rigid if you need higher R value.
Akos,
Martin is a bit more equivocal about Cut & Cobble: https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation