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Roof insulation for unvented rafter space

rawlinson | Posted in GBA Pro Help on

I was granted a building permit for a workshop, with metal roof / 1 1/2″ mineral wool board /  WPB / plywood sheathing / 2×8 joists with Rockwool batts/plywood ceiling; the intent was to avoid eave vents in a rural area prone to wildfire and to enhance fire resistance with the layer of mineral wool, while complying with IBC Section 1202.3.5. Come time to build, contractor claims no mineral wool board will be available due to industry covid shutdown, plus it will be costly. Options are to install spray foam insul on underside of sheathing and fill remainder of cavity with batt OR replace exterior mineral wool board with rigid foam OR just try and get by with 7 1/4″ of Rockwool batt. Workshop is in Zone 3 coastal California, is only sporadically occupied, has no livestock, shower, or other moisture producing activity, and will not have more than occasional portable heaters; no insulation is required but was preferred primarily for protection from heat. What alternative assembly will best serve? I have done a prior project using  interior application of spray foam, but was uneasy with the gaps in interface between the irregular surfaced foam and  batts. I look forward to your suggestions. Brian R

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  1. sunrisehomestead | | #1

    Where about in CA?

    I've typically used rigid polyiso insulation for above deck, unvented roof assemblies in northern California. This is usually when the interior ceiling is exposed with rafters and t+g decking.

    Is the "workshop" just a traditionally wood framed building? Guessing walls will be insulated as well?

    Floor? or just concrete slab floor within?

  2. rawlinson | | #2

    Sunrisehomestead: location is Sonoma County about 12 miles from coast, gets some marine fog and winds up the river valley. I preferred rock wool board instead of foam because it may offer protection in a grassfire, but I'm hearing there won't be any available until fall. Yes the building is framed like a house, plywood shear walls on insulated cavity walls, but metal roof and siding, with a slab floor(required by local code for classification as an agricultural building). I had planned on continuous insul on walls as well, but it isn't mandated in Zone 3.

  3. Kevin_Henry | | #3

    We have a similar roof in a similar region (Monterey County).

    We also wanted to use Rockwool as exterior roof insulation for fire resistance. However, it's not a common assembly, at least where we are. Neither the builder nor the roofer had experience with it, and if I recall correctly Rockwool does not promote (or explain how to use) their products for that application. (There is a This Old House segment about it that I passed along to the roofers, but understandably that wasn't sufficient for them to feel comfortable with it.)

    Instead we used wood fiberboard (Gutex). It's not as fire resistant, but we felt comfortable with that because the exterior insulation in an unvented metal roof assembly with no purlins (i.e. no air gap) shouldn't be particularly vulnerable to wildfire.

    By contrast, we did use Rockwool for the exterior wall insulation since that's a common assembly and one that's potentially more vulnerable to wildfire due to the rainscreen gap.

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