Vaulted ceiling in garage, R-39 using Rockwool and 1.5 foam board
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I have a detached shop in zone 5. I upsized the walls while framing to get R21 but not the vaulted ceiling which is 2×8…I am required R-38. I plan on using unfaced R30 Rockwool for 2×8, then placing 1.5″ foam board (R7.5) perpendicular to the rafters with 2×2 furring strips (between the foam boards/on the same plane), and finish with ceiling drywall. Is that a good solution? Should I try to fix drywall through foam board without furring strips with 3″ screws or put 1×2 furring strips over the foam board.
Replies
It all sounds good. The furring strips are unnecessary in your application, and you'll get better sealing without them (and slightly better insulation performance). Screw through the insulation right into the rafters, we do it all the time.
How will you vent the bottom of the roof deck?
I'm re-thinking my endorsement of your plan, because of potential moisture problems. I support using hot roofs in cathedral ceilings, but using rigid foam under the rock wool could be a problem, a big one. A couple of these links are promoting using smart vapor barriers which breathe, which are great, but if you follow through with your plan to use rigid XPS under the rock wool, you're creating a vapor barrier, a dumb one, trapping any moisture between the roof deck and XPS, a recipe for mold and rot. I think I would use the rock wool as you planned, but then use a rigid rock wool board below that, with a smart vapor barrier below that. (Roxul Comfortboard 80 or 110)
https://foursevenfive.com/blog/insulating-unvented-roof-assemblies-foam-free/
I posted this one not to suggest fiberglass, but to highlight the smart vapor barrier. https://foursevenfive.com/blog/yes-unvented-roof-assemblies-can-be-insulated-with-fiberglass-a-wufi-post/
Here's the 2020 IBC new code about unvented roofs, which may make you rethink your idea (which I endorsed!). https://www.constructionspecifier.com/new-ibc-rules-for-insulation-in-unvented-enclosed-roof-framing-assemblies/4/
I posted a long follow-up that somehow didn't get posted. I've changed my mind. Although I support hot roofs in general, there has to be some way for vapor to get out of the assembly. One way is venting, another is allowing the insulated area to breathe. Rigid foam under rock wool will prevent breathing back into the interior (the roof deck stops it going that way). An alternative is to add rigid rock wool (Comfortboard) under the between bay insulation, followed by a smart vapor barrier (and good air sealing). Check out Roxul's Comfortboard as one example, or use Gutex wood fiber board insulation. These guys have smart vapor barriers and insulation choices. https://foursevenfive.com/
I am confused about the venting requirement. The roof section is the lower sides of a monitor style shop. I had planned to vent to the wall then to the upper attic. In the research I have done the R-38 is allowed for unvented vaulted ceilings and R-49 is required for vented (can't find that reference now).
Just because you don't have a vent doesn't mean you should allow warm moist air to get into the insulation between the impermeable roof and the impermeable rigid foam. I guess in a perfect world if it's very dry to begin with and the insulated section is perfectly 100% sealed against warm moist air it wouldn't cause a problem, but otherwise when the warm moist air finds its way into the insulation and the cold roof under the sheathing, even through the tiniest crack, hits that warm moist air and condenses you've got rot and mold issues. The rigid foam under the rock wool is impermeable and there's no way for the moisture to come out. Using a smart vapor retarder and very good foam sealing keeps the space dry, and any moisture that may get in has a path out.
I have read some of the Green Building Advisors articles on smart vapor retarders. The assumption for the article is high humidity inside during the winters and low humidity inside/high humidity outside during the summers. I live in a high desert area so winter with 70% humidity and 30-degree temps, I would want a vapor barrier inside/under drywall. The summer is different than the scenario in the SVR article...we have humidity in the 30s and temps in the upper 80s...no need for vapor from the outside. Also this is a shop/garage so heating is set in the 40s during the winter and AC in the summer is only used when occupied on weekends for a project. What is the difference in humidity in a garage space with minimal temperature difference and no artificial sources (plumbing/cooking)?
Ah, well, new information. BTW, in non-desert climates in the US, from the humid southeast to frigid northland, winter indoor air has much lower humidity in the winter and much higher humidity in the summer. Of course a desert environment doesn't. Smart vapor retarders work in all the situations mentioned, in your case they keep moist air from getting into the insulation in your more humid winter and then open up to let any moisture out of the insulation in the drier summers. It sounds like your insulation is going to be helping keep your shop cool too. No showers, no cooking means less added humidity, but the environment still adds or takes away humidity.
Thanks for the conversation. I think I will fur out the 2x8s with a 1x2. Install the R30 (7.25") rockwool, then R7.5 foam board over the bottom. This will give me 1" of venting above the rockwool. There are vents in soffit and top of roof into wall space to upper attic.
Let us know how it goes!