Vented attic with sloped roof insulation: Vapor retarder question
I am finishing a bonus room over the garage. 8/12 pitch simple roof with ridge and eave vents, 2X10 16″ O/C. I considered bringing the entire roof into conditioned space with spray foam (unvented roof), but that’s too expensive for me even with a flash and batt method.
My plan is to keep the vented roof and install a 1.5″ vent channel using furring and probably 2″ rigid foam as a vent channel (probably XPS but if I can find some reclaimed polyiso at least 2″, faced or unfaced, I’ll use that). I will airseal the heck out of that from kneewall to ridge. Kneewalls will be insulated as an outside wall with foam on the back detailed as an air barrier.
On the sloped roof, I plan on putting either dense pack cellulose or R23 rock wool batts (probably the batts) which brings me to the underside of the rafters. This gets me to about R33 minus thermal bridging. I’m in climate zone 5 and my town uses an older code that says R38 (or R30 if under 500 sq ft). R30 will definitely not cut it for me so I’m shooting for at least R38 so I plan on placing strapping perpendicular to the rafters and covering the underside of the rafters with 1″ of foam to cut the thermal bridging a bit and bring up the overall R value. I can’t really strap it out any more because I’m facing ceiling height issues and that would reduce my ceiling height.
My question concerns the 1″ of foam and the rock wool batts. I assume that if I use the 1″ of foam, I will used unfaced rock wool batts. I’m thinking I should use a more permeable foam for the 1″ covering the rafters because with 2″ of foam sealed as an air barrier at the vent channel it will not be very permeable whether it’s XPS or faced/unfaced polyiso. I’ve read here that I don’t want to trap air permeable insulation between two layers of impermeable or low-permeable materials. Would 1″ of XPS be OK under the rafters? I will have drywall as the finished ceiling. If I’m looking at this correctly, I want it to have some drying capability to the interior if I’m using a thick piece of foam as the vent channel.
Thanks….Steve
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Steve,
I was confused about your location, but then I noticed, buried in the middle of your story, that you are in Climate Zone 5.
In a cold climate zone like yours, you don't have to worry about the vapor permeance of the foam on the interior side of your assembly. (That's not the side of the assembly where moisture tends to accumulate.) Many builders for decades have been installing interior vapor barriers in your climate zone, so you can stop worrying about the vapor permeance of the 1-inch-thick foam.
There are at least three reasons why you don't have to worry about the exterior rigid foam causing a "sandwich" problem: (1) the existence of a ventilation channel reduces risk; (2) the parts of the rafters that extend beyond the sandwich into the ventilation bays help redistribute moisture; and (3) the fact that you have paid close attention to airtightness reduces risk.
Two areas you should pay close attention to:
1. You didn't mention whether you will install blocking in the joist bays under the bottom plates of your kneewalls. This blocking is essential -- it is part of your air barrier -- and each piece of blocking needs to be installed in an airtight manner, with caulk or spray foam at the perimeter of each piece. More information here: “Two Ways to Insulate Attic Kneewalls.”
2. It's unclear whether your 1-inch-thick rigid foam will be installed as a continuous layer between the bottom of your rafters and the strapping or whether you intend to install the strapping directly to the rafters and then insert narrow rectangles of rigid foam between the strapping pieces. The former method is vastly better than the latter.
Thanks, Martin. I will be dense packing cellulose in the joist bays underneath the entire bonus room/garage ceiling. It will go from sill to sill. The joists are covered by 5/8" sheetrock on the garage side and subfloor on the bonus room floor, so we'll drill some holes from the subfloor side and pump it full of cellulose. I would think that would negate the need for blocking since it's all insulated.
Definitely will put the strapping over the 1" of foam. Otherwise, that'd be too much cutting and foam sealing for me.
Thanks for putting me at ease