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.
在倾斜的屋顶上，我计划将密集的包装纤维素或R23岩羊毛毛（可能是巴茨）放在the子上。这使我大约是R33减热桥。我在气候5号地区，我的城镇使用较旧的代码，上面显示R38（或500平方英尺的R30）。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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Replies
史蒂夫，
我对您的位置感到困惑，但后来我注意到，埋葬在您的故事中间，您处于气候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:“隔离阁楼膝盖的两种方法。”
2.目前尚不清楚您的1英寸厚的刚性泡沫是否将安装为the子底部和绑带之间的连续层，还是打算直接将绑带安装到afters上，然后插入狭窄的刚性泡沫之间的狭窄矩形绑带。前者的方法比后者要好得多。
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