Fiberglass batts in a cathedral ceiling?
I’ve been reading the GBA web site and have subscribed to the Fine Homebuilding magazine for several years but this is my first venture into the “green expert” arena. I could ask much but will try to narrow my questions to the two below.
My wife and I are building a small, hopefully energy efficient week-end/retirement home located approximately 50 miles east of Erie PA. Though the maps indicate Zone 5, having grown up there and experienced the winter lake effects, I’m considering it Zone 6.
The home is a rectangular ranch style (first floor 1360 SF) with an attached garage and a full basement under the home portion. It’s axis is situated north/south (due to the awesome view to the east). An old plantation of spruce and red pine creates somewhat of a buffer to the immediate south and southeast.
My wall assembly consists of 2″ of XPS insulation with taped seams over 3/4″ OSB sheathing with 2×6 studs which will be filled with R-21 high density fiberglass batts. Horizontal cement board siding will be attached to 1×4 furring strips which are fastened through the foam to the studs. A rolled screen insect barrier will be attached along the bottom edge. Tyvek house wrap with taped seams was placed over the foam.
The ceiling of the eastern half of the dwelling is vaulted and will be drywalled. The western half will be traditional unconditioned attic space with either blown-in insulation or double layers of unfaced R-30 fiberglass batts. The roof sheathing is 3/4″ OSB with sealed roofing paper and asphalt shingles.
The basement will be conditioned space. We are planning on gluing 1-2″ rigid foam to the concrete block, then furring the perimeter with 2×6’s and filling with R-21 prior to drywalling. Do you feel this is a reasonable approach?
My main question is centered around the best method(s) for insulating the vaulted ceiling. Our plans are hopefully commensurate with what I have read on your many articles and comments on the subject. We plan to install commercially available plastic baffles between the 2 x 12 ceiling joists from the eaves to the ridge vent (roof is 5/12 pitch) which will create approximately 1 1/2″ air channel. We have sistered 2 x 8’s to the 2 x 12’s creating 19″ for insulating space to accommodate one layer of unfaced R-30 fiberglass and a second layer of faced R-30 with the facing attached to the bottom of the joists. Assuming drywall is not completely airtight and we use latex paint, would the proposed assembly allow sufficient drying to the underside of the roof deck, or do you feel we will have a condensation problem on the fiberglass insulation and/or the underside of the baffle? Any advice would be most appreciated!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part