Cut and cobble vented cathedral ceiling
We are finishing our attic on a tight budget. We just had new roofing installed 2 years ago and unfortunately did not add continuous exterior insulation at that time since we had an insulated attic (at the time). The floor-to-ceiling height is so low that every inch counts, so we are looking at cut and cobble to maximize R-value within a very small space. House is 1927 drafty everywhere, so unvented roof seems impossible to retrofit appropriately without spending a lot of time and money. We are in Portland, Oregon (moderate climate). Here's the proposed roof/ceiling assembly going from exterior to interior:
- Existing roof: composite shingles, building paper, roof decking (1x t&g), 2x4 rafters.
- Add new ridge vent and 3.5" rafter cavity vent space with insulation: 1" vent spacers, 1 layer 1.5" rigid insulation, 2nd layer 1" rigid insulation w/ staggered joints.
- Then, to get additional R-value (even though we will still be below code): add 2x2 furring strips crosswise @ 24"oc for drywall fastening, and infill with an additional layer 1.5" continuous rigid insulation over the inside face of studs. Tape all seams - or use drywall as air barrier.
- Finish layer: 1/2" gypsum board.
For the walls separating finished from unfinished attic, we are planning 2x4 studs, 3.5" mineral wool insulation, gypsum board interior finish as air barrier (unfinished on attic side).
1. I am confused about the air barrier. Existing house ceiling is far from air tight between 1st floor finished spaces and existing unfinished attic. Does it make sense to try to make the new finished walls & ceiling of the attic air-tight, even though there still be air leaks between the existing finished and existing-to-remain attic spaces?
2. We are planning to use R-Tech EPS foam in the roof for better drying, but would XPS (Foamular) work instead? It has a better R-value.
3. What kind of tape would be best to use for sealing the insulation/wood seams?
4. Does it make sense to add the additional layer of insulation inside the roof rafters, or will this mess up the dewpoint and create moisture problems?
5. Any other comments on the proposed assemblies?
Thanks for your help!
Posted Wed, 06/18/2014 - 12:58
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Since drywall has a permeance of 0.02 liter/sec - m at 75 PA is ever possible to have sheathing dry to the interior of the home?