Small Cottage, Hoping for a Simple Cathedral Ceiling Approach
We are planning out a small cottage (under 500sf) in our backyard that we plan to live in. We Live in Sacramento, California – Climate Zone 3. Our current design is a very simple 18×26 – gable roof, rafters running in the 18’ direction, maybe a small dormer, with a cathedral ceiling and a small loft in the back third of the house. We are hoping to do most of the work ourselves and are trying to keep the build as simple and inexpensive, but tried-and-true as possible.
I’m trying to wade through the complexity and find a simple tried-and-true approach to building the cathedral ceiling. I would like to stick frame the roof with 2x material (it seems 2x12s are probably necessary) and construct a vented cathedral ceiling. Basic plan: asphalt shingles, #30 asphalt felt, plywood or OSB sheathing, 1 ½ vent from site-built vent baffles, high density “cathederal ceiling” R38 batts, drywall air barrier.
-Do I have enough room in a 2×12 rafter bay to get to R38? What if I make the vent baffles with rigid foam?
-If I have a small dormer, does this completely destroy the viability of this approach?
-If I have rafter ties cutting through the drywall, does this completely destroy the viability of this approach?
-Is there a strong advantage to using plywood instead of osb for the sheathing?
-I would like to use batt insulation because we would like to do this ourselves. Is there a strong reason to use rockwool or cellulose batts instead of fiberglass, which seems to be the only thing people use around here.
I realize I’m not at the forefront of energy efficient building with this, but I’m just trying to find a safe, good-enough assembly that we are capable of building. It seems to me that the margin of error on cathedral ceilings is razor thin. The only reason that we are taking this approach is that it allows us a sleeping loft and keeps the footprint small.
If it becomes clear that a cathedral ceiling is a risky assembly, I may simply go with the perhaps more tried-and-true approach of an old-fashioned vented attic.
I would greatly appreciate any thoughts you all have on this.
-John
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Replies
2x12 is enough for R38 HD batts, but you'll only get 1/2" or so of vent space. This is more than enough venting for such a small structure. A big safer approach is to go with mineral wool as it is 1" thinner and gives you the full 1.5" vent space. If you are careful with the install, with mineral wool you can skip the vent baffle.
Shed dormers can be easily vented and are easier to build. I would have no issues with using one of those. Treat the roof of the shed dormer as a standard cathedral ceiling (soffit to ridge venting) and the walls of it as standard walls. Dog house is more of an issue, but you are not in cold enough climate to need to worry too much about a bit of unveted roof section.
Rafter ties are very hard to seal, so try to minimize the number you need. Talk to your structural guy, they can sometimes spec a beefier top plate for your walls and reduce the number of collar ties you need.
Install solid blocking on all sides of the collar tie for something solid to sit the drywall onto. Put backer rod + caulk between the drywall and the tie. Don't skip on the backer rod, caulk by itself will not hold up. Gluing the drywall to the blocking also doesn't hurt.
You are in very forgiving climate. With a building that won't be occupied 24/7, the cathedral ceiling as you propose is not an issue.