Renovating a cottage. Porch was turned into a bedroom years ago. When it was a porch it had exposed rafters with beadboard roof sheathing. Asphalt shingles. While a bedroom it had an acoustical tile, dropped ceiling. Now they have asked to demo the dropped ceiling and sheetrock it as a cathedral/ 3 sided/hipped ceiling.
No problem, I say, thinking all I need to do is install the pink foam channels in each rafter bay, apply fiberglass insulation and then rock. I would also take off the hip shingles along the two hip rafters, saw in a kerf for venting, install vent material/ fabric and then re-shingle the hips. I would also cut in soffit vents for each rafter bay.
But after thinking about it for some time I realize that the roof sheathing is tight to the hip rafters and any kerf I saw in will have no effect. How do I vent the roof?
Is my only option to use spray on foam insulation? Cha-Ching! for such a small area. 10′ x 17′.
Can anyone advise?
F.
Replies
Why not cut the shingles and sheathing about 2 inches each side of the ridge line. That's a 4 inch wide cut, less the inch and a half ridge, leaves about 2 and a half inch vent.
Whenever you are asked if you can do a job, tell'em "Certainly, I can!" Then get busy and find out how to do it. T. Roosevelt
The problem with that is then the beadboard would be unsupported for the last bay between the hip rafter and the reg. rafters. It that ok. I am concerned that over time with the heating and cooling of the roof the beadboard will move wothout that fastening.
F
I think that you are right to be concerned about the support for the sheathing. We have put in vented hips in roofs sheathed with plywood and they have worked quite well but I would hesitate with board sheathing. I would suggest that you use dry pack cellulose insulation and fill the entire bays. This will function in a like manner to the foam and at a fraction of the cost, though with a little less insulating value.
Since the area will be open from the inside while you're working on it, it should be a simple matter to install purlins just south of the vent opening in order to support the sheathing.
Won't purlins block the air flow once the pink foam "pans"/ channels are installed? I am thinking the purlins will be 2 x 3's.
F.
Could be 2x2s or 2x4s flat. Have the styro channels cross over the purlins (with maybe a 1x spacer halfway along the length of the last channel to keep it from kinking when pressed with the insulation).
Or you could just scab a piece of 3/4" plywood to the top two feet or so of the sheathing, south of the opening.
This is one of those situations where I like to set a seperate (new) set of ceiling joists in. If the porch roof is 3 or 4/12, I'll set the new ceiling in at "one less," which would be 2 or 3/12. This does a couple of good things. It creates some "attic" space for wires & the like to run in (more importantly in my neck of the woods--a/c ducting). Also, you can then insulate the ceiling rather than futz with the very narrow former porch rafters.
You also get a nice even, straight, ceiling--with fewer odd bits than the old roof/ceiling. By being "one less" in pitch, the customers still get the height/character of the space, but you get anice professional surface to work from. A win/win all around--unless you have less than enough headroom, or the porch roof is a 2/12 . . . --the normal sorts of remodeling woes--