We have a low-pitch (2-in-12) metal roof, low ceilings overall (7 1/2′ plate height); and about 50% of the house has cathedral ceilings. We are doing an extensive remodel but can’t change the roof (due to $$), and are deliberating what to do for ceiling insulation. We’re in Northern California; moderate climate (occasional freezes; hot summers but we’re on a shady lot). To our knowledge, the best R-Value would be gained from spray foam insulation. We would like to use this without ventilation in the attic, but we’re concerned about Indoor Air Quality, and possibly condensation from the metal roof (although it should be airtight, right?). Other concerns by having no ventilation w/Rigid Foam??
Using Rigid Foam to create an airspace above the spray foam was discussed, but it sounds extremely tedious to do, and gaps would seem inevitable. We don’t have the height to shim down the roof. Our other option I suppose is a lousy RValue in the roof, using batt insulation. I imagine there is no ‘right’ answer, but would appreciate any information/opinions which can better educate me to arrive at the best solution.
A related question: we don’t plan to use many recessed lights, but we do plan to use a few line & a few low-voltage IC cans. Our electrician mentioned a possible concern of heat from the low-voltage cans rising to the metal roof and condensing. Is this a valid concern?
Thanks for your insights/opinions….
Janet
Replies
Janet,
You didn't say what the framing is like, 2x8's?, 2x10's?, 2x12's?
Attic/roof ventilation has nothing to do with indoor air quality, it has to do with preventing heat buildup in the summer and/or heat and ice dams and condesation in the winter. An unvenetd roof will, in most cases, form condesation on the underside of the sheathing and will rot in a relatively short period.
Two different thoughts on cathederals: spray in eurethane foam to eliminate any moisture in the space and get good insulation; or create an air space with rigid foam and insulate under the foam with batts or cells.
In the cathedral ceiling in my house, I put 2" rigid foam (r-10) with a 1" air gap, soffit and ridge vents, and put R-25 fiberglass batts and a vapor barrier beneath the foam. The framing is 2x12's and I end up with R-35 (plus sheathing and gwb, minus 20% for the framing) and continous venting. Even with 2x6 framing you can get R-23 or so in the space and still vent the roof.
One drawback that I have heard about (but have no personal experience with) concerning sprayed eurethane, is that over time it shrinks and draws away from the framing, allowing moisture in, with no way out.
Most will advise you against any type of recessed lights in a cathedral ceiling. If I had to have lights on/in the ceiling, I would find a suitable surface mounted fixture.
Thanks for your reply, Tim. The framing is 2 x 6. The concern re: indoor air quality is that with eurethane foam we would be sealing off one of the main areas where our home would otherwise breathe somewhat. (Although I realize the goal is to minimize heat loss/air leakage to roof, I assume the reality would be that it would be one of the ways our home would breathe). Perhaps this is not an issue, but that was our thinking...
Your application of air space, rigid foam, and batt would be ideal, however it would be a very tedious and expensive retrofit (cutting rigid foam around all the rafters). Could you explain the vapor barrier - wouldn't that keep moisture that is driven up through the ceiling from being able to be vented?
I'll have to look into the shrinkage issue with spray eurethane as that would pose a problem. Othewise, it sounds like that would be our best bet.
Thanks again...
Janet
just because the pitch is 2/12 on the roof doesn't mean it has to be 2/12 on the ceiling..
if you drop the plate ht. to say 7'4" and make your ceiling pitch 1/12.. you can use the extra space for insulation... dens-pak cells behind 1" foam would be my vote... then furr the ceiling and apply new rock
Mike Smith Rhode Island : Design / Build / Repair / Restore