Insulate roof with foam panels on top instead of blow-in insulation
I have a 1000 sq ft single story house in the SF Bay Area, and I’m paying hundreds of dollars a month to run the central AC and a window unit. And even then on hot days it sometimes gets up to 80ºF inside. Ducts run through the uninsulated “vented” attic (only vents are two holes at the gables).
All the contractors around here react to this by wanting to blow the attic full of fluffy stuff. Aside from the fact that the ducts would still be running through unconditioned space, I dislike the idea. I have this strange long-term attitude toward building that comes from growing up in a 200-year-old house in the Old Country. Basically, I assume I cannot get things perfect all at once and minor changes and repairs are going to happen.
I mean… this one is not 200 years old, but it is from 1950, and it’s not exactly top notch construction. I’ve had an electrician in to replace the old, damaged knob&tube wiring (that’s why the old insulation had to come out in the first place, and boy was it disgusting). Then another electrician had to fix a bad connection the first electrician had made. And I pulled wires for the TV, and I’ll want new bathroom fans some day, and I’ve been told the flashing is bad around some roof penetrations. Did I mention the shingles are starting to fail? The idea of doing everything that needs done and being sure nobody needs to go up there for the next 20 years and trusting that no rats will ever make it up there… that all seems far-fetched to me.
How expensive would it be to just insulate the roof deck like we do back in the aforementioned Old Country? Given that I will soon need new shingles anyway, and the roof is a *very* simple shape, could I get the roofer to put foam insulation on top of the current roof deck? Would that be a good alternative? How would I go about finding a contractor that does that sort of thing?
Everyone around here seems to want to just blow the attic full of fluffy stuff and I suspect that their advice is not entirely unconnected to that being the thing they happen to be good at…
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Is your climate considerably warmer than SF proper? Looks like they're seeing highs in the mid-60's.
To your question, yes you can insulate with rigid insulation above the roof deck. This is a good idea when there are ducts in the attic. Fluffy insulation is cheap and can be added any time, whereas you need to also replace the roof to add rigid, as you've suggested. Generally your best bet will be polyiso, ideally thick enough to meet code minimum R-value (6+ inches). You may end up with a combination of thinner polyiso and batts in the rafters to achieve the needed R-value. Over the polyiso you add another layer of sheathing for the new shingles.
It's possible that your air ducts have major leaks or that your AC needs refrigerant if it uses a lot of energy for seemingly light cooling loads.
Both rigid EPS and polyisocynurate cost about USD$0.10/R-foot^2. IRC 2012 code minimum for the SF Bay area (US cliimate zone 3 ) is R38 (see: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm ), so assuming a relatively low angle roof for about 1000 square feet of roof you're looking at about $4000 in material for the insulation alone, plus another $1000 or so in nailer-deck + timber screws for attaching the nailer deck.
If you use EPS it would add about 9" of additional thickness and more expensive timber screws, if you use polyiso you're looking at only 6" of exterior foam.
Labor rates for installation will vary, but I suspect you're looking at something on the order of $8000, possibly more.
You could also do it with 9-10" of open cell foam sprayed onto the under side of the roof deck for something like $6000 plus whatever it costs to install the code-required interior side thermal barrier.
Another possibility is to fill whatever rafter depth you have with damp-sprayed cellulose or damp-sprayed JM Spider (a fiberglass blowing wool) or rock wool batts and make up the difference with thinner rigid foam on the exterior. To insulate on the interior side of the roof deck required a minimum of R5 exterior foam (or R5 in closed cell foam under the roof deck) for dew-point control at the roof deck (or foam/fiber interface.)
If you went with interior spray foam + damp-sprayed fiber you would not need to replace the exterior shingles so it's probably the cheaper option overall, since that much damp sprayed fiber would qualify as the thermal barrier to meet fire code. But if it's a low-pitch roof and the rafters are less than 2x12s it may be difficult to achieve the requisite minimum R-values with an interior side only approach. eg: With 2x6 rafters you'd only be able to get about R20-R23 between the rafters, so you'd need something like R15-R18 above the roof deck (3" of polyiso, or 4-5" of EPS.)
Dana, you don't mean to install damp sprayed cellulose and leave it like that in open rafters? The OP is concerned about things unraveling over time. If there's no ceiling below the rafters then rockwool batts would be better. R-23 (nominal) batts in 2x6 rafters plus 3" polyiso would be a good solution. Existing fluffy insulation on the attic floor could stay, but it would be a good idea to at least temporarily excavate down the ceiling and look for air leaks between the attic and the conditioned space below. A well air-sealed ceiling goes a long way to improving the effectiveness of attic insulation.
Damp sprayed cellulose has pretty good adhesive power, even when sticking to ccSPF. Damp-sprayed cellulose in NETTING is even better (and how it's more commonly done, though it is an additional labor expense.)
Damp sprayed Spider would need to be dense-packed to at least 1.8lbs to be sufficiently air retardent, but cellulose any standard density would work in this situation.