I’m considering buying and building a kit house. The only problem I have with it: The largest roof rafter they’ll put in is a 2x10, which only allows for R-30 fiberglass batts in a cathedral ceiling. I would like to fur out the underside of the rafters with 2x2s or 2x3s to allow for R-38 batts (which need a 2x12). [I’ve already considered and rejected rigid foam insulation on the ceiling, for reasons of cost and final ceiling finish.]
Is the added weight of the furring strips (well … furring boards) going to be too much of a strain on the roof structure?
The long and short of the answer is … It Shouldn’t
Are you using white pine? Douglas Fur? Pine at all?
Why not use a foam like versi-foam or Icynene? My wife and I have an Icynene layer under our cathedral ceiling. We think it’s pretty energy efficient…That part of the home was built two years ago.
If it’s a cathedral ceiling you could go with structural insulated panels (SIPS). They provide mechanical support with insulation built in. One manufacturer is Enercept. There are others. Wiring chases for ceiling lights and fans are formed at the factory.
Is the extra couple of inches of insulation going to make that big a difference?
The extra weight shouldn’t even be noticeable. 2x4’s weigh very little. You’re talking an extra load of what, a half pound per square foot? Compare that to the weight of the drywall, sheathing and shingles and it’s miniscule in comparison… Not to mention that if you’re in an area cold enough for you to worry about insulation values, the roof structure has to be designed to withstand a considerable amount of snowpack as well.
I think your biggest problem might be aesthetics – firring out the rafters will alter the proportions of the room slightly.
I know about SIPs, but this is a kit house, so the allure is that it all comes on the back of a truck and my wife and I can put it together ourselves, without the use of large cranes, work crews, etc.
We’re on a modest budget, want to do as much work as we can ourselves (we’re kooky like that), want to minimize the impact on our lovely wooded lot by reducing the amt. of heavy equipment that has to come through, etc.
We also want to make the house as energy-efficient as we can afford, hence the insulation. The requirement for our state is only R-25 in the ceiling, but DOE recommends R-38 minimum, preferably R-49.
The room in question is lofted (i.e., right under the rafters, low ceiling). We would also like to eschew the rather stark look of a white drywall ceiling in favor of a more adventurous/modern birch plywood, which, in turn, dictates what sort of insulation we can use: if we use rigid foam under the rafters, we HAVE to put drywall over that to meet fire code, and would then additionally have to put the birch that we want over the drywall … or we could just buy some cheap lumber rather than pricey rigid foam (which would add almost 2" for an addl. R-7 to R-10) and the added trouble of hanging drywall overhead.