©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Two projects my company is currently working on involve a common problem: not enough insulation in the attic. Both homes are old; one dates from 1860, the other from 1705. In both cases we initially recommended insulating the rafter bays. In both cases, however, we were not able to get over homeowner biases against heating “storage spaces,” and instead opted for insulating the attic floor.
We had different energy auditors look at each house; interestingly, one always recommended unvented “hot roofs” and the other recommended strongly against them. We have had good luck with hot roofs, as long as the insulation is well done, but find that every remodeling project is composed of so many variables that there is never one right answer.
In the 1705 house, a two-story timber frame, not only did our clients reject the idea of heating the storage space (and could not accept our promise that it would cost very little extra to heat the space), but also it would have been a shame to destroy the beautiful hand-hewn timber frame with spray foam. The other option for insulating the rafters was dense-pack cellulose with foil-faced polyisocyanurate rigid foam facing the attic. That was probably the wisest approach, but posed other problems, including undersize rafters and a 1970s addition that would have to be cut off from the rest of the attic.
Our approach will be to remove the existing 6-in. fiberglass batts, which are no longer their original bright pink due to dirty air washing through them from the heated space, and to remove the poly-sheeting vapor barrier that was installed when the house was renovated in the 1970s. We plan to cover the joists with two overlapping layers of XPS foam and dense-pack the cavity with cellulose; then we will re-install the original board flooring. The challenge is going to be dealing with the stairs leading to the attic, which include a historic banister we cannot alter, and that makes installing a hatch nearly impossible. Our choices are to build a vestibule at the top of the stairs or modify the existing vestibule at the bottom of the stairs, either of which will be difficult to seal and insulate adequately—one of the primary reasons we would have preferred to insulate the rafters instead. We are leaning toward doing the best we can with the existing second-floor vestibule.
In the 1860 house, we are in the process of removing 6 in. of cellulose insulation to allow us to replace the mix of knob-and-tube and BX wiring and to seal any penetrations to the conditioned space below. Although we used a flash-and-batt hot roof on the addition we built onto this house last year, to save installation cost on the old part of the house our clients decided that insulating the attic floor would be adequate. In the colder months they live mostly in the new addition and will keep the thermostat turned down in the old part of the house. The existing roof system is framed in an unusual way and is dangerously inadequate; we will build a 2x10 floor system above the existing 2x6 floor joists, then we will use 2x10s to frame a new roof. Seventeen inches or so of loose-blown cellulose will provide more than R-49, and a well-insulated attic hatch similar to the one Mike Guertin built here  will complete the thermal barrier.
A third project I am working on—designing a major overhaul of my grandmother’s former house, built in 1839—will involve spray foam. The new owners wanted to make use of the greatly underutilized second floor of the Cape Cod-style house, and they had already renovated one of the “ells” using open-cell foam. As on the 1705 house mentioned above, I tried to convince them to go with dense-pack cellulose in the rafter bays with a layer of rigid foam on the inside, which would preserve the beautiful hand-hewn timber frame for future generations. They would only consider sprayed foam, however; at least I think I have them talked into closed-cell foam instead of open-cell. It will ruin the aesthetic value of the frame but will block vapor. With a damp basement and inadequate wall insulation, open-cell foam would only provide an R-value in the mid-20s and almost certainly lead to serious decay due to condensation. Closed-cell foam will provide R-40 or so, and protect the structure while saving energy costs.
The common thread in all of these projects is an old house, a limited budget, and making the best of the situation. We consider many factors, including historic integrity of the architecture and consistent employment of our dozen or so exceptional carpenters, both of which often require compromises to accomplish. We employ independent energy experts when necessary to help make informed recommendations, and we do our best to meet our clients’ needs within their always limited budgets.