I don't think my client was planning on an energy retrofit when he first talked to me about repairing and repainting the shingle siding on his 120-year-old Massachusetts home. But after crunching some numbers, we decided that re-siding the house with factory-painted fiber-cement siding would be more cost-effective than replacing, prepping, and repainting the severely weathered cedar shingles.
That's when we began to talk about improving the home's energy performance, too. I suggested that we add rigid foam underneath the siding to help eliminate drafts and reduce noise from a nearby highway. To seal the deal, I guaranteed that the new foam would pay for itself in just a few years by lowering his heating bills.
Initial Testing
To find out just how much insulation was in the walls, I scheduled an infrared (IR) camera inspection with the insulation contractor at the beginning of the job. I knew that the attic had only 6 to 8 inches of cellulose and blown fiberglass insulation, but I was surprised when the IR images showed that there was virtually no wall insulation in most of the house. Once I had explained the test results to the homeowner and gone over the benefits of cellulose wall insulation, convincing him to add it to the scope of work was easy.
I also arranged for blower-door testing to see how leaky the building envelope was. To no one's surprise, the drafty house had an air-leakage rate of more than 4,000 cfm50, more than 11 air changes per hour. But what really caught the homeowner's attention was the sight of the curtains on his first-floor bay windows being drawn out nearly horizontally while the fan was running. This prompted him to add six new double-hung sash kits with jamb liners to the project - though I don't ordinarily recommend replacement windows, because they're less effective and more expensive than insulation, with a much longer payback.
Cellulose Insulation
Our insulation sub installed the cellulose at a density of at least 3.5 pounds per cubic foot, giving the walls a solid R-13 or more worth of insulation. Though not technically an air barrier, dense-pack cellulose reduces air movement both through and within the wall, which gave us a good head start at air-sealing. After the cellulose was installed, we performed another blower-door test and found that we'd decreased the air-leakage rate to 2,550 cfm50, or about seven air changes per hour. (This wasn't tight enough to require mechanical ventilation - but we still had the exterior foam to add, so we would need to check again.) The house was also noticeably quieter.
Since it was an unusually rainy summer, we focused on one side of the house at a time when we set up our staging and began stripping shingles. To stay dry when it rained, we hung tarps off the gutters, draped them over the tops of our Alum-a-Pole staging, and staked them out in the yard behind us.
In some areas - such as below windows or where there was blocking - we discovered voids in the cellulose as we stripped off the shingles. In those places we pulled off the exterior 1x12 board sheathing and insulated with fiberglass batts. This was faster and more cost-effective than calling back the insulation sub, especially since we would be installing another blanket of rigid foam anyway.
Unfortunately, I couldn't persuade my client to air-seal the attic or insulate it with additional cellulose, partly because of the cost of removing the floorboards and all the personal items stored up there. But in the future, if energy costs skyrocket, this would be a very cost-effective energy upgrade.
Windows
We filled small voids around the windows - like unneeded sash weight pockets - with spray foam. Then we sealed the window frames to the 1x12 sheathing with 4-inch-wide Grace Vycor window flashing (866/333- 3726, www.na.graceconstruction.com), taping the bottoms and legs first. When we taped across the tops of any windows that were exposed to weather, we used wider 6-inch Vycor and left a 2-inch flap (with the release paper backing still attached) hanging down over the head jamb. Later, when we fastened the jamb extensions to the frames, we folded these flaps back, peeled off the remaining release paper, and attached the flashing to the extension. The flaps act as sheathing-level drip caps that keep water out of the joints between the jamb extensions and head jambs.
We ripped jamb extensions from Azek, a low-maintenance cellular PVC product (877/275-2935, www.azek.com), making them 1 3/8 inches wide to accommodate the 1-inch-thick rigid foam and 3/8-inch-thick plywood rainscreen battens that would be added to the wall. We fastened the extensions to the jambs with 2 1/2-inch trim nails and welded their corners together with PVC glue.
The sill extensions were ripped from 2x4 cedar. We beveled them to match the angle of the existing sills and made a shallow relief cut on the underside to serve as a drip edge. Then, after applying exterior carpenter's glue, we fastened them to the sills with 3-inch-long screws driven through predrilled countersunk holes. We filled the holes with Abatron two-part epoxy putty (800/445-1754, www.abatron.com).