The attic was insulated in two stages. The contractor installed all the mineral wool insulation before the wiring rough-in. When he returned, he installed extruded foam board insulation on the cathedral slopes and ceilings along with the vapor barrier.
The side attic
The side attic in a 1½ storey house is a cold zone that is both above and beside warm zones. Ideally, there would be a vapor barrier between the main floor drywall ceiling and the joists. However, because insulation I removed had the vapor barrier attached, we needed to install a new barrier. This was no easy task as we did not remove any ceilings from the main floor. Therefore, the new barrier had to be installed from the second floor, wrapping over the tops of the joists and across the bottom of each bay.
Blocks were installed between the joists at the point where the cold zone met the warm zone. The 2×8 floor joists allow for two layers of 2×4 insulation in between. I chose to insulate with Roxul which has a higher R value than the pink stuff. 2×4 insulation has an R-value of 13.5. With 2 layers between the joists and another layer running perpendicular over top, the R-value of the insulation in the side attic is in the range of R-40.
The Knee Wall
We treated the knee wall as an outside wall, stapling up house wrap (Tyvek, visible in the picture above) on the unconditioned side. This provides a backing to prevent the batts of insulation from falling into the attic, and also acts as a barrier to help stop the cold air from penetrating the insulation. Other options for backing for insulation: nail 1×2 strapping, or other wood strips to the attic side. Some contractors opt for chicken wire or other forms of netting to hold the batts in place.
Cathedral Ceiling Insulation
For insulating the sloped ceiling, it is absolutely crucial to maintain at least an inch or more of airspace between the insulation and roof deck to allow for adequate ventilation. Canadian home improvement expert Jon Eakes recommends against insulating between the roof members suggesting instead putting Styrofoam insulation over the structural members the same way drywall, creating a thermal break. I chose to use both batt insulation and extruded polystyrene insulation. Raft-R-Mates (extruded foam ventilation channels) prevent the insulation from coming in direct contact with the roof deck.
The contractor added 2x2s to the 2×6 roof members which not only created a larger cavity for insulation and ventilation but it also added some strength to the roof structure. Now we could insulate the sloped ceiling to R-20. With one inch extruded polystyrene foam board insulation between the drywall and the roof members, the r-value increases to R-25 with no thermal bridges. Essentially this system offers excellent insulation value because wood conducts heat and cold. The foam board prevents the outside temperatures from transmitting through the wood directly to the drywall. The foam board covers the flat ceiling as well.
The contractor’s error
All the joints were taped as was the transition between the foam boards and the 6 mil polyethylene that was installed on the vertical exterior walls. Our contractor told us that extruded polystyrene foam board insulation also acts as a vapor barrier, so we would not require additional vapor barrier on those surfaces. In fact, our contractor was not completely correct. It depends on the thickness of the board.
Warning: technical stuff
Water vapor permeance is measured in “perms.” Without getting into a technical explanation, six mil polyethylene vapor barrier is rated at 0.06 perms. Generally speaking, a vapor barrier should be less than 1 perm (in Ontario, initial permeance is not to be more than about 0.8 perm (I’m doing some rough conversions here, based on the Ontario Building Code. The one inch thick extruded polystyrene insulation used for our ceiling and cathedral slopes is 1.5 perm. So it fell short of the standard.
Paint to the rescue
I was not confident that any additional vapor barrier was used where the foam board insulation was installed. After doing some research, I remedied the situation by using a vapor barrier rated paint: Benjamin Moore’s Moorcraft Super Spec Latex Vapor Barrier Primer Sealer 260 (product not available in Canada). Most oil-based primers will measure less than one perm after two coats, even if they do not make this claim. For my little problem, the cost of a special trip to the US and a few dollars more for specialty paint was less expensive than having to re-do the drywall.