The Attic Renovation: Insulation

The Attic Renovation: Insulation

By the time I was ready to start insulating, I had become overwhelmed by the scope of the renovation as a whole. Although within my abilities as a do-it-yourselfer, finishing the attic was just too time-consuming for this weekend handyman. In talking to a contractor about other issues in the house, I asked him for a ball-park figure for finishing the attic as well as adding a dormer to the large bedroom. His price was reasonable, so I hired him to take over the project.

The side attic

side attic insulation
A well-insulated 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 drywall ceiling and the joists.  However, because the vapor barrier was attached to the insulation I removed, 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 joists.  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

House wrap (Tyvek) was stapled up on the attic side of the knee wall, treating it as an outside wall.  This not only provides a backing to prevent the batts of insulation from falling into the attic, but also acts as a barrier to help stop the cold air from penetrating the insulation, increasing its effectiveness. 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 that at least an inch or more of airspace is maintained 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) were installed to 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 being transmitted through the wood directly to the drywall. The foam board was also used for the flat ceiling as well.

The contractor’s error

Our contractor told us that extruded polystyrene also acts as a vapour barrier, so no additional vapour barrier was required on those surfaces where the panels were used.  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. In fact, our contractor was wrong.

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 that was used for our ceiling and cathedral slopes is rated at 1.5 perm.

Because I was not confident that any additional vapor barrier was used where the foam board insulation was installed, 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.

The Attic Renovation