1) Begin with ventilation. You must ensure that you have good air flow from the soffit vents right up to the ridge vent at the peak of the roof. There are two ways of accomplishing this. When batt insulation is installed between the rafters leave a 2" space between the batt and the underside of the roof sheathing to provide ventilation. Another is to fasten baffles on the underside of the roof sheathing prior to installing the batt insulation. This way, not only will the air flow better under the roof, but the cold air will not penetrate into the batts.

2) Put as much batt insulation between the rafters as possible, keeping a clear airflow and without compacting the batts. If you are using baffles, and you have at least 6" deep rafters, you should be able to install R-12 batt insulation.

3) Eliminate thermal bridging by placing 1-1/2" of STYROFOAMâ„¢ extruded polystyrene insulation across the entire inside rafter surface. This will prevent all future condensation and increase the ceiling's overall insulation values.

4) You have three STYROFOAM insulation product choices for this job; STYROFOAM SM, STYROFOAM CLADMATEâ„¢, STYROFOAM CLADMATE XL, or STYROFOAM WALLMATEâ„¢

1. STYROFOAM SM and STYROFOAM CLADMATE comes in 2' x 8' sheets with a shiplap edge. STYROFOAM CLADMATE XL comes in 4'x8' sheets with shiplap edges. These products will give you excellent insulation coverage but will require 3" drywall screws to penetrate 1" into the rafters when attaching the drywall through the STYROFOAM.
2. Alternatively, you can also use STYROFOAM WALLMATE Slotted insulation. This also comes in 2' x 8' panels but the long edges are slotted to accommodate 1" x 3" wood strapping, which fits flush into the STYROFOAM WALLMATE board edges. These wood furring strips are screwed into the rafters and in turn provide a nailing base for drywall on 24" centres. Run the 8' length of the STYROFOAM WALLMATE and the strapping perpendicular to the rafters.

5) When you are installing the boards that butt up at different angles such as wall to sloped ceiling or sloped ceiling to flat ceiling junctions, be sure to cut a beveled edge down the side of the STYROFOAM brand insulation. This will give a snug fit as well as cover the wall header. The header is another thermal bridge where heat and cold can travel right through the framing.

6) The cavity of the short knee walls need to be insulated with batt insulation. Use wood lath or strapping fastened to the exterior of the knee wall framing to ensure batts stay in the cavity and do not drop into the unheated space. Insulate the knee wall framing with STYROFOAM insulation in the same fashion as the underside of the rafters. Ensure that there is a ventilation space below the roof sheathing at the intersection of the knee wall and roof.

7) Cover the ceiling and wall insulation with a 6 mil poly vapour retarder or, if you prefer, seal all the joints between the STYROFOAM extruded polystyrene insulation with adhesive compatible with polystyrene insulation. Make sure that any electrical wiring and the plumbing stack are sealed airtight. To prevent condensation, it is very important that as little moisture as possible be allowed to pass into the cold vented roof space.