Our last post about our progress in super insulating the master bedroom left us with the first layer of R13 fiberglass insulation and the end wall sheathed in OSB as a structural upgrade.
Next up is the middle layer of insulation. This is comprised of pink XPS foam board which serves three roles. It increases the insulative value of the wall by an additional R10, it provides a thermal break between the outer walls and inner walls (since wood is a poor insulator, a lot of heat is lost through the studs that hold up typical walls), and it acts as an air barrier (when sealed properly). Most heat loss in modern homes is through unwanted air transfer via small holes in the construction. On the end wall, we attached the foam board directly over top of the OSB sheathing. To join foam panels, we applied two beads with a caulk gun: one bead of PL300 foam adhesive, and one bead of siliconized latex caulk.
On the knee walls, we nailed the panels up to the studs. We used the same 3″ cap nails that we’ve grown to hate from previous steps (because they bend so easily).
The junction between the angled and knee wall panels would be difficult to seal with caulk, so we used spray foam. Spray foam was also used to seal other large gaps and potential leak areas. It will eventually be used in the attic to seal the top plates of the inner walls to the foam layers. We trimmed back excess foam after it had dried so it would not interfere with later steps.
As a final insurance against air leaks, we taped the connections between the panels, floor, and ceiling using Tyvek tape (which is designed to bond to plastic/foam surfaces). The cap nail locations were also taped. This is really sticky tape, and should provide good insurance against cracks in the caulk which will form as temperature changes cause the foam panels to change dimension.
Our next step is to frame the inner wall which will hold wiring and plumbing. Then the inner walls will be insulated with wool batts. This will bring the ideal wall insulation value up to R33. (Not counting small insulation gains from other sources, such as the drywall and wood sheathing layers, nor the losses from thermal bridging through the stud framed wall layers.)
Next up: Inner wall framing