Am thinking of the following wall and roof assembly in BC as a cost effective way to achieve a high R-value and tight envelope.
2×6 structural wall with roxul in the cavities and gaps in sheathing (drill holes as well) to promote drying to exterior. The inside of this wall is sheathed with 1 inch of polysio (air and moisture barrier). A 2×4 wall inside this provides chase services, a cavity for fiberglass batts, and protection of air barrier.
The 2×6 walls are straightforward construction, with most of the building wrap and rainscreen installed before lifting wall. Foam insulation is also easy to install with no penetrations. The only extra work for this R-40 tight wall assembly with a thermal break is a 2×4 wall framed on the inside.
The roof is a cathedral roof framed with 2x12s and sprayed with closed cell foam (rim joist cavities sprayed at same time). In my mind the money saved by using dimensional lumber instead of I joists (material and labour), as well as the superior insulating and air sealing benefits of the spray foam makes it a good place to splurge.
Any thoughts would be much appreciated.
‘long time listener, first time caller’
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