A tight, well-sealed building enclosure ensures that heating and cooling loads are low. The walls and roof are made of SIPs. The walls are 5-5/8 inch thick with an R value of R-26 and the pitched roof is 9-3/8 inch thick with an R value of R-41, while flatter roof sections are 11-3/8 inch thick with an R value of R-50. The slab is well insulated with R-20 of rigid XPS foam under the entire slab and R-10 rigid XPS foam installed vertically on the interior side of the foundation walls.
In addition to providing insulation with almost no thermal bridging, the SIP enclosure contributes to an exceptionally airtight enclosure. In this case the home measured 1.32 ACH50 with a blower door test. SIP panels also provide sound proofing, structural stability (no nail pops), and shear strength, which is a consideration in coastal areas like the Puget Sound where wind storms are a more-than-annual event. The roof is covered with 40-year composition shingles that are dark colored because light-colored shingles will not warm up enough in the typically cool climate to soften the tar to self-seal the shingles.
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