How important when insulating with foam board is it to be for the joints be overlapped, or pieces be foamed in place?
My project is a retrofit of foam over the sheathing of a geodesic dome. Every triangle has a gentle angle, which will interrupt the five inches of XPS foam that’s being added. The sheathing below the foam will be primed with an acrylic sealer and taped. Over the insulation will be a water barrier, and strapping, 2 3/8 inch vent space, sheathing – with Densdeck Prime (sealed and taped) and a single ply adhered roofing of TPO. My feeling is that the foam insulation in this case is not the primary air barrier, and that I could use mineral wool slivers to fill the gaps, as this adds some fire resistance laterally between foam panels and a vapor conduit for any tiny amounts of condensation, that may occur. Which leads to the question of what perm rating should the weather barrier (underlayment, wrap, or slip sheet ) be over the foam? Should I be thinking also of the fire rating of this foam board cover? Even using two types of wrapping, like Tyvek, with a perforated aluminum reflective cover? The strapping is to be notched at the corners, and the intermediate to allow water draining in the event of moisture getting past the outer roofing material. Retrofit will start from the top, working down, weathering and leaving exposure to a bare minimum.
The weather zone is a 5 in central WA, high desert, with cold winters, and hot summers, generally low humidity. Interior existing poly air barrier to be removed during retrofit. The TPO roofing material is highly reflective, and there will be a sprinkler system on top of roof, as this is an area with annual wild fires. There is existing fiberglass insulation in the triangular strut cavities, and the retrofit will bring the insulation value from R21 to a R46. While the existing home was under insulated to the default code, it was allowed per the alternative energy code, by computer analysis, due to the buildings high volume to skin ratio. Comments, answers, recommendations, etc, highly appreciated. Thanks!
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