I have a weirdly designed house.
The attic in the lean-to addition has 2×6 joists (actual 2×6 – old wood). The joists sit directly on top of the wall framing for the ouside wall, and extend outward from there. They’re notched to sit flat on the outside wall header. – there’s less room for insulation there, as teh joists are no longer the full six inches.
There was never any room for both insulation and ventilation, so we have a ‘hot roof’. It was created using three sheets of XPS foam sandwhiched together, and sealed with copious amounts of canned sprayfoam.
In the winter, we saw striping on the roof when the first snowfall happened, so we were thinking of getting a DIY sprayfoam kit and adding more insulation – and encapsulating the joists.
The issue is where the roof joists meet the outside wall – there’s no way to add insulation to those without lowering the ceiling on the entire roof.
How big of a thermal bridging issue will it create if I sprayfoam the joists on the entire roof, except at the bottom?
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