How to use rigid foam insulation in roof space
Context: Central Pennsylvania. Want to convert vented (sofit and ridge vent) cathedral attic space into conditioned space. No plans to replace the roof. Roof cavities are 7.25″. Cost is important, but so is DIY-ability.
Plan so far: Cut and cobble XPS into cavities between trusses, sealing all the way to exterior wall. Assembly stack spray foamed in place:
2 layers of 3” XPS
1.25″ ISO on inside face covering roof trusses
Comments: R~35-40 is lower than we want, so tempted to remove ventilation space. XPS R-value goes up in cold (crucial for us), but will it suffer in heat under a shingle roof? Will 1.25″ ventilation be enough to help with heat? ISO on inside of assembly avoids its R-value drop in cold, but will that risk trapping moisture in the assembly?
Another possible alternative:
3.5” fibre-faced ISO OR 4″ double foil-faced ISO
1.25″ fibre-faced ISO on inside face covering roof trusses
Comments: R~35-45. Worried about R-value drop in cold, possible moisture issues from double foil face on ISO, and (as above) ISO on inside surface. Heat is fine for ISO, so maybe replace ventilation with more insulation?
The key questions about this are:
i. What boards to use? In what configuration?
ii. Is ventilation useful in this situation or not?
iii. should I consider blowing in cellulose between foam boards? How thick would my board vapor barrier have to be?
iii. how important is the inner layer for thermal bridging? Will using ISO here cause problems?
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