Deep energy retrofit from the inside or out?
We have a 2 x 4 house with standard framing in Calgary (very cold and semi-arid – zone 6).
It is a full gut job, so we can insulate from inside or out, aiming for R-50 walls. We’re thinking an offset frame wall with 4.5″ spray foam;1.5″ of XPS; synthetic stucco with appropriate water control. Existing 2 x 4 stud cavities would get cellulose. The foam is the air/vapor control layer so no housewrap or poly. But building on the exterior with a sloped roof means little space to get R-50 above the top place and still provide adequate roof ventilation.
1. What do you think the trade-off on less than R-50 on the top plate vs building the wall on the inside is (aside from loss of floor space)?
2. 2″ of foam is enough for an air and vapor barrier, but if I use less foam and more cellulose how do I know where the dew point is in the wall and if I can still go without 6 mm poly from the interior?
3. If the roof has enough ventilation (based on area calculations and 1:300 rule) in general, do you need to vent every rafter bay? If you don’t, is the roof in that area as risk of moisture problems?
Thank you in advance your thoughts.
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