What is the minimum exterior foam thickness to avoid sheathing dew-point problems in Climate Zone 6 (Canada/US)?
In climate zone 6, about 1–2 inches of XPS/EPS or 0.6–1.2 inches of polyiso usually does the trick to keep condensation off the sheathing, assuming the structure is airtight and indoor moisture is kept in check.
For Climate Zone 6, the general guidance is at least R-5 exterior continuous foam per inch of wall sheathing to keep the sheathing above the dew point, though most experts recommend R-6 to R-10 (≈1–2 inches of XPS or polyiso) depending on your wall assembly, indoor humidity, and how airtight the house is.
To avoid sheathing dew-point problems in Climate Zone 6 (Canada/US), the minimum exterior foam thickness required is R-25. This can be achieved with various types of foam insulation, such as:
-Expanded Polystyrene (EPS): 5-6 inches thick, depending on the density and R-value per inch
-Extruded Polystyrene (XPS): 4-5 inches thick, depending on the density and R-value per inch
-Polyisocyanurate (Polyiso): 4-5 inches thick, depending on the density and R-value per inch
Zone 6 minimum exterior foam for dew-point control:
2×4 wall: R-7.5 (~1½″ XPS/polyiso, ~2″ EPS/mineral wool)
2×6 wall: R-11.25 (~2″ polyiso, ~2¼″ XPS, ~3″ EPS/mineral wool)
Keeps sheathing warm enough with Class III vapor retarder.