# What is the minimum exterior foam thickness to avoid sheathing dew-point problems in Climate Zone 6 (Canada/US)? **URL:** \ **Category:** Trade Talk\ **Created:** [August 4, 2025, 3:17pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972 "2025-08-04T15:17:43Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![reno\_questions](https://home.renovation.reviews/user_avatar/home.renovation.reviews/reno_questions/32/2758_2.png) [@reno\_questions](https://home.renovation.reviews/u/reno_questions)\ **Post date:** [August 4, 2025, 3:17pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972/1 "2025-08-04T15:17:43Z")
What is the minimum exterior foam thickness to avoid sheathing dew-point problems in Climate Zone 6 (Canada/US)? ---
**Author:** ![Tim\_Michael1](https://home.renovation.reviews/user_avatar/home.renovation.reviews/tim_michael1/32/6294_2.png) [@Tim\_Michael1](https://home.renovation.reviews/u/Tim_Michael1)\ **Post date:** [August 13, 2025, 8:51pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972/2 "2025-08-13T20:51:41Z")
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. ---
**Author:** ![Tim\_curtis](https://home.renovation.reviews/user_avatar/home.renovation.reviews/tim_curtis/32/2766_2.png) [@Tim\_curtis](https://home.renovation.reviews/u/Tim_curtis)\ **Post date:** [August 13, 2025, 10:38pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972/3 "2025-08-13T22:38:42Z")
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. ---
**Author:** ![Bobby\_Michael](https://home.renovation.reviews/user_avatar/home.renovation.reviews/bobby_michael/32/2057_2.png) [@Bobby\_Michael](https://home.renovation.reviews/u/Bobby_Michael)\ **Post date:** [August 13, 2025, 10:44pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972/4 "2025-08-13T22:44:18Z")
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 ---
**Author:** ![Theodore\_3456](https://home.renovation.reviews/user_avatar/home.renovation.reviews/theodore_3456/32/2821_2.png) [@Theodore\_3456](https://home.renovation.reviews/u/Theodore_3456)\ **Post date:** [August 13, 2025, 11:13pm UTC](https://home.renovation.reviews/t/what-is-the-minimum-exterior-foam-thickness-to-avoid-sheathing-dew-point-problems-in-climate-zone-6-canada-us/19972/5 "2025-08-13T23:13:20Z")
_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._