# For unvented roofs with spray foam in heavy-snow climates, how do you control condensation without over-insulating abov… **URL:** \ **Category:** Trade Talk\ **Created:** [August 4, 2025, 3:18pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976 "2025-08-04T15:18:05Z")\ **Posts on this page:** 8\ **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, 3:18pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/1 "2025-08-04T15:18:05Z")
For unvented roofs with spray foam in heavy-snow climates, how do you control condensation without over-insulating above the deck? ---
**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, 8:43pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/2 "2025-08-13T20:43:24Z")
Use the right amount of closed-cell spray foam, keep inside moisture under control, and have a continuous air/vapor barrier. This stops condensation, but don’t overdo the insulation. ---
**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, 9:01pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/3 "2025-08-13T21:01:41Z")
Vapor Permeable Membranes: Using vapor-permeable underlayment membranes can allow for drying towards the exterior, reducing the risk of condensation. Adequate Insulation: Ensuring sufficient insulation while avoiding over-insulation is crucial. This balance helps maintain a stable temperature and reduces the risk of condensation. Air-Tight Construction: A well-sealed roof assembly is essential to prevent warm, moist air from entering the roof cavity and condensing on the deck. ---
**Author:** ![kent\_0902](https://home.renovation.reviews/user_avatar/home.renovation.reviews/kent_0902/32/2825_2.png) [@kent\_0902](https://home.renovation.reviews/u/kent_0902)\ **Post date:** [August 13, 2025, 10:17pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/4 "2025-08-13T22:17:43Z")
Use enough closed-cell foam for dew-point control, keep interior airtight, add vapor control, and allow outward drying. ---
**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 14, 2025, 3:29am UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/5 "2025-08-14T03:29:01Z")
For unvented spray foam roofs in heavy snow: seal air leaks, use closed-cell foam, avoid over-insulating above sheathing, and control indoor humidity; a small vented gap can help if allowed. ---
**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 14, 2025, 10:51am UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/6 "2025-08-14T10:51:58Z")
Use closed-cell spray foam under the deck, control interior humidity, and limit foam thickness to meet R‑value without trapping moisture. ---
**Author:** ![Joyboy03j](https://home.renovation.reviews/user_avatar/home.renovation.reviews/joyboy03j/32/2900_2.png) [@Joyboy03j](https://home.renovation.reviews/u/Joyboy03j)\ **Post date:** [August 14, 2025, 12:23pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/7 "2025-08-14T12:23:07Z")
Use closed-cell spray foam at the roof deck for sufficient warm-side R-value per code ratio, seal all air leaks, and manage indoor humidity so you meet condensation control without excess above-deck insulation. ---
**Author:** ![Joe\_clintonnn](https://home.renovation.reviews/user_avatar/home.renovation.reviews/joe_clintonnn/32/2820_2.png) [@Joe\_clintonnn](https://home.renovation.reviews/u/Joe_clintonnn)\ **Post date:** [August 14, 2025, 2:43pm UTC](https://home.renovation.reviews/t/for-unvented-roofs-with-spray-foam-in-heavy-snow-climates-how-do-you-control-condensation-without-over-insulating-abov/19976/8 "2025-08-14T14:43:20Z")
It is controlled by maintaining a warm roof deck temperature, which can be achieved by limiting the amount of insulation above the deck and ensuring proper air sealing and ventilation in the conditioned space below. Over-insulating above the deck can lower the deck temperature, increasing the risk of condensation