Metal Building & Barndominium Spray Foam Guide: How to Stop Condensation & Heat Gain
Quick answer
Closed-cell spray foam on the underside of a metal roof deck and walls stops condensation (the 'sweating roof'), adds real R-value, and turns a steel shell into a controllable space. For living quarters, an open/closed-cell hybrid balances cost and performance.
Steel buildings in the Tennessee Valley sweat and cook all summer. Here is how foam fixes both the moisture and the heat.
Why metal buildings sweat
Steel sheeting cools below the dew point on humid valley nights, and water condenses on the underside — dripping onto insulation, tools, and floors. The fix is to keep the metal above the dew point with closed-cell foam against the skin.
Closed-cell vs. open-cell for steel
Closed-cell is the default for steel: it bonds to metal, adds a vapor retarder, and has the highest R-per-inch. Open-cell is cheaper and good for sound but is vapor-open, so it is not the right call against a sweating steel roof.
| Goal | Closed-cell | Open-cell |
|---|---|---|
| Stop roof condensation | Yes | No |
| Maximum R/inch | ≈6.7 | ≈3.7 |
| Vapor retarder | Yes | No |
| Cost | Higher | Lower |
Living quarters (barndominiums)
For conditioned living space, a hybrid works well — closed-cell on the roof deck for condensation control, open-cell or batt on the walls for cost, with the whole shell air-sealed.
Frequently asked questions
- Do I need a vapor barrier with closed-cell on steel?
- Closed-cell foam itself is the vapor retarder, so a separate barrier is usually not needed on the steel skin.
- Can I spray foam over my existing metal insulation (batt)?
- Usually we remove the old foil-faced batt first so the foam bonds to the steel and we don't trap moisture. Site-specific call.