Pole barn condensation in Liberty Township: closed-cell vs vented

The problem

A 36-foot by 60-foot metal pole barn in Liberty Township, built 2018, used as a workshop and storage. Standard exposed-purlin construction, painted-steel roofing on 2x6 purlins, no insulation, no ceiling. The homeowner heated occasionally with a portable propane heater for winter weekend project work.

Every cold morning, water dripped from the underside of the metal roofing. Tools rusted, OSB shelves warped, project lumber cupped. The homeowner thought the roof leaked. It did not. The problem was condensation.

Why this happens

Cold metal roofing combined with warm interior air (whether from a heater, body heat, or stored equipment that warmed during the day) produces dew on the metal surface every time the metal temperature drops below the air's dew point. In a Cincinnati winter that means almost every cold morning. The dripping is the dew running down the underside of the roof.

Two paths fix this:

1. Vent the underside aggressively. Add ridge vents and soffit vents so outside air flushes the underside of the roof continuously. The metal stays at outdoor temp; no dew forms because the inside-air dew point cannot reach the metal surface. Works well for unconditioned, unheated structures. 2. Insulate the underside. Spray closed-cell foam directly to the underside of the roof and the inside of the wall sheeting. The foam isolates the warm interior air from the cold metal surface; no condensation can form because the metal never contacts humid interior air. Works well for any structure the homeowner wants to use comfortably.

For a barn the homeowner uses occasionally with portable heat, choice 1 is cheaper but choice 2 is the structural fix. The homeowner picked closed-cell foam because they wanted to upgrade the barn to year-round-usable workshop in the next two years anyway.

The install

The partner-network installer scoped 3 inches of closed-cell foam (R-21) directly to the underside of the metal roof and the inside face of the wall sheeting. Total: 3,400 board feet at $1.95/bf, $6,630 fixed.

Closed-cell spray foam adheres to clean metal and forms a vapor barrier in addition to the air seal and R-value. Critically, it will not allow vapor to migrate behind it, which is what would have caused continued corrosion if the roof had been left vented and humid air kept circulating.

Crew handled the project across two days. Day one was the roof; day two was the walls plus a 2-inch overspray on the gable ends and door framing for thermal continuity. Foam cured to a dry tack within 4 hours and was fully cured at 24.

Result

• No more morning condensation. Period. Within a week of install.
• The homeowner ran a portable heater for one weekend and found the barn held heat overnight at a 14°F outdoor low; previously it was at outdoor temperature within 90 minutes of shutting the heater off.
• Wall sheeting interior temperature jumped from outdoor-equivalent to within 3-5°F of conditioned space when the heater was running.
• Stored tools and lumber stopped showing surface rust and cupping.
• The barn is now a candidate for being a year-round-conditioned workshop. The homeowner is planning to add a mini-split HVAC unit in spring 2026.

When to choose vented vs foam

Vented (ridge plus soffit) is the right call when: the building stays unconditioned year-round, no occupied use that introduces humidity, no stored items that would suffer from temperature swings.

Closed-cell foam is the right call when: the building will be heated even occasionally, used for workshop or shop activities that introduce humidity, stores equipment or material affected by temperature swings, or the homeowner ever plans to upgrade to a conditioned space.

For Liberty Township and other Cincinnati exurbs where pole barns commonly evolve from "garage" to "workshop" to "second living space" over a decade of ownership, foam is the path that does not require redoing the roof later.

One thing to never do: open-cell foam on a pole barn. Open-cell is vapor-permeable. On a metal-roof structure, the moisture migration through open-cell will corrode the back of the metal sheeting from the inside out. Closed-cell only.