Condensation Under a Metal Roof: Where It Comes From

"Metal roofs sweat." I hear it a lot, usually from someone who has stood inside a pole barn on a cold morning and watched water drip off the underside of the panels. That happens, and it is real. It is also not what happens under a metal roof on a house that was built correctly, and the difference is worth understanding because it is the difference between a roof problem and an attic problem.

Where the water comes from

Condensation forms when warm, moist air touches a surface colder than its dew point. The moisture is already in the air. The cold surface just makes it visible.

In a house, that moist air comes from inside. Showers, cooking, breathing, a crawl space, a dryer vented into the attic. Warm air carries it up through every gap in the ceiling, and on a winter night the underside of the roof deck is the coldest surface it finds. Metal is a good conductor, so a metal panel gets cold fast, but the deck under an asphalt roof gets cold too. The roofing material is not the cause. The moisture and the path are.

Why the barn is different

A pole barn has no ceiling, no insulation, and no deck. The panels are the ceiling. Warm air from animals, equipment, or a heater meets cold steel directly, and there is nothing between them. That is the picture most people have of condensation and metal, and it is a picture of a building with no assembly, not a picture of metal.

What a house needs

Three things, and they matter in this order.

Air sealing at the ceiling. The less warm moist air that gets into the attic, the less there is to condense. Can lights, attic hatches, plumbing chases, and bath fans that end in the attic instead of outside are the usual leaks. This is cheap to fix and it is rarely on a roofing quote.

Balanced ventilation. Intake at the soffit, exhaust at the ridge, in roughly equal amounts. Air moves in low, sweeps the underside of the deck, and leaves high, carrying moisture with it. If the soffit vents are painted shut or buried under insulation, the ridge vent has nothing to pull from and the attic sits still. If a contractor never asks about your attic, they are quoting a product rather than a roof.

Insulation where it belongs. At the ceiling plane in a vented attic, so the attic stays cold and close to outdoor conditions. That is the point. A cold attic with dry air does not condense. A warm attic with wet air does.

Where it gets harder

Cathedral ceilings and finished attics. There is no attic to vent. The assembly has to either breathe through a channel above the insulation in each rafter bay or be built unvented with the insulation tight to the deck and a vapor retarder on the warm side. Both work. Neither is a place for guesswork, and this is where I see the most condensation trouble in houses, with any roofing material.

Full peel-and-stick underlayment. A self-adhered membrane across the whole deck is a vapor barrier. On a vented attic that is fine. On an unvented assembly it can trap moisture in the deck if nobody thought about which way the vapor is trying to go.

Powered attic fans. A solar or electric fan at the roof can pull air from the house through ceiling leaks faster than the soffits supply it, which moves conditioned air into the attic and can pull moisture with it. They have a place on a poorly vented roof that cannot get more passive ventilation. They are not a substitute for doing the first three things right.

What this means for a metal roof

Nothing about metal creates condensation, and nothing about it prevents condensation. It is a roof over an assembly, and the assembly decides. What is true is that a metal roof over a well-ventilated deck will last a very long time, and a metal roof over a wet deck will sit on wood that is slowly failing underneath it. Same roof, different outcome.

Every install of ours starts with the attic, because the roof cannot do its job over a deck that cannot dry. If you have a damp attic now, fix the air sealing and the ventilation first. That is true whatever roof you end up with.

This is why the assembly matters as much as the material.