How to Stop a Metal Roof Sweating: Houses, Barns, Sheds and Carports

Water dripping from the underside of a metal roof on a cold, clear morning is one of the most common complaints about metal buildings, and it is almost never a leak. The roof is sweating: moisture in the air is condensing on the cold steel and running off it. Sealing the outside will not touch it.

The fix depends on the building. A heated workshop, a hay barn, a garden shed, an open carport and a house all sweat for the same basic reason, but they need different answers.

Leak or condensation?

A few signs point firmly to condensation:

• It happens on cold, clear nights or early mornings, including when it has not rained.
• Drops form across the whole underside of the panels, not at one spot under a fastener or a flashing.
• In winter there is frost on the underside of the steel that turns to dripping water when the sun comes out.
• It is worst after something adds moisture inside: animals, a wet load of equipment, washing down a floor, or running a heater.

A leak shows up during or just after rain, in the same place each time, and usually below a seam, a penetration or a flashing. If you are not sure, finding a roof leak goes through how to trace one.

Why metal roofs sweat

Air always carries some water vapour. Cool that air down far enough and it cannot hold all of it; the temperature at which it starts to give up water is the dew point. Any surface colder than the dew point of the air touching it will collect water.

Steel sheet is thin, has almost no thermal mass and conducts heat well, so it follows the outdoor temperature closely. On clear nights it does more than that. A roof radiates heat to the open sky, and on a cold, clear, still night the panel can drop below the temperature of the air around it, and below its dew point. The Australian Steel Institute puts it plainly: on open sheds, carports and awnings, where night air circulates freely under the roof, condensation on the underside of the sheeting is expected.

So the question is never whether a bare metal roof will get cold enough. It will. The question is how much moisture is in the air that reaches it.

Where the moisture comes from

In most problem buildings the moisture comes from inside:

• Livestock. Animals and manure put a great deal of water into the air.
• The ground. A dirt or gravel floor, or a slab with no vapour barrier under it, keeps feeding moisture into the building.
• Wet equipment and crops. A tractor brought in covered in snow, or hay stored before it has dried.
• Unvented heaters. Burning propane or kerosene produces water vapour as well as heat. A space heater with no flue is a humidifier.
• Washing down. Hosing a floor or equipment inside on a cold day.

On open structures the moisture is simply the outdoor air, which is why a carport can drip on a humid autumn night with nothing inside it at all.

Step one: put less water in the air

The cheapest fix is the one that removes the source. Put a heavy polyethylene vapour barrier over a dirt floor. Fix gutters and grading so water is not running in under the walls. Let wet equipment drip outside or near an open door. Replace an unvented heater with a vented one. Store crops dry.

None of that changes the roof, and in a lot of sheds and machinery buildings it is enough on its own.

Step two: ventilate

Ventilation swaps humid inside air for drier outside air before the moisture reaches the steel. For livestock buildings, University of Illinois Extension calls ventilation the main way to reduce condensation. In practice that means openings low at the eaves and high at the ridge, so warm, moist air can rise and leave, plus fans where the moisture load is high.

Ventilation has limits. It cannot make the outside air drier than it is, and in an unheated building on a humid night the outside air may be the problem. It works best combined with the other steps, not instead of them.

Step three: insulate, with vapour control

If the underside of the roof never gets as cold as the dew point, it cannot sweat. Insulating the roof keeps the inner surface warmer. But insulation done carelessly makes things worse, not better.

University of Illinois Extension is clear on this: insulation stops condensation on the inner surface, but not inside the insulation or between the insulation and the metal, unless there is a properly installed vapour barrier or closed-cell insulation. Moist air moves straight through fibreglass, reaches the cold steel behind it and condenses there, where you cannot see it, soaking the insulation and rusting the back of the panels.

That leaves two reliable approaches. Closed-cell spray foam applied to the underside of the roof both insulates and blocks vapour in one layer. Alternatively, insulation with a continuous vapour barrier on the warm, inside face, with every seam taped and every penetration sealed, and usually a liner to protect it. Which is right depends on whether the building is heated and what it is used for.

Anti-condensation membranes on new panels

When a building is being roofed or re-roofed, panels can be supplied with an anti-condensation membrane: a felt-like layer bonded to the underside of the steel at the factory. It does not stop condensation forming. It catches the water, holds it, and releases it back into the air as the building warms, so it does not drip.

It suits unheated, ventilated buildings such as implement sheds, carports and storage barns, where a small amount of condensation is acceptable as long as it does not land on anything. It is not a substitute for insulation in a heated building, and it generally cannot be added to panels that are already up. If you are planning a new agricultural roof, ask whether the panel you are being quoted can be supplied with one.

Sheds and carports

An open carport will sweat on clear nights; that is physics, not a defect. If the drips land on a vehicle, the realistic options are an anti-condensation membrane when the roof is replaced, or a ceiling or insulation layer under the panels.

An enclosed garden shed or garage is easier. Deal with the floor first, add a vent at each gable or along the ridge, and keep wet things out of it overnight. If it is heated, it needs insulation with proper vapour control, as above.

On a house, the problem is the attic

A metal roof on a house is a different assembly. Between the steel and the rooms there is underlayment, a roof deck, a ventilated attic, insulation and a ceiling, and on a properly built roof the panels sit on strapping with a drained, ventilated air space underneath. Condensation on the back of the panels themselves is rarely the issue.

What does happen in houses, under metal or shingles alike, is condensation in the attic: frost on the underside of the deck and on nail tips in winter, which melts into drips and ceiling stains in the first warm spell. The causes are almost always on the house side. Warm, humid air leaking up through gaps around light fixtures and attic hatches, bathroom fans that vent into the attic instead of outside, and soffit intakes blocked by insulation, so the attic cannot breathe. Why a metal roof needs an air gap covers the roof side of this, and ice dams come from the same warm-attic problem.

What not to do

• Do not caulk or coat the outside of the roof to stop drips that are condensation. The water is forming inside.
• Do not staple fibreglass batts against cold steel with no vapour control. It will get wet and stay wet.
• Do not seal up every opening to keep the cold out. Closing off ventilation traps the moisture in.
• Do not run an unvented propane or kerosene heater in a closed building and expect the roof to stay dry.

When the roof is being replaced

A re-roof is the cheapest moment to fix a sweating building, because the panels are coming off anyway. That is the time to decide on an anti-condensation membrane, add ventilation at the ridge, or plan insulation and vapour control. Metal roofing for barns, shops and farm buildings covers the other decisions for agricultural roofs.

Highland roofs agricultural and commercial buildings as well as homes. Tell us what the building is used for, whether it is heated and where the water is landing, and we can talk it through.

Sources

• R. S. Gates, Metal-walled building condensation, University of Illinois Extension.
• Australian Steel Institute, Metal roofing and condensation, technical note, 2020.