Why Pole Barns in the Bakken Sweat, and How to Stop It

The short answer: A pole barn sweats because warm, moist air inside the building touches roof steel that is as cold as the outside air, and the water in that air condenses on the metal the same way it does on a cold can. The fix is to keep warm air from ever reaching cold steel, and the only material that does that reliably on a metal roof is closed-cell spray foam applied directly to the underside of the panels. Vapor barriers, batts, and extra vents all fall short for reasons we explain below.

Drive any section road between Williston and Watford City in February and you will see them: post-frame shops with a line of icicles along the eave and a dark wet band on the concrete inside the overhead door. The owner has usually already tried a roll of bubble wrap under the purlins or a couple of ridge vents.

Why does my pole barn sweat?

Air holds water vapor, and warm air holds a lot more than cold air. Every air mass has a dew point, the temperature at which it starts to drop its moisture as liquid on whatever surface it touches. In a Williston shop in January, the inside air might be in the 40s or 50s with a truck melting off in the corner, a propane heater running, and two people working. The steel roof panel a few feet above it is at outdoor temperature, which is often well below zero with a north wind pulling heat off the ridge.

The warm air rises, touches the panel, and drops below its dew point in an instant. Water forms on the steel. It runs down the slope and drips off the purlins, or freezes into frost on the underside of the roof. When the sun hits the south slope at midday, the frost melts and the whole roof rains for an hour. It is not a leak and no roofer can fix it.

Metal makes this worse than any other roofing. Steel has almost no insulating value of its own, so the underside of the panel is the same temperature as the outside. It is also perfectly smooth and non-absorbent, so every drop that forms stays a drop and falls instead of soaking in.

Where the moisture comes from

A pickup driven in with snow packed in the wheel wells can shed gallons as it thaws. A propane or kerosene heater that vents into the room puts water into the air as a direct byproduct of burning fuel. New concrete releases moisture for months. Hay breathes. A horse or a few head of cattle in a lean-to add more. Open dirt floors in older barns are a constant source. And in the Bakken, a lot of shops get a hot crew truck parked inside every night all winter, which is a wet cycle every day.

Why a vapor barrier under the purlins fails

The most common thing already installed when we walk into a dripping shop is a layer of reflective bubble insulation or a white vinyl-faced fiberglass blanket laid over the purlins with the steel screwed down through it. In practice it fails three ways.

First, it is not sealed. Every screw, every lap, every purlin crossing, every place the blanket got torn during erection is a path for warm air. Air moves moisture far faster than diffusion ever could, and post-frame buildings are full of air paths. The warm air gets behind the barrier, touches the steel, and condenses in a place you cannot see or reach.

Second, the barrier is on the wrong side of the cold. With the blanket sagging between purlins and the steel above it, the cold face is the top of the blanket itself. Moisture condenses there, soaks the fiberglass, and the wet blanket loses its R-value and starts to stain and sag.

Third, there is no way to make it continuous. A vapor retarder that is not continuous is mostly a tarp.

Ventilation versus insulation

The second thing a lot of owners try is more ventilation: ridge vent, eave vents, a gable fan. In a cold storage building that is never heated, this is actually a reasonable strategy. If the inside air is nearly as cold as outside, it holds little moisture, and a well-vented roof handles the small amount that shows up.

The moment you heat the building, ventilation stops being a solution and starts being a heating bill. You are warming air, loading it with moisture, and blowing it out the ridge. A heated shop cannot be vented out of a condensation problem. It has to be insulated so the steel is never cold on the inside face.

There is a middle case: a shop heated only when someone is working in it. Those buildings sweat the worst, because every heating cycle loads the air and every cool-down dumps the water. If that is your building, treat it as heated for the purpose of this decision.

Closed-cell foam directly on the metal

What works is closed-cell spray foam applied straight to the underside of the roof panels and the inside of the wall steel. It does three jobs at once that no other material manages together.

It is an air barrier. Sprayed foam fills the flutes of the panel, wraps the purlins, and bonds to the steel with no laps, no screws through it, no seams. Warm air cannot get to the metal because there is no path.

It is a vapor retarder. Closed-cell foam reaches that threshold at around 2 inches. At that thickness shop air cannot push moisture through the foam to the cold steel behind it.

It moves the cold plane. With foam bonded to the panel, the inside face of the assembly is the face of the foam, and that face is close to room temperature. The steel is still cold, but it is sealed off from the air that carries the water.

It also stiffens the building and quiets it. A foamed roof takes a lot of the oil-can flex out of the panels in a wind and cuts the drumming in a hailstorm.

Open-cell foam is not the right material here. It is not a vapor retarder and it can hold moisture against the steel. On metal, in this climate, it is closed-cell or nothing. Our pole barn and shop page covers how we approach the roof, walls, and the eave and ridge details on a post-frame building.

What thickness

Two inches gets you the vapor retarder and the air seal, which is the moisture job. That is the right target for cold storage you want to keep dry. For a heated shop, the roof and walls should carry the R-value the building needs to hold heat, which means going thicker. Williams County is Climate Zone 7, and the heat loss through 2 inches on a large roof adds up when it is well below zero with the wind up for a week.

Heated shops versus cold storage

A heated shop (anything with a furnace, a hanging unit heater, in-floor heat, or even a wood stove that runs most days) needs closed-cell foam on the roof and walls at a thickness that reflects the heating load. It also needs the big leaks addressed: the gap under the overhead door, the gaps where the wall steel meets the grade board, and the eave where the roof panel ribs are open to the outside.

A cold storage building (a machine shed, a hay barn, a place the camper and boat live for the winter) can sometimes get by on ventilation if nothing wet goes in and no one heats it. If you park a warm truck in it, or the floor is dirt, or you store anything you care about, 2 inches of closed-cell on the roof is the minimum that will keep the ceiling from raining on your stuff.

If you are not sure which one you have, a building that gets heated “just for a few hours” counts as heated.

What a winter of sweating does

The dripping is the part you see. The damage is the part that shows up in April.

Tools and equipment. Water dripping on a drill press table, a welder, a compressor, or the hood of a tractor all winter rusts everything it touches. Electrical connections corrode. Paint on equipment bubbles at the edges. Anything with a bearing picks up water.

Hay. Condensation dripping into stacked bales soaks the top layer, which molds, which heats, which can spread into the stack.

The building. Water running down the inside of wall steel finds the grade board and the bottom of the posts, which is exactly where you do not want a post-frame building to stay wet. Rust lines form along the purlins and concrete gets a permanent tide line at the door.

The people. A shop that rains on you when the sun comes out is a shop you stop working in. Around here, where a lot of people run a side business out of the shop, that is a real cost.

Getting it done

The best time to foam a post-frame building is before the interior goes in and while it is warm enough for the foam to cure on the steel. That means late spring through early fall in our climate. Retrofits work fine as long as we can clear the walls and roof and get a lift in. We would rather schedule a cold, full shop for spring than spray on frozen steel in December.

We work from our shop in Williston out to Watford City, Tioga, Stanley, Crosby, and across into Sidney and Fairview. If your building is in Watford City or out a section road from it, it is inside our normal range.

Common questions

Will ridge and eave vents stop my pole barn from sweating?

Only in a building that is never heated and stores nothing wet. Venting works by keeping inside air as cold and dry as outside air. The moment you add heat, a thawing truck, or livestock, the inside air carries more water than the vents can remove, and the cold steel condenses it anyway.

Is the bubble-wrap insulation under my roof steel doing anything?

Very little, and it may be hiding a problem. Reflective bubble products have almost no R-value and cannot be sealed around screws, purlins, and laps, so warm air gets behind them and condenses on the steel where you cannot see it. The wet spots on the blanket are the evidence.

Can you spray foam over the existing fiberglass blanket?

No. Foam has to bond to clean, dry steel or it will not seal. The blanket comes out first, along with any wet or stained material and whatever the mice left behind. Removal is part of the quote on most retrofit shops, and it is worth doing right.

How thick does the foam need to be on a cold storage shed?

Around 2 inches of closed-cell on the roof is the threshold where the foam becomes a vapor retarder and stops the condensation. That is the minimum for a building you do not heat. If you heat the building, it needs more for the heat loss, and the walls need attention too.

Does closed-cell foam on the roof stop ice dams at the eave?

It helps a great deal. Ice builds at the eave when heat from inside warms the roof, melts snow, and the water refreezes over the cold overhang. Foam bonded to the panel keeps the roof surface cold and even, so the snow stays snow. Deep drifts on the north side can still need clearing.

If your shop roof rains on you every sunny afternoon in February, send us a quote request through our contact page with the building size and how you heat it, and we will tell you what it needs.