Why is my metal building dripping water inside?
Almost certainly condensation rather than a leak. On a clear calm night a metal roof radiates heat to the sky and cools a few degrees below the outdoor air temperature. It keeps cooling until it reaches the dew point, and at that moment it starts making water out of the humid air touching it. The giveaway is that it drips when it has not rained, it is worst in the early morning, and the water appears across the whole panel rather than in one spot. More vents and more fans do not fix it in Houston, because the air you are bringing in is the wet air causing it. The fix is to remove the cold surface, which means closed-cell foam bonded directly to the panel with no air gap behind it.
On this page
If your shop, barn or warehouse rains on the inside when it is dry outside, nothing is broken. The building is working exactly the way bare steel works in a humid climate. You have a condensing surface over your head.
This is the single most common reason people call us about a metal building, and it is usually described the same way: water on the floor, rust streaks under the screws, boxes damp on top, and a roofer who came out, found no leak, and left.
First, is it condensation or a leak?
Work this out before you spend money, because the two problems have nothing to do with each other.
| Condensation | Roof leak | |
|---|---|---|
| When | Clear calm nights, worst at dawn | During and just after rain |
| Where | Across whole panels, fairly even | One place, every time |
| Fastener heads | Beads of water on many screws | Stain around a few |
| Season | Worst in spring and fall here | Follows the weather |
| Underside | Damp sheen over a wide area | Rust trail from one point |
The clearest test is rain. If the water shows up on a dry night, it is condensation. If it only ever appears when it rains, call a roofer, not us.
You can have both. A building that has been sweating for years often has rust around the fasteners that eventually becomes a real leak, which is the main reason this is worth dealing with rather than living with.
Why steel does this and wood does not
Two things are happening at once, and the second one surprises people.
Steel has almost no insulating value and almost no thermal mass. A panel is a fraction of an inch thick, so it takes the temperature of whatever is touching it, almost instantly. There is no lag and no buffer.
On a clear night the roof radiates heat to the sky and gets colder than the air around it. The sky at night is effectively very cold, and anything with a clear view of it loses heat upward. Field measurements put a roof surface a few degrees below the outdoor air temperature overnight, and studies in dry clear conditions have measured considerably more than that.
Put those together and you get the part that matters:
The panel keeps cooling until it reaches the dew point. Then it stops cooling and starts making water instead.
That is not a malfunction. That is a dehumidifier. You have built a very large, very efficient dehumidifier into your roof, and it drains onto your floor.
It is also why clear, calm, still nights are the worst ones. Cloud cover blocks the radiant heat loss and wind mixes warmer air across the panel, so an overcast breezy night may produce nothing at all.
Why spring and fall are the bad seasons in Houston
Houston's dew point sits in the 70s through the summer, which sounds like it should make July the worst month. It is not, usually, because the panel is also warm.
The problem is the gap between the two. You want the panel well above the dew point, and the dangerous setup is a cool surface meeting warm wet air:
- A cold front passes and chills the building and the slab.
- Two days later Gulf air returns and the dew point jumps 20 degrees overnight.
- The steel, the slab and everything in the building are still cold, and now the air touching them is saturated.
That is the classic spring sweat, and it is why a building can drip for a week in March and behave all August.
Where the water is actually coming from
The dew point inside your building is not the same as the dew point outside, and some of what you do in there makes it worse.
- A slab without a vapor barrier under it. Bare concrete gives off moisture continuously. In an older shop this is often the largest single source.
- Open doors. Every hour a roll up door is open in July you are filling the building with wet air.
- Unvented combustion. Propane heaters, torpedo heaters and forklifts. Burning fuel produces water vapor, and an unvented heater puts every bit of it into the room you are standing in.
- Washing anything. Pressure washing a truck inside is a few gallons of water going into the air.
- Welding, curing concrete, stored green lumber, plants.
None of this is a reason to change how you use the building. It is context for the next section, which is about why the obvious fixes disappoint people.
What does not fix it
This is the part worth reading before you buy anything.
More ridge vents and more fans. Ventilation works by swapping inside air for outside air. In Houston the outside air is the wet air causing the problem. You can move a lot of air through a building and still have every surface below the dew point, because you have not changed the dew point or warmed the panel. The metal building trade's own guidance is that increased ventilation on its own is typically not enough. It is a reasonable supporting measure and a poor primary one.
A dehumidifier on its own. It genuinely helps, and in a conditioned space it may be enough. In a large shop with doors that open, you are dehumidifying the neighborhood. Run the numbers before you commit.
Painting or coating the underside of the panel. Paint does not insulate. The surface is still at the same temperature, so it still condenses. Some coatings hold the water instead of dripping it, which looks like a fix for a season and traps moisture against steel.
Vinyl faced fiberglass batts, which is what most of these buildings came with. They work when they are new, intact and continuous. Fifteen years later the facing is torn, the batts are compressed flat where the purlins pinch them, there are gaps at every lap, and humid air is reaching the panel through every one of them. Then it condenses where you cannot see it and runs down inside the assembly. A building with sagging grey batts is not an insulated building.
What does fix it
Take away the cold surface. There is no other category of answer.
Closed-cell foam sprayed directly onto the inside of the panel does three things at once:
- It removes the air gap. There is no space behind the foam for humid air to reach the steel, because the foam is bonded to the steel. No contact, no condensation.
- It moves the dew point into the foam rather than onto a surface where water can form and run.
- It is a vapor retarder in its own right, so moisture is not passing through it to reach the panel from the room side.
That is why closed-cell is the only foam we will put on a metal skin, and the detail on how it is specified and what it costs is on the metal building insulation page and in what it costs to insulate a metal building.
Open-cell on a metal panel is the mistake to avoid. It is cheaper per inch and it is vapor open, which means moisture travels through it and condenses on the steel behind it, out of sight, where it rusts the panel from the inside. By the time you see a stain the damage has been happening for years. We go through the difference assembly by assembly in open-cell or closed-cell for a Houston home.
Air sealing matters as much as thickness here. Uncontrolled leakage drives wet air into the assembly no matter how much R-value is in front of it, which is a point in favor of a material that seals and insulates in one pass.
If your building is already insulated
Usually the old batts come out. Foam has to bond to the panel to work, and it cannot bond through a vinyl facing, so anything between the foam and the steel defeats the whole mechanism.
That is labor and disposal, and it is a real line on the quote rather than something we discover on the day. If the existing insulation is genuinely intact and continuous, there are cases where a flash of foam over the purlins and a different strategy makes sense, but that is uncommon and we will say so after seeing it.
If the building cannot stop operating while this happens, it can be phased by bay. That is routine and it is covered in insulating a warehouse that cannot shut down.
Frequently asked questions
Can condensation really put that much water on my floor?
Yes. A roof is a large surface and condensation forms across the whole of it, so even a thin film adds up to gallons on a bad night. People routinely assume the volume means a leak, because a leak is the only thing they have seen produce that much water.
Why does it drip from the screws first?
A screw is a piece of metal passing straight through the assembly, so it is a thermal bridge and often the coldest point on the inside surface. Water forms there first and hangs there, which is also why those fasteners rust before anything else does.
Will insulating my metal building stop the sweating completely?
If it is closed-cell, bonded to the panel, at the right thickness and continuous, yes. The failures we get called out to look at are almost always one of those four conditions not being met, most often continuity: a good job everywhere except the areas that were awkward to reach.
Is condensation damaging my building?
Over time, yes, and the fasteners go first. Water sitting on screw heads and panel laps rusts them from the inside out, and a sweating building eventually becomes a leaking building. It also damages whatever you store in there and makes a slab floor slick, which matters if you have people and a forklift in the same space.
My building is not heated or cooled. Is foam still worth it?
Often this is where it pays best, because an unconditioned building has nothing fighting the humidity and no equipment to blame. Removing the condensing surface is the whole job, and it does not require you to start conditioning the space afterwards.
Does the foam need to be covered up in a shop?
Usually there is a requirement for an ignition barrier or thermal barrier over exposed foam, and what applies depends on the occupancy and how the building is used. We include the right covering in the scope rather than leaving it to the inspection, and the general rules are in thermal barriers and ignition barriers.
What to do next
Check it against rain first. If the water appears on clear dry nights, you have condensation and more ventilation will not solve it.
Then book a free assessment or call (281) 703-2700. We will look at the panel from underneath, tell you what is already rusting, and put a scoped number in writing. If it turns out you have a leak rather than a condensing roof, we will tell you that and you should call a roofer instead.