You walk into your pole barn on a cool morning and the floor is wet. Drips are falling from the roof panels, but it didn't rain. In a lot of metal buildings, that "leak" is the building sweating. After 45 years on roofs, we've seen it in shops, barns and barndominiums. It's the same "beer can effect" we talk about with sheds and containers, only bigger.
Why metal buildings sweat
Condensation needs two things. The Metal Building Manufacturers Association (MBMA) Condensation Fact Sheet spells it out: "Two things must be present for condensation to occur: warm moist air, and cool surface temperatures below the dew point."
Metal roof panels cool off fast at night. When the warm, damp air inside touches the cold underside of the steel, the water comes out of the air and beads up, just like a cold can on a summer day. We covered the shipping container version in Container Rain.
Common moisture sources:
- Wet vehicles, tractors and equipment pulled in after rain
- Animals, manure and stored hay or feed
- Fresh concrete in a new building
- Damp ground under a dirt or gravel floor
The University of Illinois Extension's page on metal-walled building condensation puts it plainly: "If the metal interior temperature is below the dew-point temperature of the interior air, condensation occurs on the wall."
The two kinds of condensation
MBMA separates the sweat you can see from the sweat you can't.
- Visible condensation shows up as water, frost or ice on ceilings, walls, windows, doors and skylights.
- Concealed condensation happens when moisture gets inside the roof or wall system. MBMA lists the signs as "damp spots, stains, mold and/or mildew on walls or ceilings," along with peeling paint and damp insulation.
MBMA calls the hidden kind "the most difficult to deal with and can be the most damaging." It also names the two big costs of trapped moisture: "(1) corrosion of metal components and (2) degradation of the thermal performance of insulation." Rusting purlins and soggy insulation are not cheap fixes.
Metal building condensation control: the three tools
There's no single fix. It takes three things working together.
1. A real vapor retarder. In most climates, insulation in a metal building should have a facing that keeps damp inside air away from the cold steel. MBMA notes that "Plain white vinyl with a perm rating of 1.0 is not an effective vapor retarder, especially in buildings with a high relative humidity."
2. Insulation, done right. More insulation alone won't solve it. Lamtec, a metal building insulation facing maker, says in its Moisture Control Guidelines that "an increase in the insulation levels in metal buildings will not reduce moisture control problems." Thicker insulation can actually leave the roof sheets colder.
3. Ventilation. This is the one people skip. MBMA says, "All metal buildings require some level of ventilation," and warns that a lack of it "can also contribute to condensation problems." The Illinois Extension is even stronger: "It must be stressed that ventilation is the primary means of reducing condensation potential." Lamtec agrees: "In all metal buildings, natural or mechanically powered ventilation is the single most effective means you can take to control moisture in interior spaces."
Pole barn ventilation: intake low, exhaust high
Here's where our usual rule comes in. Moist, warm air rises. Give it a way out up high, at the ridge, and give fresh air a way in down low, at the eaves or lower walls. Then keep the two balanced.
It's the approach we laid out in Shed Ventilation, on a bigger building. A couple of louvers on one end wall isn't a system. Air needs a clear path from low intake to high exhaust.
Where a solar fan fits, and why bigger is not better
A solar fan at the peak helps the natural flow along. On a sunny day it pulls out the hot, damp air that collects under the roof so it doesn't sit there waiting for the steel to cool off at night.
But bigger is not better. A high-CFM fan with weak intake starves for air. It pulls from wherever it can: gaps, cracks, the ridge vent beside it, or the living space in a barndominium. Size the fan to the space and give it intake. Our product pages use a 10'x10'x10' space, or 1,000 cubic feet, as the example:
- The Solar Metal RidgeBlaster is built for metal-roofed sheds, garages and houses with ridge venting. It moves 35.2 cfm, about one air exchange every 28.4 minutes with proper intake ventilation.
- The Solar Mega RoofBlaster moves 96 cfm, about every 10.4 minutes with proper intake ventilation.
A pole barn is far bigger than 1,000 cubic feet, so it may need several fans spread along the ridge plus plenty of low intake. We explain why in Why Solar Blaster Bets on Balance, Not Brute Force.
What about Arizona?
MBMA notes that condensation problems "are most likely to occur in climates where temperatures frequently dip to 35°F or colder over an extended period of time." Most of the Valley doesn't live there. But desert nights cool off fast, the high country gets real cold, and monsoon season brings damp air. Add a wet truck or a fresh slab and a metal building can still sweat.
Climate also changes how the wall and roof should be built. Hansen Pole Buildings, writing about ceiling vapor barriers in post frame construction, says, "In hot dry climates a vapor retarder should also not be installed, but attics can be vented." Get local advice before you add plastic anywhere.
Being honest about the limits
- A fan doesn't replace a vapor retarder or proper insulation. Ventilation is one of three tools, not all three.
- It won't fix a real leak. Loose fasteners, bad flashing and failed seams need repair, not air.
- Heavy moisture loads need more. Livestock barns, wash bays and heated shops may need powered ventilation designed by an HVAC professional. Lamtec says a design professional "should be consulted."
- We won't tell you a fan kills mold or fixes every moisture problem. It doesn't. Clean up existing mold properly and deal with the water source.
The bottom line
Metal building condensation is warm, damp air meeting cold steel. Cut the moisture you bring in, insulate with a real vapor retarder, and ventilate the right way: balanced intake low, exhaust high, with right-sized fans instead of one giant one.
As Daniel Rheaume of Solar Blaster puts it, "Our products move air, which is a natural cleanser. Clothesline fresh."
Questions about your attic? Call us at (480) 747-7097 or email sales@solarblasterfans.com.
