Condensation & Ventilation

Why it happens

Air holds water vapor, and warm air holds more of it than cold air. When warm humid air touches a surface below its dew point, it cannot hold that moisture any more and the water condenses out onto the surface.

A steel roof panel is an almost perfect version of that surface. It is thin, it conducts heat readily, and on a cold clear night it can drop below the temperature of the air inside the building. The moisture in that air condenses on the underside of the roof and eventually falls.

The humidity does not have to come from anywhere exotic. A concrete slab releases moisture for a long time after it is poured. A vehicle driven in wet brings water with it. So does stored hay, damp ground under a partial floor, and any unvented combustion heater.

The three fixes

1. Vapor barrier

Stop the moist air reaching the cold steel. A continuous vapor barrier on the warm side of the assembly is the most direct fix, and it is the one most often compromised in installation — a barrier with gaps at the seams and penetrations is not continuous, and moisture finds the gaps.

2. Insulation

Keep the steel from getting cold enough to condense on in the first place. Insulation raises the interior surface temperature toward the air temperature, which moves it above the dew point. Faced blanket insulation is the common approach; spray foam is the most effective because it delivers insulation and a continuous air seal at once.

3. Ventilation

Move the humid air out before it can condense. This is the cheapest of the three, works in unheated buildings where insulation is hard to justify, and is the one people install incorrectly.

Getting ventilation right

Ventilation needs a path. Air has to come in somewhere for air to go out somewhere else — an exhaust vent with no intake is trying to pull a vacuum, and it moves very little.

• Exhaust high. Ridge vents or turbine vents at the peak, where warm moist air collects.
• Intake low. Wall louvers, eave vents or gable intakes near the bottom of the building.
• Balance the two. Roughly matched intake and exhaust area. Undersized intake is the usual failure.
• Keep the path clear. Stacked inventory against a wall louver stops it working.

Turbine vents are worth noting because they need no power at all — wind spins them and they exhaust continuously. Paired with adequate low intake, they solve a great many condensation problems on their own.

Unheated buildings

A common assumption is that condensation is only a heated-building problem. It is not. Unheated storage buildings sweat readily, because the temperature swing between day and night is exactly what drives the effect, and there is usually a moisture source inside — a slab, damp ground, or wet equipment.

For unheated storage, ventilation plus a vapor barrier is normally the sensible package. Full insulation is hard to justify if you are not conditioning the space, but doing nothing at all means finding rust on whatever you stored.