Why condensation forms on commercial roofs
Condensation forms when warm, moisture-laden air meets a colder surface. On a metal or fibre-cement roof, the underside can sit well below the temperature of the air inside the building. When that air cools against the cold deck, it can no longer hold its moisture as vapour, so water droplets form on the surface.
Industrial and agricultural buildings are especially prone because processes, machinery, livestock and washdowns all release large volumes of water vapour into the air. Combined with a thin, uninsulated metal roof, the conditions for condensation are almost permanent through the colder months.
Understanding the dew point
The dew point is the temperature at which air becomes saturated and water vapour turns back into liquid. If any surface in the building falls below the dew point of the surrounding air, condensation will form on it. A cold steel roof deck is usually the first surface to reach that point.
The goal of any condensation control strategy is to keep internal surfaces above the dew point. That means either warming the surface, removing the moisture from the air, or creating a barrier so warm internal air never reaches the cold deck in the first place.
Surface vs interstitial condensation
There are two types to understand. Surface condensation is the visible kind, the droplets and drips you see forming on the underside of the roof sheets. It stains ceilings, drips onto stock and pools on floors.
Interstitial condensation is more insidious. It occurs inside the build-up of the roof itself, where warm vapour passes into the structure and condenses out of sight against a cold layer. Left unchecked it corrodes fixings and purlins, degrades existing insulation and rots timber, often going unnoticed until structural damage appears.
- Surface condensation: visible drips, staining and pooling on internal surfaces
- Interstitial condensation: hidden moisture forming within the roof build-up
- Both accelerate corrosion of steel sheets, fixings and supporting structure
- Both can ruin stock, damage equipment and create slip and electrical hazards
Why traditional fixes often fall short
Anti-condensation paints and felt-backed membranes can buffer small amounts of moisture, but their capacity is limited and they do nothing to address the underlying temperature differential. Loose-laid quilt insulation can sag, leave gaps and trap moisture against the deck, sometimes making interstitial condensation worse.
Increased ventilation helps remove humid air but is difficult to control in large industrial volumes and conflicts with the desire to retain heat. The most reliable approach is to stop warm internal air ever reaching the cold roof deck, which is exactly where spray foam excels.
How closed-cell spray foam stops condensation
Closed-cell spray foam is applied directly to the underside of the roof and self-bonds to the substrate as it cures, forming a continuous, gap-free layer. Because it bonds seamlessly across sheets, laps and fixings, it leaves no cold bridges and no voids where warm air can slip through to the cold deck.
The foam keeps the inner surface warm and well above the dew point, so condensation simply cannot form. As a closed-cell material it also resists moisture movement through the build-up, addressing interstitial condensation at the same time as surface condensation. ThermoFoam uses Enverge closed-cell foam, with Nexseal LE manufacturer-certified (Agrément 09/4610).
- Self-bonds to metal, fibre-cement and most roof substrates as it cures
- Forms a continuous, joint-free layer with no cold bridges or voids
- Keeps the internal surface above the dew point so condensation cannot form
- Resists moisture movement, tackling interstitial condensation too
- Adds structural rigidity by bonding loose sheets and fixings together
What to consider before treatment
A proper survey is essential. The condition of the existing roof, the level of humidity generated inside, the presence of any asbestos and the building's use all influence the right specification. Where asbestos cement sheets are involved, spray foam can encapsulate and stabilise them as part of the works.
Air-tightness also matters. Sealing the roof line as a continuous layer supports better overall building performance and complements the air-tightness expectations set out under Part L of the Building Regulations. A specialist contractor will assess all of this and recommend the correct system for your building.