How to Stop Condensation on a Corrugated Metal Roof

Why Condensation Forms on Metal Roofs

Condensation on corrugated metal roofs is not a defect in the roofing — it is basic physics. Metal is an excellent conductor of heat. On a cold night or during a cold morning, the metal roof panels lose heat rapidly to the sky by radiation. The roof surface temperature drops below the dew point of the air inside the building. When warm, moist air from inside the building contacts the cold metal surface, the moisture it carries condenses into liquid water on the underside of the roof sheets.

This condensation drips onto everything below — machinery, stock, vehicles, electrical equipment, and workers. In agricultural buildings, condensation dripping on livestock causes stress and increases disease risk. In warehouses, it damages packaging and contents. In workshops, it causes tools and metal components to rust. The problem is worst in buildings that generate internal moisture (livestock respiration, washing processes, swimming pools) and in buildings with large temperature differences between inside and outside.

The Dew Point Explained

The dew point is the temperature at which air becomes saturated and can no longer hold its moisture as vapour. When a surface is at or below the dew point temperature of the surrounding air, moisture condenses on that surface. For example:

At 20°C and 60% relative humidity (typical conditions inside a heated commercial building), the dew point is approximately 12°C. If the metal roof surface temperature falls below 12°C, condensation will form. On a winter night when outside temperatures are 0–5°C, an uninsulated metal roof will be at approximately the same temperature as the outside air — well below the dew point of the internal air. The condensation is unavoidable without intervention.

The solution, therefore, is to either: (a) reduce the moisture content of the internal air so the dew point is lower than the roof surface temperature, (b) raise the roof surface temperature above the dew point by insulating it from the cold outside air, or (c) both.

Solution 1: Ventilation

Increasing ventilation replaces warm, moist internal air with drier external air, lowering the internal dew point. Ridge vents, eaves vents, and mechanical extract fans are used to create airflow through the building.

Advantages

• Lowest upfront cost
• Simple to install retrospectively
• Effective in buildings with moderate moisture loads

Disadvantages

• Ventilation means heat loss — you are deliberately exhausting warm air from the building and replacing it with cold outside air. In heated buildings, this directly increases energy consumption.
• Ventilation alone is often insufficient in high-moisture environments (livestock buildings, swimming pools, wash-down areas) where moisture generation exceeds the rate at which ventilation can remove it.
• Does not address the fundamental problem: the metal roof is still cold, still below the dew point for much of the year, and will still condense moisture whenever ventilation rates drop (overnight, weekends, during calm weather).

Solution 2: Traditional Insulation

Traditional insulation methods for corrugated metal roofs include anti-condensation felt (bonded to the underside of the roof sheets at manufacture), mineral wool draped between purlins, and rigid board insulation fixed to the underside of the purlins.

Anti-Condensation Felt

Anti-condensation felt (also called condensation control fleece) is a thin fibrous membrane bonded to the underside of new metal roof sheets during manufacturing. It absorbs condensation droplets and holds them until conditions change and the moisture evaporates. It does not prevent condensation — it manages it. Over time, the felt becomes saturated, contaminated with dust and debris, and loses its ability to absorb moisture effectively. It cannot be retrofitted to existing roof sheets.

Mineral Wool Between Purlins

Mineral wool batts draped between purlins provide some insulation but create significant problems in condensation-prone environments. Mineral wool absorbs moisture — when condensation forms on the metal roof above, the wool becomes saturated, loses its insulating properties, adds weight to the purlin fixings, and creates a constantly damp environment that accelerates corrosion of the steel structure.

Rigid Board Below Purlins

Rigid insulation board (PIR or EPS) fixed to the underside of the purlins provides better thermal performance than mineral wool but creates a cavity between the board and the metal roof where condensation continues to form. This trapped condensation corrodes the purlins and fixings, and the insulation boards themselves can delaminate over time as adhesives fail in the damp conditions.

Solution 3: Closed-Cell Spray Foam

Closed-cell spray foam insulation (Nexseal LE) is applied directly to the underside of the corrugated metal roof sheets. The foam expands into every corrugation, around every purlin, and over every fixing — creating a continuous, gap-free insulation and vapour barrier layer bonded directly to the metal surface.

This approach is fundamentally different from the other solutions because it addresses both the thermal and the moisture problem simultaneously:

Thermal barrier: The foam raises the internal surface temperature above the dew point. With 40mm of Nexseal LE (thermal conductivity 0.027 W/mK), the internal surface of the insulation stays warm even when the external metal surface is at 0°C. Condensation cannot form on a surface that is above the dew point.

Vapour barrier: Closed-cell foam has very low vapour permeability. Warm moist air from inside the building cannot pass through the foam to reach the cold metal surface behind it. This eliminates concealed condensation — the type that corrodes structures and degrades other insulation types from behind.

No cavity: Because the foam is bonded directly to the metal, there is no cavity or air gap where condensation can form and accumulate. This is the critical difference from rigid board and mineral wool systems, which always create a condensation-prone cavity between the insulation and the metal.

Advantages

• Eliminates condensation completely (not just manages it)
• Provides thermal insulation and vapour barrier in one application
• Conforms to corrugated profiles — no gaps or bridges
• Bonds directly to metal — no cavity for hidden condensation
• Structural reinforcement of roof sheets
• Single-visit installation, no scaffolding typically required
• 25+ year service life

Disadvantages

• Higher upfront cost than ventilation or anti-condensation felt
• Requires professional installation by trained operatives
• Must be coated with DC315 or covered with non-combustible lining where TGD B fire safety requirements apply

Cost Comparison

While spray foam has the highest upfront cost, it is the only solution that completely eliminates condensation rather than merely managing or reducing it. When the cost of condensation damage (corrosion repairs, stock damage, cleaning, maintenance) is factored in, spray foam frequently has the lowest total cost of ownership over a 20-year period.

Which Solution Is Right for Your Building?

The right solution depends on your building's use, moisture load, and budget:

Ventilation only: Suitable for unheated storage buildings with low moisture loads where some residual condensation is acceptable.

Ventilation + spray foam: The ideal solution for heated buildings, livestock buildings, and any application where condensation must be completely eliminated. Ventilation provides fresh air and reduces internal humidity; spray foam prevents condensation on the cold roof surface and reduces heat loss.

Spray foam alone: Effective in buildings with moderate moisture loads where the foam's vapour barrier properties are sufficient to manage the internal humidity without additional ventilation beyond normal air exchanges.

Duratite offers free site surveys to assess your building's condensation problem and recommend the most effective and economical solution. Email info@duratite.ie.