Anti-Condensation Roof Films
Anti-condensation roof films capture and temporarily retain condensation beneath metal roofing in an absorbent fibrous layer. They are commonly used in unheated or partially heated buildings to help prevent condensation from dripping onto the structures below.
How do anti-condensation roof films work?
When warm, moisture-containing air meets a cold metal roof, condensation can form on the underside of the roofing. An anti-condensation membrane uses a fibrous, felt-like layer to absorb these droplets before they can drip onto the structures below.
The moisture remains within the fibrous layer temporarily. As conditions become warmer and ventilation moves air through the roof space, the retained moisture evaporates and is carried away. Anti-condensation roof films are commonly used:
- under metal roofing;
- in unheated or partially heated buildings;
- in industrial and agricultural buildings.
An anti-condensation membrane does not perform the same function as a vapor-permeable diffusion membrane. It is designed to manage condensation forming beneath the roofing, not to allow water vapor to escape from thermal insulation. For this reason, it should not be installed directly against the insulation. A ventilated air gap is required between the membrane and insulation.
Where are anti-condensation roof films used?
Anti-condensation roof films are particularly useful in buildings where metal roofing is installed above spaces that are unheated or only partially heated. Metal responds quickly to changes in temperature, increasing the likelihood that condensation will form on its underside when conditions change.
Typical applications include warehouses, hangars, garages and agricultural buildings. These structures often provide enough space to incorporate the ventilation needed for the membrane to dry after absorbing condensation.
Anti-condensation membranes are not intended for wall structures.
How does an anti-condensation membrane manage moisture?
The moisture-control process changes as roof conditions change:
- during cold conditions, condensation forms on the underside of the metal roofing;
- droplets are captured and absorbed by the fibrous layer of the membrane;
- warmer temperatures and ventilation allow the retained moisture to evaporate;
- ventilation carries the resulting humid air out of the roof space.
This cycle allows the membrane to temporarily manage condensation without allowing droplets to fall directly onto the structures below. For insulated roofs, a vapor-permeable diffusion membrane is generally used instead because it allows water vapor to move out of the insulation.
Why is a ventilated air gap required?
Because anti-condensation roof films are not vapor-permeable, they should not be installed directly against thermal insulation. Air needs to circulate beneath the membrane so the moisture stored in its fibrous layer can dry between periods of condensation.
Without adequate ventilation, the membrane may remain damp and have less capacity to absorb additional condensation. Providing the required air gap can also make the roof design more complex than a system that uses a vapor-permeable diffusion membrane.
How much condensation can the membrane hold?
The fibrous layer can absorb and temporarily retain condensation, but its capacity is not unlimited. The membrane needs an opportunity to dry before additional moisture exceeds the amount its fibers can hold.
If condensation forms faster than the retained moisture can evaporate, the membrane can become saturated and water may begin to drip onto the structures below. This is more likely where ventilation is inadequate or indoor humidity remains high.
The membrane should therefore be selected with the expected moisture conditions and ventilation of the building in mind.
Why are anti-condensation roof films a good option?
Anti-condensation roof films can provide a simpler, more economical way to manage condensation beneath metal roofing in large unheated or partially heated buildings. Instead of using a more complex moisture-control system designed for an insulated roof, the film captures condensation where it forms and releases the moisture as ventilation conditions allow. In warehouses, hangars and agricultural buildings, the ventilation space required for this drying process can usually be incorporated into the roof design without significantly complicating construction.
Insulated residential roofs have different moisture-control requirements. Vapor-permeable diffusion membranes are generally preferred because they allow water vapor to escape from the insulation. Using an anti-condensation membrane in an insulated roof requires additional ventilation space and careful consideration of how moisture will leave the roof structure.