# Vapour Retarder (Vapour Control Layer)

> A layer placed on the warm side of roof insulation to limit water vapour moving from the building into the roof, where it could condense.

- URL: https://roof.wiki/vapor-retarder/
- Topic: Low-Slope & Flat Roof Systems
- Also known as: vapour control layer; VCL; vapor barrier; vapour barrier; air barrier
- Regions: Global
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Vapour Retarder (Vapour Control Layer)." roof.wiki. https://roof.wiki/vapor-retarder/

## Key facts

- **Class I permeance:** 0.1 US perm or less
- **UK term:** Vapour control layer (VCL)
- **UK condensation standard:** BS 5250 (referenced by BS 6229)
- **Placement (cold climate):** Warm side of insulation

A vapour retarder, called a vapour control layer (VCL) in the UK and often a vapour barrier, is a membrane or coating with low permeability to water vapour. In a roof it limits the amount of indoor moisture that diffuses upward into the insulation, where it could condense on the cold underside of the membrane, wetting insulation, corroding fasteners and degrading adhesives.

## Classes

North American practice groups retarders by permeance measured to ASTM E96:

- **Class I:** 0.1 US perm or less (about 5.7 SI perm or less), such as polyethylene sheet, sheet metal, rubber membranes and bitumen membranes
- **Class II:** 0.1-1 perm
- **Class III:** 1-10 perm
- **Permeable:** over 10 perm

## Roof materials used

- self-adhered or torched modified bitumen sheets
- hot-applied asphalt with felts on concrete decks
- polyethylene or reinforced foil-faced sheets
- aluminium-faced bitumen VCLs common in Europe

## Where they are needed

Need depends on climate, indoor humidity and the roof type. In heating-dominated climates the retarder goes on the interior side of the insulation; in hot, humid climates exterior vapour drive matters more. Buildings with high indoor humidity such as pools, laundries and food plants usually need robust retarders. In the UK, BS 6229 refers to BS 5250 for condensation risk analysis and uses an external temperature of minus 5 C over 60 days of the heating season.

## Vapour retarders and air barriers

A vapour retarder slows diffusion; an air barrier stops air leakage. Air movement carries far more moisture than diffusion, so on roofs, especially mechanically attached roofs over steel decks, the retarder layer is often designed and sealed to act as an air barrier too. Inverted roofs place the membrane itself on the warm side, so it serves this function.

## Sources

1. Vapor barrier — Wikipedia. https://en.wikipedia.org/wiki/Vapor_barrier
2. BS 6229:2018 and flat roofing systems — IKO. https://ikogroup.co.uk/?p=820
3. Built-Up Roofing (BUR): What It Is, Types, Issues & More — RoofOnline. https://roofonline.com/built-up-roofing-introduction/
4. Fit for Mod-Bit — Roofing Contractor / ARMA. https://roofingcontractor.com/articles/95892-fit-for-mod-bit