What is a vapour barrier?

A vapour barrier (more accurately a vapour control layer or VCL) is a membrane installed on the warm side of insulation in walls, roofs, and floors to restrict the movement of water vapour from the warm, moist interior into the building fabric. Without it, warm moist air diffuses into the construction and condenses when it reaches the cold dew-point zone within the insulation - a problem called interstitial condensation, which can saturate and degrade insulation and cause structural rot.

Where should a vapour barrier be installed?

The vapour barrier must always be installed on the warm (interior) side of the insulation - never on the cold side. In a warm flat roof, it goes between the deck and the insulation. In a timber-frame wall, it goes between the insulation and the plasterboard lining. In a floor build-up above a cold subfloor, it goes beneath the insulation. The key rule is to position the VCL so that no significant thickness of cold construction lies between it and the warm interior.

What is the difference between a vapour barrier and a breathable membrane?

A vapour barrier (VCL) resists the passage of water vapour and goes on the warm side of insulation to prevent interstitial condensation. A breathable membrane (also called a breather membrane) has high water vapour permeability but is water-resistant - it lets vapour out but keeps liquid rain out, and it goes on the cold side of insulation (typically on the outside of a timber frame, under the cladding or tiling). Using them in the wrong positions causes serious condensation and moisture problems.

Do I need a vapour barrier in a flat roof?

Yes - a vapour barrier is essential in all warm flat roof construction in the UK. The typical warm flat roof build-up is: deck (structural), then vapour barrier, then insulation, then waterproofing membrane. The VCL is installed on top of the deck and prevents warm moist air from the building rising into the insulation layer. Without the VCL, interstitial condensation will occur within the insulation, significantly reducing its performance and eventually causing the deck to rot.

Thermal & Energy noun

Vapour Barrier

/ ˈveɪpə ˈbarɪə /

Also known as: vapour control layer (VCL), vapour check, vapour retarder

Definition

A vapour barrier (formally a vapour control layer or VCL) is a layer of material with low vapour permeability installed on the warm side of insulation in walls, roofs, and floors. It restricts the movement of water vapour from the warm, humid interior of a building into the building fabric, preventing interstitial condensation - condensation that forms within the construction when warm vapour cools to its dew point.

In practice

Common vapour barrier materials include polythene sheeting (typically 500 gauge / 125 micron), foil-faced PIR insulation boards (where the foil acts as a VCL if joints are taped), and specialist foil laminates. The VCL is only as good as its continuity - any punctures from service penetrations or torn joints will allow vapour to bypass it. All joints must be lapped at least 150mm and taped, and service penetrations sealed with proprietary grommets or tape.

A breathable membrane (breather membrane) is the opposite of a vapour barrier - it is vapour-open and water-resistant, installed on the cold side of insulation (outside a timber frame) to let vapour out while keeping rain out. It is a very common error to install these in reversed positions. In a pitched roof with mineral wool between rafters, the typical build-up is: breathable felt underlay at the top, insulation between rafters, VCL below the rafters, then plasterboard.

Building Regulations

Relevant PartsPart C (moisture), Part L (energy)

Condensation risk assessmentBS EN ISO 13788 (Glaser method) or WUFI dynamic hygrothermal

Position ruleAlways on the warm (interior) side of insulation

Approved Document C requires that construction shall not promote surface or interstitial condensation. Condensation risk must be assessed in line with BS EN ISO 13788 for new construction. Approved Document L indirectly requires VCLs where airtightness is critical to achieving energy targets - the VCL often serves double duty as the air barrier layer. In timber frame and SIPS construction, the VCL position is absolutely critical to moisture safety.

Full Building Regulations guidance

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Vapour Barrier

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