This guide covers what a vapour barrier is, which build-ups need one, which build-ups are better off without one, and how to install it so it actually performs. Everything here is anchored to BS 5250:2021 and the NHBC Standards, so you can specify with confidence rather than guesswork.
What a Vapour Barrier Actually Is
A vapour barrier, correctly called a vapour control layer (VCL), goes on the warm side of the insulation. Its job is to slow water vapour and moist air from inside the heated space moving out into the building fabric, where it can cool, condense and cause damage.
The proper terminology matters when you are reading a datasheet. BS 5250 uses vapour control layer, and where the same sheet is also doing airtightness duty, air and vapour control layer (AVCL). That is the term NHBC uses throughout. "Vapour barrier" and "vapour check" are the trade words, and they are what most people search for, but no membrane on the market stops vapour completely. "Barrier" oversells it, and as you will see below, that overselling encourages the exact design mistake BS 5250:2021 now warns against.
The Five Sheets That Look The Same
| Layer | What it stops | Where it goes |
|---|---|---|
| Vapour control layer (VCL / AVCL) | Water vapour and air moving out of the heated space into the fabric | Warm (inside) face of the insulation |
| Damp proof membrane (DPM) | Liquid water rising from the ground | Under or over the ground floor slab |
| Breather membrane / vapour permeable underlay | Liquid water and wind driven rain, snow and dust from outside, while letting vapour out | Cold (outside) face of the insulation |
| Gas / radon membrane | Radon, methane and other ground gases | Below the slab, as a sealed system |
| Screeding / separating membrane | Nothing. It is a slip and separating layer so screed does not bond or lose water | Between insulation and screed |
Note the third row in particular. A breather membrane is not a low-grade vapour barrier, it is the functional opposite, and the two are separated by a factor of roughly a thousand in vapour resistance. If you see a product advertised as a "breathable vapour barrier", the description is a contradiction and the listing cannot be trusted.
How Vapour Actually Moves
Two mechanisms move vapour through a wall or roof.
Diffusion is vapour pushing through solid material under a vapour pressure difference. Air movement is warm moist air physically travelling through a gap, a lap, a fixing hole or a downlighter cut-out.
Transfer by diffusion happens, but vapour transfer by air movement is very much more significant.
That single fact reorders how you should choose a product. An unsealed membrane with an excellent resistance figure will perform far worse than a modest membrane that is properly lapped and taped. It is why NHBC specifies lap dimensions and fixing centres rather than simply quoting a resistance number, and it is why tape belongs on the same order as the membrane.
Reading The Numbers
Three different figures describe the same property, and all three are in circulation across the category.
| Figure | Unit | What it is |
|---|---|---|
| Vapour resistance | MNs/g | The resistance of the product as supplied. Usually quoted on VCL datasheets. |
| sd value (equivalent air layer thickness) | metres | The thickness of still air that would resist vapour the same amount. Usually quoted on breather and intelligent membrane datasheets. |
| µ value (water vapour resistance factor) | dimensionless | The material's resistance relative to air, independent of thickness. Usually quoted on boards and masonry. |
sd in metres = vapour resistance in MNs/g × 0.2
Divide the sd value by material thickness in metres to get µ.
Applied to the thresholds that matter:
| Product | Vapour resistance | Approx. sd |
|---|---|---|
| Type LR pitched roof underlay (maximum) | 0.25 MNs/g | 0.05 m |
| Vapour permeable wall membrane (BS 5250 band) | 0.25 to 0.6 MNs/g | 0.05 to 0.12 m |
| 500 gauge (125 micron) polythene VCL | Hundreds of MNs/g | Tens of metres |
| NHBC minimum AVCL, warm roof supporting traditional hard metal | 4,000 MNs/g | around 800 m |
Two conclusions fall out of that table. First, breather membranes and vapour control layers are not variants of one product. Second, resistance requirements vary enormously by application, so "high vapour resistance" on its own tells you nothing. What matters is whether the figure suits the build-up you are working on.
When You Need A Vapour Control Layer, Build-Up By Build-Up
Timber Frame Walls
Yes. Standard build-up, inside to out: plasterboard, VCL, studs with insulation between, sheathing board, breather membrane, ventilated cavity, outer leaf. The VCL and the breather membrane work as a pair, high resistance inside and low resistance outside. NHBC accepts 500 gauge polythene, vapour control plasterboard, or a third-party assessed proprietary membrane.
Pitched Warm And Hybrid Roofs
Yes, and NHBC requires it. AVCLs should be placed on the warm side of the insulation in warm and hybrid roof construction. This is the loft conversion build-up, with insulation at or following the rafter line. NHBC treats a missing continuous AVCL as something you have to ventilate your way out of: where a continuous air and vapour control layer is impractical to install, an additional 25,000mm²/m of eaves ventilation and 5,000mm²/m of ridge or high level ventilation should be provided below the underlay.
Cold Pitched Roofs
Usually not a separate membrane. With the insulation on the flat ceiling, the job here is a well-sealed ceiling plus loft ventilation. NHBC distinguishes a normal ceiling, typically 300 mm²/m² air permeability, from a well-sealed ceiling conforming to BS 9250 at not more than 30 mm²/m². Moving from one to the other roughly halves the ventilation required.
Where the ceiling below a cold pitched roof includes an air and vapour control layer, the design should ensure adequate ventilation is provided to the habitable areas. Sealing the ceiling without addressing ventilation to the rooms below moves the problem rather than solving it.
Warm Flat Roofs
Yes, and this is the highest-consequence application in the category. The build-up runs deck, AVCL, insulation, waterproofing. Warm flat roofs do not incorporate ventilation, so they rely entirely on the AVCL to avoid interstitial condensation. NHBC's language is firm: a good quality AVCL must be used, all laps, joints and penetrations fully sealed, and the AVCL fully checked for damage just before it is covered over. Where the AVCL is penetrated by mechanical fixings it should be self-sealing, for example a self-adhesive aluminium foil-backed modified bitumen membrane.
For warm roof constructions supporting traditional hard metal roofs, NHBC 7.1.10 sets a minimum vapour resistance of 4,000 MNs/g, fully supported. Polythene is not the answer on a flat warm deck.
Cold Flat Roofs
Discouraged. Where used, BS 6229:2018 and BS 5250:2021 recommend a minimum 50mm air gap between the insulation and the cold deck, with cross-ventilation to the void equivalent to a continuous opening of not less than 25mm on each side. The AVCL goes on the warm side and is fully sealed at laps, penetrations and abutments.
Inverted Warm Roofs
No separate AVCL. The waterproofing layer also acts as the AVCL.
Internally Insulated Solid Masonry Walls
Often no, and this is where a lot of published advice is now out of date. BS 5250:2021 states that the warm-side vapour barrier recommended by its own earlier editions can prevent the wall drying, creating rot and mould in joist ends. The standard instead recommends allowing the wall to dry in both directions. Solid wall retrofit needs a condensation risk assessment and usually a vapour-open system, not 500 gauge polythene.
Agricultural, Industrial And Unheated Buildings
Different job. Here the membrane is doing anti-condensation duty rather than working towards Part C compliance in a heated dwelling, and the specification is far simpler.
Put the materials with the highest vapour resistance on the warm side and the lowest on the cold side. Where insulation is built up in more than one layer, the thinnest layer goes on the warm side. Where different insulation materials are combined, the one with the greater vapour resistance goes on the warm side.
The Benefits Of Getting It Right
- Protects the structure. Interstitial condensation attacks timber studs, rafters, joist ends and sheathing boards, in a location where nobody sees it until the damage is done.
- Protects the insulation. Insulation that gets damp does not deliver the performance it was specified to deliver, so the VCL protects the money already spent on the boards or rolls.
- Contributes to airtightness. A membrane that is lapped, taped and sealed at penetrations is doing airtightness work as well as vapour work, which is why the standards call it an air and vapour control layer.
- Reduces mould risk in the fabric. Keeping moist internal air out of the cold parts of the build-up removes the conditions mould needs.
- Supports compliance. Building Regulation requirement C2 covers protection from interstitial and surface condensation, and it is far cheaper to detail correctly at first fix than to open a roof back up.
- Low cost, high consequence. Membrane and tape are among the cheapest items in the build-up and among the most expensive to have got wrong.
Worth stating plainly: every one of those benefits is conditional on installation. The product on its own does very little.
The Regulations, Briefly
In England, the driver is Building Regulation requirement C2, which requires walls, floors and the roof to adequately protect the building and its occupants from ground moisture, precipitation, interstitial and surface condensation, and water spillage. Approved Document C sets the principle and hands the detail over, stating that a roof will meet C2 if it is designed and constructed in accordance with BS 5250, with paragraph 5.34 doing the equivalent for walls. Approved Document F covers ventilation and Approved Document L covers fabric performance. Scotland works to the Building Standards rather than the Approved Documents.
BS 5250:2021 came into effect on 31 July 2021, superseding BS 5250:2011+A1:2016, with a corrigendum in October 2021. It is a code of practice, so it takes the form of recommendations rather than requirements, and it says of itself that it should not be quoted as if it were a specification. Anyone deviating from it should be able to justify the alternative.
The 2021 revision was a full rewrite, with major changes to the clauses on floors, walls and roofs, and particular emphasis on managing moisture risks in buildings undergoing energy saving measures. It also introduces a framing worth borrowing: the distinction between a building as designed in theory and as built or in service. Very few buildings escape residual construction moisture and imperfect workmanship, so detail generously rather than to the minimum the calculation allows.
Vapour control layers themselves are covered by the product standard BS EN 13984, plastic and rubber vapour control layers, and should carry CE or UKCA marking against it. Ask for the Declaration of Performance if a listing does not show one.
Installing It So It Works
NHBC clause 7.2.15 is the most specific published guidance in the category, and five points decide the outcome.
The AVCL sits on the warm side of the insulation, behind the plasterboard and inside the insulation zone.
Joints need 100mm minimum laps, should be located on rafters, and may be sealed with adhesive tape for enhanced airtightness. Joints should still occur over rafters even where taped.
Fixed at 250mm centres to framing members, including at laps and around openings.
Seal around service penetrations, specify and seal downlighters to limit air leakage, and lap the membrane into openings such as roof windows and dormers. Downlighters are the classic loft conversion failure point.
Install once the building is weathertight and once framing timbers are below 20% moisture content, and make good any damage. Sealing wet timber behind a membrane manufactures the problem the membrane was bought to prevent.
Where vapour control plasterboard is used instead of a membrane, which is the other route NHBC accepts, joints should be positioned on rafters, cut carefully to avoid displacing the vapour control material, then filled, taped and finished.
One detail that is easy to miss: access hatches to cold roof voids should have an air leakage rate of not more than 1m³/h at 2 Pa when tested to BS EN 13141-1, or use a push-up cover weighing at least 5.5kg compressing a closed cell seal. See our loft hatches range.
How Much To Allow
With 100mm laps, a 4m wide sheet loses roughly 2.5% of its width per lap, so nominal coverage is close to actual on a long clear run. Real waste comes from cutting around openings, dormers and rafter ends. A 10% contingency is reasonable on a simple ceiling. On a cut-up loft conversion, allow closer to 15%.
Products That Do The Vapour Control Job
A separate membrane is not the only route, and several products already on site do the same work:
- Polythene vapour barriers. 500 gauge (125 micron) polythene is the accepted specification for timber frame walls and pitched warm and hybrid roofs. Green tinted sheet is translucent, which lets the installer see the framework through it during fixing.
- Vapour control plasterboard. Boards with a metallised polyester film on the reverse deliver board and VCL in one fixing, and this is the alternative route NHBC accepts in place of a membrane.
- Foil-faced PIR boards. The foil facing on PIR insulation acts as a vapour control layer within the build-up.
- Multifoil insulation. Multifoils function as a vapour control layer once all laps and joints are sealed, which is a condition rather than a footnote.
- Foil and membrane tapes. Foil tape is what turns a sheet of plastic into a functioning air and vapour control layer. Order it at the same time.
Still Not Sure Which Layer You Need?
Get the build-up right on paper before anything goes on the wall or the rafter. If you know your construction and want to check you are ordering the correct membrane, the team can talk it through on 03003 034 578.
