Airtightness and Vapour Membranes: Insulation’s Invisible Layer

The invisible layer

Two facts of building physics drive everything on this page. First, warm air holds more moisture than cold air. Second, when warm, moist air meets a cold surface, that moisture condenses. Insulate a wall or roof and you deliberately create a temperature gradient through the build-up — warm on the room side, cold on the outside. If moist indoor air can get inside that build-up, it will find a cold surface partway through and condense there, invisibly. The trade calls it interstitial condensation.

The vapour control layer (VCL) and the airtightness detailing exist to stop that air getting in. Kitchens, bathrooms and even breathing load the indoor air with moisture every day; the membrane is what keeps that moisture on the room side of the insulation, where ventilation can remove it.

What a vapour control layer actually does

A VCL is a sheet material — polyethylene membrane, or the foil facing bonded to boards like PIR insulated plasterboard — installed on the warm side of the insulation. Its job is to resist water vapour diffusing from the room into the build-up.

But diffusion is the minor route. The major route is air leakage: a 5mm gap at a board joint or a socket cut-out lets through far more moisture-laden air than will ever diffuse through the intact sheet. This is the detail most failed jobs get wrong — the membrane was fitted, but nobody sealed it. A VCL with untaped laps and untaped service penetrations is largely theatre.

Airtightness tapes and sealing — where jobs are won or lost

The unglamorous consumables are the ones that matter. On a properly detailed job you will see:

• Lap and joint tapes — every membrane overlap and board joint taped with purpose-made airtightness tape, not packing tape, which lets go within a couple of years.
• Perimeter sealing — the membrane sealed to floor, ceiling and adjoining walls with flexible airtight sealant or gaskets, because the edges are where air sneaks around an otherwise perfect sheet.
• Penetration details — grommets or patch-taping around cables, pipes and socket back-boxes. Every socket cut into an insulated wall is a hole in the airtight layer; on our internal wall jobs, moving and resealing sockets is routine work (electrical adjustments appear on 37 of our quotes).
• Window and reveal junctions — taping the membrane to the frame so the airtight line is continuous where wall meets window.

An installer's tell: dust and surface preparation. Airtightness tape only bonds long-term to clean, sound, often primed substrates. A crew that tapes over dusty brick without primer has fitted tape that will peel off inside the wall — you just will not know for a few years.

Intelligent membranes

A fixed-resistance polythene VCL is one-way thinking: it blocks moisture getting in, but also blocks any moisture already in the build-up from drying inwards. Intelligent (humidity-variable) membranes solve this: their vapour resistance changes with the surrounding humidity — vapour-tight in winter, when the pressure is from inside pushing out, and vapour-open in summer, when the build-up wants to dry back into the room.

We specify intelligent membranes where drying capacity is precious: rafter-level and room-in-roof insulation, timber-framed walls, and internal wall insulation on older solid brick where some seasonal moisture movement is inevitable. On fully breathable build-ups — wood fibre with lime plaster, the approach in our breathable insulation guide — the strategy is different again: the whole wall manages moisture by staying vapour-open, and a plastic VCL would be wrong.

Where the VCL goes: IWI vs warm roof

Internal wall insulation

In IWI, the VCL goes on the room side of the insulation, directly behind the plasterboard — warm side, always. With foil-faced PIR insulated plasterboard the facing is the VCL, which is why the joints must be taped and the perimeter sealed rather than relying on the board alone. Get it backwards — membrane against the cold masonry — and you have built a condensation trap that collects water on the wrong side of the insulation.

Warm roof

In a warm flat roof, the insulation sits on top of the deck, and the VCL goes on the deck, underneath the insulation — again the warm side — with the waterproofing above. The commonest warm-roof error we see is a beautifully waterproofed roof with no VCL under the insulation: watertight from above, condensing from below.

Same rule both times: the vapour control layer belongs on the warm side of the insulation. Only the geometry changes.

What failure looks like, years later

Missed or badly sealed vapour layers do not fail loudly. The job looks perfect at handover, and for the first winter or two the wall itself is still warm from the room. Then the pattern starts:

• Mould blooming along board joints and screw lines — the cold, leaky paths;
• A musty smell in rooms that never smelled before insulation;
• Damp staining at skirting level, where condensate inside the build-up drains down and puddles;
• In roofs: drips in cold snaps mistaken for a leak, and eventually stained ceilings and soft, rotten timber.

By the time it is visible, the fix means opening up the build-up — which is why we treat the invisible layer as non-negotiable, and why condensation control appears on 54 of our quotes and damp or mould remediation on 32. A worrying share of that remediation work is putting right insulation someone else fitted without moisture control. Our condensation, damp and mould guide covers the diagnosis side in detail.

The honest caveat: airtightness without ventilation causes the problems it should prevent

Here is the trap. Seal a leaky old house properly and you also seal in the moisture its draughts used to carry away. All the water vapour from cooking, showers and breathing now stays indoors — and it will condense on whatever is still cold: window frames, the uninsulated party wall, the cupboard on an external corner. We have seen airtight, well-insulated rooms develop mould that the draughty original never had.

The rule we work to: build tight, ventilate right — and plan both at the same time. Extract fans that actually move air, background ventilation, and in some homes continuous mechanical ventilation. It is why ventilation and extract works appear on 70 of our quotes — not as an upsell, but because airtightness and ventilation are two halves of one moisture strategy. The full picture is in our ventilation after insulation guide.

If you are planning wall or roof insulation and want the invisible layer designed properly rather than hoped for, get a free, fixed, itemised quote — every membrane, tape and extract fan we intend to fit appears as a line you can read.

Airtightness & vapour membrane FAQs