Vapour barrier
or vapour retarder.

Cooking, showering, breathing: all of this loads the indoor air with water vapour. ADEME gives telling orders of magnitude: a pan of boiling water releases about 400 g of water per hour, a hot shower 200 g. Ventilation removes part of it; the rest tries to cross walls and ceilings to reach the outside.

During this crossing, the vapour meets increasingly cold layers. Its relative humidity climbs, to the point where it becomes liquid water again in the heart of the insulation or against the roof frame. The CSTB installation guide, published by ADEME in 2023, describes the consequences: materials that lose their performance, weakened timber, mould that spreads into the rooms.

The membrane therefore fulfils two functions, which ADEME distinguishes: reducing the amount of vapour that enters the wall, and cutting off unwanted air leaks through the junctions. Fitted without interruption around the heated volume, it does both. Pierced or poorly connected, it does neither.

The Sd quantifies a material's resistance to the passage of vapour. It is expressed in metres: it is the thickness of air that would slow the vapour as much as the product. It is obtained by multiplying the material's μ coefficient by its thickness: a 12 mm OSB board, with μ = 217, thus shows an Sd of about 2.6 m.

Standard NF DTU 31.2, revised in May 2019 for timber-frame houses, draws the line: a vapour retarder has an Sd of between 1 and 18 m, a vapour barrier an Sd of 18 m or more. So-called humidity-variable membranes have no single value: their Sd changes with the ambient humidity, and may even differ depending on the direction of vapour flow. Their fitting then falls under their Avis technique (technical approval), not just the general rules.

A vapour retarder is not a cut-price vapour barrier. It is chosen when the wall must be able to dry towards the inside, or when its outer layers themselves let vapour through. On an old stone or cob wall, it is the rule, as our guide to insulating walls from the inside explains.

The membrane is fitted on the room side, between the insulation and the plasterboard or any other interior facing. This is where it stops the vapour, before it reaches the cold layers of the wall.

When the insulation is fitted in two layers, the membrane can slip between them, on a condition set by the CSTB guide: the inner layer must not exceed a third of the wall's total thermal resistance, and a quarter in a very cold zone. Thicker, it would put the membrane too close to the cold.

The same guide describes another arrangement: a service void left between the membrane and the plasterboard. It takes ducts and pipes, and protects the membrane if the board is pierced later. In a timber-frame house, NF DTU 31.2 even makes it essential.

On a timber-frame wall, the interior membrane is not chosen in isolation: it depends on what is on the outside. NF DTU 31.2 allows two methods. One requires a vapour barrier with an Sd of at least 18 m. The other, known as the "factor 5", accepts a more open wall provided that the Sd on the inside is at least five times higher than that of the outer layers, bracing board and weather barrier included.

An educational sheet on this standard, published by the Ministry of National Education, gives two examples. A 9 mm OSB (Sd 1.5 m) and a flexible weather barrier (Sd 0.18 m) total 1.68 m: the interior vapour retarder must exceed 1.68 × 5, i.e. 8.4 m. With bracing in fibreboard and a rigid weather barrier, also in fibre, the total falls to 0.46 m, and 2.3 m is enough.

This calculation makes it possible to build so-called breathable walls without exposing the frame timbers to condensation. In our timber-frame wall quotes, the membrane is a vapour barrier bonded at the junctions with a sealing adhesive.

Almost. ADEME considers it necessary, on the heated-volume side, for insulating a timber-frame house, and for the roof spaces (unconverted or converted) of houses in a very cold zone, except unconverted lofts over a concrete floor. Everywhere else, it strongly recommends it for airtightness.

In Savoie and Haute-Savoie, the question therefore rarely arises in that form. Our teams do not ask whether a membrane is needed, but which one: a vapour barrier on a timber frame, a smart (humidity-variable) membrane on an internal lining or a ceiling, a humidity-variable vapour control layer on old masonry that must keep drying out.

Unconverted lofts follow their own logic. ADEME places the membrane on the attic floor, before the insulation is rolled out or blown in. When the ceiling is redone, the CSTB positions it just above the plasterboard, and the hangers that pass through it receive dedicated accessories. Our guide to unconverted lofts covers this case in detail.

And what about the kraft facing on wool rolls?

Many mineral wools are sold with a kraft paper facing. The CSTB guide is clear: on a wall or a rafter slope, this kraft, even when taped on site, is not considered a vapour-sealing layer. It does not replace a continuous, bonded membrane. Most of our internal lining specifications therefore combine a faced wool with a separate membrane, as our comparison glass wool or rock wool points out.

A poorly joined membrane is hardly better than no membrane at all. The defects to watch for come up again and again from one site to the next:

• Strips simply laid over one another. The CSTB requires an overlap of at least 100 mm between two strips, and a compatible adhesive tape to restore continuity.
• Forgotten penetrations. Cables, electrical conduits, ventilation ducts and hangers each receive an accessory: adhesive grommet, tape or sealant-adhesive from the same system.
• Poorly connected windows and doors. On a timber frame, the membrane is folded at least 100 mm into the opening, or connected to the frame by a tape with at least an equivalent Sd.
• Adhesives from another system. The CSTB points out that a membrane and its accessories are assessed together, and that this assessment only holds with those same accessories.
• A sealed membrane on an old wall. Masonry that takes up moisture from the ground must dry towards the inside; an airtight vapour barrier creates a condensation zone there.
• A damp wall or no ventilation. ADEME prohibits insulating a wall showing signs of damp, and the CSTB requires ventilation compatible with an envelope that has become airtight.

The CSTB installation guide, produced on the initiative of the housing, planning and landscape directorate, details the common junctions with diagrams: wall base, intermediate floor, window or door, flue, loft hatch.

We reviewed our 2024-2026 quotes in Savoie and Haute-Savoie: the membrane appears in 143 lines across 36 quotes, and it is never priced on its own. It is part of the per-square-metre price of the work that carries it, with its adhesives and the treatment of penetrations. No quote therefore allows the price of the installed membrane to be isolated honestly, and we do not publish one.

Here are the two items of work that include it most often, with the prices published in our guides:

To compare two quotes, look less at the price than at the description: type of membrane and Sd value, brand and reference, jointing adhesives, treatment of penetrations and of windows and doors. A line that says only "vapour barrier" tells you very little about what will be installed.

The choice of membrane goes hand in hand with the choice of insulation: our guide Thermal insulation: which one to choose? compares the insulation families, notably on how they behave with water vapour.