Vapour-open construction
A build-up whose layers become progressively more permeable towards the outside, so moisture that gets in can dry out. It is not a breathing wall.
What does vapour-open construction actually mean?
A vapour-open build-up is one whose layers become progressively more permeable to water vapour towards the outside. The small amount of moisture that gets into the construction, whether build moisture from the site, winter diffusion or a minor leak, has somewhere to go. The point is not that the wall should pass vapour in quantity, but that no layer should trap it.
The layer-order rule is simple and can be put in numbers. Vapour resistance, expressed as the equivalent air-layer thickness Sd, must fall from inside to outside, roughly by a factor of five or six across the build-up. The inner layer is a vapour retarder with an Sd of roughly 0.2 to 10 m, or a variable membrane moving between about 0.25 and 10 m. The external layers, a diffusion-open sarking board or a roof underlay, have an Sd of 0.3 m or less, and values below 0.1 m are common for underlays. Tight inside, open outside.
| Position | Typical layer | Sd |
|---|---|---|
| Internal finish | Plasterboard or lime plaster on a service void | Low, not counted as a control layer |
| Control plane | Vapour retarder or variable membrane | 0.2 to 10 m |
| Structural layer | Wood fibre or mineral wool between studs or rafters | Negligible |
| Sarking or sheathing | Wood fibre board, open sheathing | 0.3 m or less |
| External membrane | Diffusion-open underlay | Often below 0.1 m |
| Finish | Ventilated cladding or mineral render | Must not close the build-up |
What does vapour-open construction not mean?
This is the part clients need most, because the phrase breathing wall has done more damage than any other in building physics. A vapour-open construction is not a breathing wall: walls do not breathe and air does not flow through them. It does not ventilate the house or change the air in the rooms, it does not replace window airing or mechanical ventilation, it is not automatically healthier, and it does not release the building from having a continuous airtight layer.
The decisive argument is one of quantity. The moisture that passes by diffusion through the entire area of an external wall is a tiny fraction of what a household produces every day by cooking, showering, washing and breathing. Diffusion and ventilation work on completely different scales. Build vapour-open and fail to ventilate, and you get humid air and mouldy corners exactly like anyone else.
Where is a vapour-open build-up the right answer?
It makes sense where the external layer is permeable and the construction has somewhere to dry to. In Slovak practice that means two situations above all.
- A timber frame with wood fibre insulation and a control layer on the inside, where the build-up is designed to dry outwards through an open sarking board.
- Old solid masonry finished in lime plaster, left free to dry in both directions rather than sealed behind a cement render.
It makes no sense, and is an error, where the external layer is vapour-tight: fully supported sheet metal on solid boarding, a conventional flat roof with bitumen membranes, an inverted or ballasted roof. Nor is it used in wet rooms and swimming pool halls, where the moisture load is exceptional. In those cases the right answer is a full vapour barrier and an annual balance assessment to STN EN ISO 13788.
How is diffusion openness different from airtightness?
Diffusion is the slow migration of vapour molecules through the substance of a layer, driven by a difference in partial pressure. Air flowing through a leak transports orders of magnitude more water. Diffusion openness gives no protection whatever against the second mechanism, which is why a vapour-open timber building must still have a continuous airtight layer, taped joints and verification by a blower door test.
| Property | Diffusion openness | Airtightness |
|---|---|---|
| What moves | Vapour molecules through a material | Air, carrying vapour with it |
| Quantity of water | Small | Orders of magnitude larger |
| Set by | The choice of products and their order | The quality of the taping and detailing |
| Checked by | Moisture balance calculation | Blower door measurement |
| Failure mode | Slow accumulation over years | Concentrated wetting at one leak |
The build-up is therefore designed in three steps, none of which substitutes for another: an order of Sd values falling towards the outside, an airtight layer executed without interruption, and ventilation sized for the moisture the occupied house actually produces. A design that gets two of the three right still fails, and it usually fails at the third.
Why is a vapour-open build-up more forgiving?
Because it has a drying reserve, and every real construction eventually needs one. Timber arrives on site with moisture in it, screeds release water for weeks, rain gets in before the roof is closed, and a flashing somewhere will leak at some point in fifty years. A build-up that can dry in at least one direction absorbs these events without consequence.
A build-up sealed on both faces stores them instead, and stores them where nobody will look. That is the honest case for designing vapour-open where the construction allows it: not that it is healthier or more natural, but that it converts a class of unavoidable accidents from permanent damage into a temporary condition. The check that proves it is the annual condensation and evaporation balance, and the failure it prevents is interstitial condensation.
How is compliance demonstrated?
By calculation, not by the description of the build-up. Compliance with the moisture requirements of STN 73 0540 Part 2 is shown through an assessment to STN EN ISO 13788, and the criterion is the annual balance rather than the absence of any condensation. A build-up is not compliant because it is vapour-open, and it is not non-compliant because a small amount of water forms in January.
Two practical consequences follow. First, the calculation is done with the actual products and their declared Sd values, so substituting a membrane on site invalidates it. Second, the assessment belongs in the design stage, when the layer order can still change at no cost, rather than in the dispute stage when it can only be used to assign blame.
Frequently asked questions
- Does a vapour-open wall ventilate the house?
- No, and this is the most damaging misunderstanding in the whole subject. The moisture that passes by diffusion through an entire external wall is a tiny fraction of what a household produces daily by cooking, showering, washing and breathing. Diffusion and ventilation work on completely different scales, and one cannot substitute for the other.
- What is the layer-order rule?
- Vapour resistance must fall from inside to outside, roughly by a factor of five or six across the build-up. The inner layer is a retarder at about 0.2 to 10 m, or a variable membrane at roughly 0.25 to 10 m. External layers such as an open sarking board or a roof underlay sit at 0.3 m or less, and often below 0.1 m.
- Does a vapour-open build-up still need an airtight layer?
- Yes, without exception. Diffusion openness gives no protection against air movement, and air flowing through a gap carries orders of magnitude more water than diffusion through a whole wall. A vapour-open timber building must still have a continuous airtight layer, taped joints and verification by a blower door test.
- Where is a vapour-open build-up the wrong choice?
- Wherever the external layer is vapour-tight: fully supported sheet metal on solid boarding, a conventional flat roof with bitumen membranes, an inverted or ballasted roof. It is also wrong in wet rooms and pool halls, where the moisture load is exceptional and a full vapour barrier is the correct answer.
- Is a vapour-open house healthier to live in?
- Not by itself. Indoor air quality is decided by ventilation, filtration and the emissions of the finishes, not by the diffusion resistance of the wall. A vapour-open build-up is a durability strategy for the construction, and selling it to clients as a health feature sets an expectation the wall cannot meet.