Vapour barrier
A layer on the warm side of the insulation that all but stops vapour diffusing into the build-up. A true barrier starts at an equivalent air-layer thickness of 100 m.
What is a vapour barrier and what does the Sd value mean?
A vapour barrier is a layer that all but stops water vapour diffusing out of the heated interior into the construction. Its performance is stated as the equivalent air-layer thickness, written Sd and expressed in metres: the value says how thick a layer of still air the sheet behaves like. A layer counts as a true barrier from an Sd of about 100 m upwards. Plain polyethylene film sits around 100 m, and aluminium-laminated films reach 1500 m and above.
Sd is never read on its own but as part of an order. The rule is that vapour resistance must fall from inside to outside, roughly by a factor of five or six across the build-up: tight inside, open outside. Diffusion-open external layers, a roof underlay or a wood fibre sarking board, have an Sd of 0.3 m or less, and values below 0.1 m are common for underlays. The barrier is the extreme, tightest member of that chain, and choosing it means accepting that the build-up loses its ability to dry inwards.
| Layer | Typical Sd | Consequence for the build-up |
|---|---|---|
| Roof underlay or open sarking board | 0.3 m or less, often below 0.1 m | Outward drying is possible |
| Vapour retarder | 0.2 to 10 m | Inward drying reserve retained |
| Polyethylene barrier | Around 100 m | Inward drying effectively closed |
| Aluminium-laminated barrier | 1500 m and above | Used only where the moisture load is extreme |
Where does a vapour barrier belong, and which build-ups need one?
A vapour barrier belongs on the warm side of the insulation, that is the inside face, always. The reason is physical: vapour has to be held back before it reaches the depth at which the temperature falls to the dew point. A sheet placed part-way through the insulation, or behind it on the cold side, creates exactly what it was meant to prevent, a plane on which condensate forms.
Build-ups where a full barrier is the right answer share one feature: they cannot dry outwards, so they must not be allowed to get wet from inside.
- Insulated pitched roofs with a vapour-tight covering, typically fully supported sheet metal on solid boarding.
- Warm flat roofs, where the waterproofing above the insulation is effectively vapour-tight.
- Timber-framed walls whose external skin is diffusion-closed.
- Bathrooms, laundries and drying rooms with a permanently raised humidity.
- Swimming pool halls and wellness rooms, where the moisture load is an order of magnitude above a flat.
In a vapour-open build-up a full barrier would needlessly cut off the drying reserve, and there the correct choice is a vapour retarder instead. Specifying a barrier because it sounds safer is one of the most common errors in Slovak timber construction.
Why is one unsealed penetration worse than the whole area?
A barrier works as a whole or it does not work at all. If the sheet is broken by a single unsealed downlight, a socket box or an untaped lap, the moisture flow from the entire wall or roof area funnels into that one point. The damage is not diffuse and mild but local and severe: rot in the rafter directly above the box, and wet insulation in a band a few hundred millimetres wide, discovered years later when the lining is opened.
That dictates the site procedure. Laps are taped with the system tape, pipe and cable penetrations get proprietary collars, and the junction to masonry is sealed with adhesive under a batten. Electrics should not perforate the barrier at all: they run in a service void inside the sheet, which stays intact. That void is the cheapest insurance in the build-up, set against the price of opening a finished ceiling to find a leak.
How is a vapour barrier different from an airtight layer?
The same sheet usually performs both jobs, but these are two different moisture transport mechanisms and confusing them causes real failures.
| Question | Vapour barrier | Airtight layer |
|---|---|---|
| Mechanism | Diffusion of molecules through the material | Bulk air flow through a gap |
| Driving force | Difference in vapour partial pressure | Difference in air pressure |
| Quantity of water moved | Small and calculable | Orders of magnitude larger |
| Property that governs it | Declared Sd of the product | Continuity of the workmanship |
| How it is verified | Moisture assessment to STN EN ISO 13788 | Blower door test on site |
| What it says nothing about | Whether the tape was applied properly | How the build-up behaves in diffusion |
A design must pass both sieves. The build-up has to satisfy the moisture requirements of STN 73 0540 Part 2, and its execution has to satisfy the airtightness measurement. Passing one and failing the other produces a building that looks correct on paper and gets wet in practice.
How is a vapour barrier installed and verified?
The sheet is laid on the warm side with generous laps, taped continuously, and dressed into every junction rather than cut around it. Penetrations are minimised before they are sealed, which is why the service void matters more than the tape: a detail that does not exist cannot leak. The barrier is then inspected while it is still visible, photographed for the record, and verified by pressurisation before the linings close it in.
Timing is the part most often got wrong. Once plasterboard is fixed, a defect in the barrier is no longer repairable at reasonable cost, and the blower door test that would have found it becomes a diagnosis rather than a remedy. Booking the test at the right moment, with the layer accessible, is a scheduling decision made months earlier.
When is a vapour barrier the wrong choice?
Whenever the build-up has a genuine drying route that the barrier would close. A diffusion-open timber wall with a wood fibre sarking board, an old masonry wall finished in lime plaster, a ventilated roof over open sarking: in all of these the design intent is that small amounts of moisture can leave, and a barrier converts an event that would have dried out into permanent damage.
The diagnostic question is therefore not how tight the inner layer should be, but where the moisture in this construction is expected to go. If the answer is outwards, use a retarder and keep the outer layers open. If the answer is nowhere, because the outer layer is metal or bitumen, use a barrier and make it continuous. Getting that question the wrong way round is the root of most interstitial condensation cases found in Slovak roofs.
Frequently asked questions
- What does the Sd value of a vapour barrier mean?
- Sd is the equivalent air-layer thickness in metres: it says how thick a layer of still air the sheet behaves like. A layer counts as a true barrier from about 100 m upwards. Plain polyethylene film sits around 100 m, and aluminium-laminated films reach 1500 m and above.
- Which side of the insulation does a vapour barrier go on?
- The warm side, which in a heated Slovak house means the inside face of the insulation, without exception. Vapour has to be stopped before it reaches the depth at which the temperature falls to the dew point. A sheet placed part-way through the insulation creates exactly the condensation plane it was meant to prevent.
- Is a vapour barrier the same thing as an airtight layer?
- Usually the same sheet performs both jobs, but they are two different transport mechanisms and they are verified differently. Diffusion follows from the declared Sd and from a moisture calculation; air leakage follows from workmanship and is measured with a blower door test. A design has to pass both checks.
- What happens if the vapour barrier is punctured?
- The moisture flow from a whole area of wall or roof funnels into that one point, so the damage is local and severe rather than diffuse and mild. A single unsealed downlight or socket box can produce rot in the rafter directly above it and a band of wet insulation a few hundred millimetres wide.
- Does every house need a vapour barrier?
- No. A full barrier belongs where the external layer is vapour-tight, in timber structures with a closed outer skin, and in rooms with a permanently high moisture load. In a build-up designed to dry out, a barrier removes the drying reserve and a vapour retarder is the correct layer instead.