Vapour barrier and airtightness per BR18
From sealing class to tight joints — how the climate barrier becomes tight
Introduction
A vapour barrier has two jobs at once: it must slow the moist indoor air from penetrating the insulation and condensing there, and it must make the climate shell airtight so heat does not escape through cracks. Both have become increasingly important as houses are insulated thicker and heated more expensively. A hole in the vapour barrier the size of a ballpoint pen can let large amounts of moist air escape over a heating season.
Why air tightness matters
- 01Energy — leaks cause uncontrolled air exchange and heat loss that district heating or heat pump must cover
- 02Moisture — warm, humid indoor air that seeps out, cools and condenses in the construction and can cause rot and mold
- 03Comfort — cold draughts along floors and at joints are perceived as poor indoor climate
- 04Sound and smell — a tight structure also dampens sound and odour transmission between rooms and building parts
How to install the vapour barrier
The vapour barrier is always installed on the warm side of the insulation — that is, towards the dwelling. The sheets are laid with sufficient overlap, and all joints are taped or clamped tight. A widespread and robust solution in pitched roofs is to screw a batten layer of approximately 45–70 mm to the underside of the rafters and clamp the film tight against it. The batten layer also forms an installation layer where electrical cables and pipes can be run without perforating the vapour barrier itself.
What BR18 requires
The Building Regulations set requirements for buildings' air-tightness to ensure both energy consumption and a healthy indoor climate. The tightness is documented by a pressure test (blower door test) where the building is put under a pressure of 50 Pascal and the air exchange is measured. The requirement is expressed as an allowable air permeability through the climate envelope at this pressure difference. The building owner can demand a pressure test as documentation and a tight structure is a prerequisite for reaching the building's energy framework.
| Typical error | Consequence |
|---|---|
| Poor overlap or bad tape | Damp air seeps out and condenses |
| Missing clamping to purlin/rafter | The foil moves and opens the joint |
| Perforations from nails and screws | Many small leaks add up to one big loss. |
| Missing connection to adjacent structure | Leaks at floor, wall and penetrations |
| Incorrect foil type | The vapor brake does not fit the construction |
The installation layer – cheap insurance
One of the best habits a carpenter can develop is to keep installations outside the vapor barrier. By placing electrical and plumbing in a separate installation layer on the warm side of the film, you avoid the many perforations that outlets, spots, and piping routes would otherwise create. This makes the structure both tighter and easier to build correctly — and it saves large amounts of taping work.
Air tightness is ultimately a matter of care. Materials are rarely the problem; it is the joints, penetrations and connections that determine whether the house passes the pressure test. Therefore it is a discipline where the thorough carpenter really makes a difference to the building's durability and operating economy.