From statements to practice
In a high-performance timber structure, nothing is random.
A house is not a mere collection of materials, but a system in which every layer has a precise function. Moisture control is one of the essential components of this system.
Why is a vapour barrier is necessary?
Warm indoor air contains water vapour. During the cold season, this vapour naturally migrates toward the exterior. If it encounters a cold layer within the wall assembly, it condenses.
Condensation does not produce immediate visible effects.
Over time, however, it cand lead to:
- loss of thermal performance
- insulation degradation
- mould growth
- structural damage
The vapour barrier is designed to control this transfer. It is not about “sealing” the house, but about managing vapour diffusion toward the exterior of the structure.
For this reason, it is a mandatory component of both wall and roof assemblies, and the way it is installed is critical to ensuring proper performance.
A high-quality vapour barrier, correctly installed, protects the structure over the long term.
Exterior vapour-permeable layers also contribute to durability; however, their ability to keep the structure dry is significantly lower compared to the interior vapour control layer.
How do we choose the right vapour barrier?
The market offers a wide range of solutions, and selection criteria vary depending on the project type and client requirements.
We typically consider:
- the type of structure (timber frame, mass timber, pitched roof, flat roof)
- the complete wall build-up
- the climatic zone
- the targeted performance level (nZEB, passive house)
- the outcome of the hygrothermal calculation
In certain cases, we use membranes with high vapour diffusion resistance (high Sd value), typically when sufficient outward diffusion cannot be ensured. Their installation requires a higher level of precision and execution discipline; therefore, we recommend their use only by experienced professionals.
In other situations, we opt for intelligent membranes with variable permeability, which allow controlled drying of the structure under specific conditions.
In some projects, a medium Sd value represents the necessary and sufficient solution.
Regardless of the chosen solution, we recommend using certified products, supported by technical documentation that proves their performance, preferably from established brands with a proven track record. Equally important is precise installation, strictly following the manufacturer’s guidelines.
Where is the vapour barrier installed?
The basic rule is clear: the vapour barrier is installed on the interior side of the building envelope, on the warm side.However, correct positioning alone is not enough. Continuity is essential. A membrane interrupted at corners, poorly sealed at structural joints, unsealed at service penetrations, or carelessly perforated cannot deliver the performance stated in the technical datasheet. In practice, proper installation matters more than price, brand, or declared characteristics.
Beyond visual inspection, a blower door test performed before installing the interior finishes helps identify any air leakage or installation errors. This allows timely corrections and ensures the intended performance is achieved.
What happens if it is installed incorrectly?
Improper installation of the vapour barrier leads to uncontrolled moisture accumulation within the structure and insulation layers. Structural elements and materials become exposed to condensation, and the resulting damage often involves substantial repair costs. What may appear as a short-term saving can ultimately compromise the entire project and significantly reduce the return on investment.
Let us build homes that maintain their performance over time.
Correct and complete installation of materials means respecting and honouring the principles of timber construction — the very reasons why these buildings are among the most efficient and comfortable homes for our families.
We simplify, we explain, we deliver. Write to us or give us a call — and let’s build with care for a sustainable future
