A frequently asked question when selecting a bituminous vapour barrier is whether there is a major difference between V3 and V4. The short answer: no, functionally they are very similar, but there are some technical nuances that can be decisive in certain situations.
According to Buildwise TV 280 (table 13), both V3 and V4 bituminous vapour barriers belong to the same E3 class. This means that, from a normative and performance perspective, they are equivalent in their application as vapour barriers.
So where is the difference?
The differences are mainly found in three aspects:
1. Thickness
- V3: 3 mm
- V4: 4 mm
The greater thickness of a V4 results in a more robust product, which is particularly relevant for less even substrates.
2. Vapour diffusion resistance
For IKO Base products (V3/V4 T/F 10 m), we see:
- V3: µ ≈ 150
- V4: µ ≈ 200
The V4 therefore has a slightly higher vapour diffusion resistance, but this difference is generally not decisive for the overall roof performance.
3. Substrate and roughness
In practice, the choice between V3 and V4 is often determined by the roughness of the substrate. TV 280 (table 9, page 43) provides clear guidelines:
- b ≤ 2 mm → use of V3
- b ≤ 3 mm → use of V4
- b ≥ 4 mm → application with hot bitumen or cold bituminous adhesives
b = flatness deviation
In other words: the rougher the substrate, the thicker the vapour barrier required to ensure proper and durable adhesion.
For more information, contact your IKO advisor.