When choosing a diffusion film, the most important indicator is not the weight, but the functional membrane layer thickness (FMLT). It is this layer that ensures that the material will perform its intended function – it will allow vapour to pass through, but will block wind and water. If the functional layer is too thin or almost non-existent, the film will quickly lose its properties, so it is simply not worth installing.
Comparison with other fields
The importance of FSS is easiest to understand when looking at the pharmaceutical or cosmetics industries. In these industries, everything revolves around active ingredients: if there are too few of them, there is almost no effect. The same is true for diffusion film – its “active ingredient” is the thickness of the membrane layer.
Why is this important?
The thickness of the membrane layer determines:
- how permeable it is to vapour, but resistant to water and wind;
- how much energy you will use for heating and cooling;
- how many years you will be able to live comfortably without additional worries.
The functional layer of most membranes available on the market is only 20-40 microns thick. In comparison, Tyvek FSS film is 175 microns thick. Thinner membranes are easily damaged, have poor UV resistance, and therefore have a shorter service life.
Common confusion: weight and FSS
Buyers often confuse two indicators:
- Weight (g/m²) – important only during installation to prevent the film from tearing;
- Functional layer thickness (microns) – determines whether the membrane will perform its function for a long time.
How to check FSS?
You cannot see the functional layer with the naked eye, but you should know that its thickness should be at least 175 microns. DuPont has developed a device that instantly shows how the membrane allows vapor to pass through and retains water – you can ask the seller for a demonstration.
Durability – not just on paper
Sometimes we hear people say, “I don’t care, I won’t live that long.” However, durability is important not only for you, but also for the structure itself: the membrane must retain its properties for decades and not deteriorate on the roof or walls. Only then will it truly ensure peace of mind and comfort.
Conclusion
The quality of a diffusion film is determined not by its weight, but by the thickness of the functional membrane layer. When choosing a material for your roof or walls, always check this indicator – it will ensure that your investment pays off in terms of longevity and comfort.