What is the purpose of kraft paper on glass wool?

When you insulate the attic or walls of your renovated home, you use glass wool in rolls - that's the basis. There are other types of insulation, but this is the most widespread.

You've noticed a film of kraft paper on one side of the roll of glass wool you bought at Leroy Merlin, Brico Dépôt or perhaps online at Mano Mano, and you're wondering what it's for.

The kraft paper on the glass wool has 2 uses:

  • Vapor barrier (vapour barrier) to avoid condensation in the wool
  • Have a cutting grid and make clean cuts

Kraft paper over glass wool: a basic vapour barrier

Also known as vapor film or vapor membrane. The role of the vapour barrier is to prevent condensation within the insulation of a dwelling.

In fact it prevents or limits the penetration of water vapour into insulated walls. Air always contains a certain amount of water vapor, depending on the room's humidity. If this water vapor passes through the glass wool and condensation occurs, then Mould can form, degrading insulation and indoor air quality, which is bad for the energy bill as well as for the health of the occupants.

The vapour barrier also improves the airtightness of the building envelope, thus limiting heat loss due to air leakage.

Given that the vapour barrier prevents water vapour from escaping, and that the production of water vapour by a family of 4 is estimated at 12 liters per day, it will therefore be necessary to Ideally, set up a good ventilation system to extract all that water vapour from the house! You can either use manual natural ventilation, by opening the window regularly, or mechanical ventilation, via a double-flow VMC for example. And ideally both!

The strength of a vapour barrier is measured by its water vapour resistance value: Sd in metres. This is the water vapour diffusion resistanceIt is obtained by multiplying the mu coefficient of resistance to water vapor diffusion by the thickness of the insulating material in meters.

  • If the Sd is higher than 18m, we talk about a vapour barrier
  • If the Sd is between 2 and 18m then we talk about a vapor barrier

In the case of kraft paper, the Sd is around 2m, which makes it more of a vapor barrier. Mineral wools with kraft paper can be combined with vapour barriers that are more resistant to water vapour diffusion, to avoid condensation in the glass wool in winter due to the higher water vapour pressure inside than outside.

Kraft paper on glass wool also provides a clean 10×10 grid.

The great thing about kraft paper is that it gives a clean surface, usually with a nice 100mm x 100mm grid for orientation when cutting.

What other types of vapor barriers are there?

As kraft paper is not really sufficient in terms of steam braking according to French regulations and precisely according to the DTU 31.2 which recommends an Sd value greater than or equal to 18mcan be combined with other types of membrane to increase the overall Sd value.

The idea behind setting Sd = 18m is to respect a factor of 5 rule. In other words, vapour diffusion on the inside must be 5 times greater than on the outside.

Intelligent membranes are the latest development in vapour barriers. These are hygro-variable vapour barriers. In other words, they adapt their Sd value according to humidity. This ensures a dry wall in all conditions. The Sd can vary from 0.5m to several dozen meters, transforming the membrane from vapor barrier to vapor barrier, depending on the season and water vapor pressures.

Some examples of available products are listed below:

[all4affiliates id="1977″ title="Membrane Vario® Xtra hygro-régulante"] [all4affiliates id="1979″ title="Pare-vapeur/frein vapeur 1,5 m x 50 m hygrorégulant Alfa Rufol"]

About the author

Juliena mechanical engineering graduate and specialist in climate engineering since 2009, has become a writer specializing in renewable energies, with expertise in heat pumps and photovoltaic solar panels for individual housing.