μ
d
= μ * thickness of the material Unit m where μ = = resistance coefficient for water vapor dissemination (in relation to a layer of still air)
As resistance is also dependent on the thickness of the element crossed, the value μd (or Sd) [m] is used, where d is the thickness of the material in the wall. An μd of 1 m is equivalent to the water vapour diffusion resistance that a 1m-thick stationary layer of air would exert.
A vapour barrier is a barrier with a relatively high μd value (generally higher than 10 m). Suitably placed, it can greatly reduce vapour diffusion, regardless of how it is applied. A vapour brake generally has a lower μd value (often lower than or equal to 10 m). However a "vapour brake" is a commercial term without any set μd threshold value, meaning that users should pay attention to the values listed in its technical specifications. The use of a vapour brake rather than a vapour barrier maintains a wall's ability (at least in part) to dry out inwards in summer. Material
Thickness (e)
Sd (μ x e ) value
Air – reference value
1m
1m
EN 12524
Plaster
0.015 m
0,06 to 0,015 m
EN 12524
Glass wool
0,2 m
0,2 m
EN 12524
Cellulose
0,2 m
0,4 m
EN 12524
Under-the-roof wood fibre insulating panel
0,022 m
0,11 m
EN 12524
OSB panel
0,022 m
0,66 à 4,4 m
EN 12524
Vapour brake
0,001 m
4,5 m
Vapour brake with variable Sd
0,0002 m
0,25 to 10 m
Vapour barrier
0,0002 m
20 to 50 m
Reinforced concrete
0,2 m
10 m
Polyethylene
0,00015 m
50 m
Aluminium sheet
0,00005 m
1500 m
PE sheet
0,00015 m
8m
Glass
0,006 m
+∞
chap2 propreUK - Copy.indd 117
Source
design
The ideal situation is to give priority to water vapour moving outwards through designing walls made up of different layers with decreasing μd values from inside to outside.
02
EN 12524
117 24/10/2014 19:23:46