Vapour control layer

Frequently asked questions

Which side does the vapour control layer go on?

The warm side — and the reason is worth holding onto, because it is what tells you what to do when the warm side changes. Vapour moves from warm, humid air towards cold, dry air, so in a heated building in winter it pushes outward through the construction. Somewhere in that path is a plane cold enough for the air's dew point, and if vapour reaches it faster than the construction can dry, water accumulates inside the wall where nobody sees it until the timber rots. Putting the control layer on the warm side stops most of the vapour before it gets there. The corollary is the dangerous case: in a hot, humid climate with air conditioning, the warm side is OUTSIDE, and a layer installed on the inside face is now on the cold side, trapping moisture driven inward from the outside. That is why the American classes are assigned by climate zone rather than given as one rule, and why a detail copied across climates is a genuine hazard rather than a stylistic difference.

What does an sd-value or a perm rating actually say?

Both express how much a material resists vapour diffusion, in opposite directions of counting. The SD-VALUE, used in Britain and Europe, is the thickness of still air that would resist diffusion as much as the material does — measured in metres, so a bigger number means more resistance, and a polythene sheet might be fifty metres while a breather membrane is a fraction of one. A PERM RATING counts the transmission instead, so a bigger number means MORE vapour passes: that is why an American Class I retarder is defined by a very LOW perm rating. The two scales are therefore inverted with respect to each other, which is a reliable way to specify the exact opposite of what was intended when a datasheet crosses markets. Neither number is useful alone: what matters is the ratio between the resistance on the warm side and the resistance on the cold side, because that is what decides whether a wall that gets wet can dry.

Why does a wall need to dry, and how does a barrier stop it?

Because every construction gets wet — from a leak, from building moisture, from vapour that got past the control layer through a gap at a socket or a joist end — and the question is never whether moisture enters but whether it can leave. A construction dries by vapour diffusing outward through its cold side, which is why the layer there should be MORE open than the layer on the warm side. Put an impermeable layer on both faces and the wall cannot dry in either direction, so the first leak is permanent. This is the reasoning behind the general rule that vapour resistance should decrease from inside to outside, behind the American prohibition on a Class I retarder in assemblies that need inward drying, and behind the damage done by fitting an impermeable cement render or a vinyl wallcovering to an old solid wall that was relying on drying through that face. A perfectly installed barrier on one side is fine; two of them is a trap.