Vapor retarders help slow the diffusion of water vapor through a building assembly. During the winter, a vapor retarder on the interior of a wall will slow down the transfer of water vapor from the humid interior of the home into the cool stud bays. During the summer, a vapor retarder on the exterior of a wall will slow down the transfer of water vapor from damp siding towards the cool stud bays.
However, a vapor retarder is a double-edged sword: while under some circumstances it can have the beneficial effect of helping to keep a wall or ceiling dry, under other circumstances it can have the undesirable effect of preventing a damp wall or ceiling from drying out.
Q. How often does water vapor diffusion through walls and ceilings cause problems? A. Very rarely. In many cases, in fact, an interior vapor retarder does more harm than good. The main mechanisms by which moisture enters a wall are from the exterior (usually due to flashing defects that admit wind-driven rain) and via air leaks that carry “piggy-backing” moisture that condenses in a wall cavity. Vapor diffusion is a relatively insignificant cause of moisture problems in walls.
Q. What’s the difference between a vapor barrier and a vapor retarder?
A. A vapor barrier stops more vapor transmission than a vapor retarder. A vapor barrier is usually defined as a layer with a permeance rating of 0.1 perm or less, while a vapor retarder is usually defined as a layer with permeance greater than 0.1 perm but less than or equal to 1 perm.
Vapor Retarder - Polyethylene sheeting, aluminum foil, kraft paper facing, and vapor-retarding paint
The vapor retarder shall be installed on the warm-in-winter side of the thermal insulation.
Anton TenWolde: “The calculations show that even with very low air pressures across the assembly, and even with a very good air barrier, sufficient moisture can bypass a poly vapor retarder, degrading its performance. In practice it doesn’t matter what the permeance of the vapor retarder is, because the air leakage will go around it for moisture transfer. I came to the conclusion that the idea that we need a vapor barrier to keep our walls dry doesn’t hold a lot of water, so to speak.”
André Desjarlais: “We can’t assume that the building envelope is perfect. We have to assume some level of failure: some rain will get into the wall, and there will be imperfections in the air barrier.” Golden, CO USA