Building Science

UNDERSTANDING BUILDING PHYSICS – Vapor Retarders vs. Vapor Barriers

You will need to know a little bit more about material properties in order to understand the drawbacks of construction methods that require the use of a vapor barrier as part of the insulation. If you use a vapor barrier you cannot allow any moisture. With the smallest air gap and a poly barrier on the inside there is no controlled diffusion or no way for the moisture to be expelled. A vapor retarder on the other hand enables a controlled release. First air pressure and then a certain amount of moisture/steam pressure will release through the permeable vapor retarder. This is the natural law of building physics.

When Central Europe experienced an energy crisis in the 1980s they started building houses with more insulation. They used dense sheeting such as plywood or OSB on the outside. Dense sheeting is impermeable, there is always moisture behind the sheeting and a poly vapor barrier is required on the inside. As a result, mould developed in the insulation cavity inside the exterior cladding.

In Europe they realized that rather than try to circumvent the law of physics, they can be used to advantage and this is why a new cladding system was developed for permeability regarding steam pressure. Agepan understood that based on known science, the key is to develop the building envelope is similar to the way people dress for warmth and insulation in the winter time – in layers so that moisture is wicked away from the skin. For optimum permeability the new system includes:
1. Moisture absorption
2. A membrane that moisture can “steam” through
3. Insulation without vapor resistance
4. A membrane that releases steam but does not allow liquid moisture to penetrate from outside.

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 insulation cavity. 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 insulation cavity.
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. “Permeance” is the property of a material that prevents fluids (such as water or water vapor) to diffuse through it to another medium.
A lot more energy is expended to heat and cool buildings if shiplap cladding is used on outside walls and the building envelope is not sealed but in this environment, if it is not sealed mould is not a problem.
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