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Q.In a mild climate, should the vapor barrier be on the interior or the exterior of the walls?

A.Frank Vigil responds: The answer is simple. Or, more accurately, the question is irrelevant since codes in most states with mild climates require a vapor barrier on the side which is warm in winter — the inside. However, it is important to remember that walls built in this climate must have the potential to dry.

Mild (or "mixed") climate zones are those areas requiring both heating and cooling for several months of the year. During hot, humid, summer months, the predominant vapor drive will be from the outside in during the sunny daytime. At night, the wall should dry to the outside, if given the opportunity.

An exterior sheathing of plywood or OSB will act as a mild vapor retarder, but will still allow the wall to dry to the outside at night.

To fully understand why this is important, you must understand what a vapor barrier does. First, let’s be technically accurate and call this a vapor diffusion retarder! A vapor shield, such as polyethylene. sheeting, can only slow the flow of vapor diffusion — it can’t stop it.

Vapor retarders are intended (at least in theory — probably not too well in practice) to limit the amount of moisture — in vapor form — that passes through the building. The rate of vapor diffusion is determined by the permeabilityof the building material and the driving force, which is air pressure. The higher the pressure or the lower the permeability, the greater the vapor diffusion will be.

Vapor diffusion is also a function of surface area. A vapor retarder that has 30% holes would only be 70% effective. However, this may not be as critical as you think.

Diffusion is only one way moisture migrates through a wall, and the least important. The others, in order of importance, are bulk moisture (rain and snow), capillary action (wicking), and air-transported moisture (leakage of humid air). Thus, having a few small tears in the vapor retarder of a wall is less important than sealing all the wall penetrations, such as electrical outlets, light switches, and gaps between the interior wall board and the framing.

If all this seems confusing, remember, given the opportunity, most conventional wall systems are somewhat forgiving. Control the moisture in the order of importance, and you should have no problem.

— Frank Vigil is senior project manager with the North Carolina Alternative Energy Corporation, currently developing a voluntary, statewide performance standard for new construction.