The Battle of the Barriers

In theory, it's simple: When water vapor from inside or outside a building gets inside wall assemblies, the water vapor condenses on cool surfaces, such as pipes or cavity walls. As most of you know, water in walls equals big trouble, including mold, corrosion and rot, as well as increased heating and cooling costs and uncomfortable living conditions.

Want to avoid all that? Just keep the water vapor out. Do that one thing and you and your client's buildings are protected.

Doing it, though, is where things get complicated. That's because products and procedures vary for keeping water vapor out of walls. If you're trying to make sense of the often conflicting information, the first question you run into might be: Which is best for keeping water vapor out - a vapor barrier or an air barrier? And the fact that correctly installed vapor barriers also act as air barriers doesn't make the question any easier.

Construction professionals have long used impermeable vapor barriers, such as rubberized asphalt sheets, to waterproof below-grade foundations and roof assemblies. So when they found that water vapor moving through walls was a problem, they turned to that same type of product to provide a similar protection.

In a mostly cold climate, an impermeable vapor barrier installed on the warm side of the insulation stops living-space water vapor from diffusing into the wall assembly where it can condense. A phenomenon called vapor drive, which moves water vapor from areas of high density to low density, is what drives the water vapor through the walls. However, in hot, humid climates, the vapor barrier should be installed on the warm exterior side of the insulation to stop vapor from diffusing through the wall from outside.

The problem is that in climates with hot summers and cold winters, the vapor barrier will be on the wrong side of the insulation at least part of the year. In summer, hot humid air from outside may condense on a vapor barrier placed for a cold climate, but cooled by air conditioning. And warm, moist air from living spaces can condense on vapor barriers placed for a hot climate but cooled by winter temperatures. Both conditions contribute to water inside the wall assembly.

Recent studies have shown that comparatively little water actually gets into the walls through diffusion. It is mostly carried in through air leaks in the wall assembly. So a vapor barrier also has to stop those leaks.

Since stopping air movement turns out to be what's important, some systems have dispensed with the problematic vapor-impermeability of vapor barriers. Air barriers just concentrate on stopping air movement.

Effective air barriers stop air from leaking through the sheathing or CMU backup by swathing it 100 percent, from top to bottom. If air, which always carries water, can leak in or out anywhere, the entire installation is no good.

That's the key to air barriers: they don't have to be impermeable. Many will let a little vapor diffuse through them as part of stopping water buildup in the cavity. In fact, letting water vapor diffuse out of a cavity contributes to keeping the cavity dry.

But air barriers must stop air movement - period.

Sort through this crowded field of products by looking for these basic characteristics:

Continuous and Seamless The air barrier must be able to cover all the nooks, crannies, cracks and transitions of the wall assembly, 100 percent, with no gaps, holes or seams.

Structural It must stick like blue on sky to the sheathing or CMU backup so it won't come off in wind during construction, or from air pressure differentials after the walls are completed.

Durable The air barrier must be able to resist weathering unprotected in case of construction delays, with no rips, tears or breaches of any kind or size - for months, if need be.

Some other features to consider in an air and moisture barrier:

• Is it easy and fast to install, or labor intensive?

• If fluid-applied, is it water-based or solvent-based? Water-based systems are more likely to comply with VOC (volatile organic compound) regulations.

• Is the installed cost, including labor, competitive? Some products may be cheap to buy, but difficult and time-consuming to apply correctly.

When you multiply all these different air and vapor barrier products by all the different types of wall assemblies, you get an intimidating array of possibilities. Some will work great. Some will accelerate the very problems they're meant to stop.

Following the simple guidelines of "continuous, structural and durable" will help you choose the right air barrier.

The right air barrier will do that one simple thing - keep uncontrolled air and the destructive water it carries out of walls. That means the ultimate victors in the battle of the barriers are your clients and you.