Back to basics: Differences between air and vapor barriers

More than 80 per cent of premature building deterioration is the result of moisture damage resulting from the dissolution of materials, reduction in effectiveness of insulation, and more. For this reason, building industry professionals know the importance of moisture control methods.  But as our friend Dr. Joe Lstiburek has said before, misunderstanding vapor barriers and air barriers is common.

To make the right decision on which building envelope methods and materials to use, it is critical to understand the differences between air and vapor barriers, and their role as part of an effective wall system. Addressing industry confusion, Dörken’s Peter Barrett goes back to the basics to explain the differences between air and vapor barriers in this month’s issue of Construction Specifier.


Air Barrier Function


The function of an air barrier is to stop air leakage and resist differences in air pressure by keeping the indoor climate regulated. Essentially, it keeps the outdoor air out and the indoor air in. An air barrier should be:

  • durable enough to withstand construction pressures and handling;
  • impermeable to airflow; and
  • continuous, enclosing or enveloping the entire conditioned space.

While air barriers are key to limiting the airflow in and out of a living space, air from either a conditioned or an unconditioned side may still find its way into walls. So, even though it is critical for a living space to have walls that are as airtight as possible, the assembly of the wall itself should always be moisture vapor permeable to allow for incidental moisture to escape, or diffuse, rather than be trapped and make the structural materials of the wall wet.

As Peter explains in Construction Specifier, a vapor barrier’s most important function is to be impermeable to moisture in vapor form, thereby preventing the movement of water vapor through the wall cavity to the outside, and vice versa.


Rainwater Wall Penetration


One of the trickiest elements coming into the wall from the outside is rainwater, also known as bulk water, which almost always causes trouble. Water moving inward and reaching the vapor barrier causes failure of the wall in the form of rot and mold. One solution is to eliminate the vapor barrier altogether, allowing the wall to dry from both sides—a flow-through assembly. While possible in many cases, the elimination of the vapor barrier is not always an option.

Leaving out the vapor control layer is only feasible in structures with non-extreme environmental loads and continuous insulation (ci) on the outside. As noted by building industry educator Dr. Joe Lstiburek, a non-extreme environmental load essentially means a building enclosure on an office, house, or apartment as opposed to the façade of a natatorium, museum, hospital, or art gallery. The enclosures subjected to an extreme environmental load are operated at high interior humidity (50 percent or higher year round) and are pressurized. Therefore, for natatoriums, museums, hospitals, or art galleries, a wall incorporating a vapor barrier layer to the interior is essential to maintain moisture vapor impermeability.


High-performance Solutions


Vapor barriers can be placed on the warm side of the insulation without being sealed airtight, as long as there is an uninterrupted air barrier somewhere else in the wall and ceiling assemblies. The reason for this is simple: diffusion of moisture vapor is slow, while moving air carries moisture vapor faster. However, you don’t always have to have a combination of materials to achieve this effect. Some materials come as a combination of these qualities already. It is possible to find a high-performance air and moisture barrier that resists air movement and, in turn, the moisture that moving air would have carried, by increased airtightness.

Making your building as airtight as possible is an important first step when determining which barriers to use; the more airtight the building, the fewer vapor issues you’ll have. That said, airflow isn’t the sole cause of vapor issues, so an air barrier is a must, but a vapor barrier can still significantly benefit your building. Now that the differences in functionality and requirements are clear, the question that might be lingering is which material(s) to choose. Fortunately, there are products on the market that encompass all of these qualities. What’s more, adding the benefit of self-adhesion only works to improve the performance of a multi-functional barrier. Look at DELTA®-VENT SA, for example – a high-performance, three-layer, vapor permeable air- and water-resistive barrier that prevents moisture from getting into buildings.

By also allowing moisture already in to escape, DELTA®-VENT SA eliminates the risk of mold and other problems caused by water pooling where it should not. Its vapor permeability and airtightness make DELTA®-VENT SA ideal for building and maintaining healthy and comfortable interiors while letting moisture out and improving energy efficiency.

It is very expensive to install moisture control measures after a building is complete; it’s much cheaper to do so at the time of initial construction. By putting the emphasis on air and vapor control as part of the design, rather than attempting a retrofit, the building will be much more secure against mold, mildew, and other problems, and there will be fewer issues and expenses down the road.

Designed with applicators in mind, DELTA®-VENT SA comes fully adhered for simple and straightforward application, no fasteners required. With the self-adhesive edge-lap feature, ensuring an airtight overlap is easier than ever, taking the guesswork out of the application process. DELTA®-VENT SA comes with clear and comprehensive installation guides and videos and technical documents that help reduce installation time and labor costs.