One of gypsum board's properties is that it allows water vapor to pass through-it has a permeance of 24 to 35 perms. Contrast that with a vapor retarder, which, by definition, has a permeance of one perm. One of the physical properties of water vapor is that it will move from areas of high concentration to areas of lower concentration; or stated differently, it will migrate from a warm, humid environment to a cool, dry environment. Additionally, warm air will hold more water vapor than cool air; so as moisture-laden warm air cools, it quickly loses its ability to support the water vapor and deposits it as condensation on any relatively cooler surface over which it passes.

Gypsum board is occasionally mistakenly identified as the culprit when water vapor has been found to have condensed and collected in a wall cavity between the interior wall surface and the exterior sheathing; some builders have even asserted that evaporation from drying joint compound is sufficient to result in condensation in wall cavities-more on that theme later. But it is not the role of gypsum board to prevent water vapor from accumulating in a wall cavity-preventing the movement of water vapor is the role of a vapor retarder (although gypsum board can be finished or treated with a coating system or an impermeable laminate, like aluminum, so that it will serve as a vapor retarder).

Keep in mind the facts

Builders and designers need to keep these facts in mind when designing and constructing exterior walls, because humidity and temperature ranges vary in different regions, and therefore the use and placement of a vapor retarder may need to be factored in to ensure that condensation in the wall cavity is avoided.

In other words, failure to properly provide for a vapor retarder can result in condensation in a wall cavity.

Also, an exterior wall cavity will almost always contain some type of insulation. If too much water vapor enters an insulated wall cavity, the half inch or so of the insulation closest to the cold side of the wall may become damp. In extremely cold weather conditions, this can result in the formation of frost or ice in the wall cavity. Ice in a wall cavity can result in damage to the framing, the sheathing, and the insulation. And because icy insulation has no R-value, there are energy losses that may also occur when water vapor enters a wall cavity.

So, how does one successfully build exterior walls using gypsum board and a vapor retarder to prevent condensation from occurring in the wall cavities? The 2003 International Residential Code states in section R318.1:

"In all framed walls, floors and roof/ceilings comprising elements of the building thermal envelope, a vapor retarder shall be installed on the warm-in-winter side of the insulation."

It is therefore most important to first determine what the prevailing conditions are in the region where the building is to be located when deciding where in the exterior wall system the vapor retarder should be placed.

Weather conditions

In the north, cold winter conditions suggest that the inside of the house is going to be considerably warmer than the outside, so the vapor retarder should be located as close to the interior surface as possible and, as such is usually placed between the back of the interior gypsum board and the insulation. In the south, however, air conditioning is running much of the year and the exterior side is usually the warmer side. In humid areas like the Gulf Coast, condensation on the interior side of an exterior wall could very easily accumulate if there is no vapor retarder present to prevent the water vapor from migrating to the air-conditioned interior.

So, is it a clear-cut case that in the north the vapor retarder should be situated between the wallboard and the insulation, while in the south, the vapor retarder should be situated between the insulation and the sheathing? Not necessarily. In fact, a mechanical engineer should be consulted to determine the correct placement of a vapor retarder.

What about playing it safe and placing a vapor retarder on both sides of the insulation? Unfortunately, this seemingly reasonable solution courts disaster: Water that intrudes from leaky windows, plumbing, or faulty flashing can become trapped between two vapor retarders and can condense on the insulation and framing where it can create an environment favorable to fungal growth that might only be detected once odors or stains are noticed, or worse, structural damage occurs.

Regarding the water vapor contribution of joint treatment materials: depending on atmospheric and ventilation conditions, joint treatment products typically require at least 24 hours between subsequent applications of material for the water contained in the material to dissipate. If product instructions are followed during the mixing and application process, a minimal amount of water will be contained in the material. Under proper and normal conditions, the water will dissipate harmlessly during the joint treatment drying cycle.

To illustrate the point, the water content of ready-mixed joint compound is roughly 50 percent of the total material volume and it takes approximately 9.5 gallons of ready-mixed joint compound to finish 1,000 square feet of gypsum board.

According to the National Association of Home Builders, the average new home has 2,200 square feet of floor space and contains approximately 8,500 square feet of finished gypsum board surfaces. So, the total potential water contribution from the finishing of gypsum board in a typical new house is roughly 40 gallons (9.5 gallons of joint compound per 1,000 square feet of gypsum wallboard times 8,500 square feet of finished gypsum board surfaces equals 80 gallons of joint compound X 50 percent water content equals 40 gallons) of water released over the several days necessary to finish the gypsum wall board.

In fact, using the figure offered above on a job that requires four days to complete, the joint compound would add only about 10 gallons of water per day to the house's atmosphere. Compare that to another common source of construction-related water: a paint crew with a paint spray rig that can spray paint the house in less than four hours. To paint the house will require between 25 and 40 gallons of paint, a quantity that will add between 15 and 25 gallons of water-considerably more than the drying joint treatment-to the environment, in a much shorter period of time.

So, though joint treatment materials introduce some water vapor to a construction project, it is a minimal amount. Indeed, several other sources of water and water vapor-all much more significant than joint treatment-are found on a typical construction job. Lumber is often still quite green when purchased from the supplier and delivered to the site, and has quite possibly been rained on before the roof went up. If a concrete floor has been poured in the basement or if there are masonry walls or poured concrete walls, the concrete will produce a considerable amount of water vapor. If the work is being done in cold conditions, space heaters using combustible fuel will create up to six gallons of water for every gallon of fuel burned.

Additionally, daily household activity by a family of four can produce as much as five gallons of water (S.C. Mite and J.L. Bray, "Research in Home Humidity Control." Research Series No. 106, The Engineering Experiment Station, Purdue University, November 1948.), and a crawl space under a house can contribute 10 gallons of water (William B. Rose and Anton Ten Wolde, "Moisture Control in Crawl Spaces." Wood Design Focus, Winter 1994). So, even once the construction is long finished, water vapor is still present and can enter unintended places that can create a variety of problems. Clearly and comparatively, the amount of water vapor produced by drying joint treatment is insignificant.

For these reasons it is important for designers, builders and occupants to understand that gypsum board is used as part of a complete wall or ceiling building system. Consequently, it must be used with other appropriate building components, including a vapor retarder where necessary, to ensure that condensation of water vapor does not become a problem during or after the construction of a building.