COVER STORY, MARCH 2005

FOLLOWING THE VAPOR TRAIL
Although mold is the industry buzz word, moisture vapor is just as dangerous to buildings.
Steve Flaten

Flaten

Industry experts believe the building materials and methods used decades ago were superior in quality compared with today’s construction. Today, fast-track construction is blamed for many building woes because of its “get it done as fast as possible at the lowest price” philosophy.

But there are many other changes in building construction that have led to increased damage from water vapor. Different adhesives required by environmental law changes, new energy codes and new building materials make buildings tighter but are also reasons for some of the recent problems. Other culprits that trap unwanted moisture include leaks during construction, moisture saturation in all building materials, weather-tight shells and cold-weather construction. Even the type of temporary heating used during construction can have a major effect on increasing moisture vapor inside a building. Also, buildings do not lose as much heat through their exterior skins, which used to create a drying action that would carry moisture to the outside.

Because of its physical properties, moisture vapor is potentially capable of more movement through building materials than bulk water leakage. Think of moisture vapor as steam coming out of a tea pot. It contains pressure and affects a broadly diffused area. How do moisture vapor and  humidity travel?  

• from higher to lower vapor pressure

• from high to low dewpoint; also known as condensation

• from high to low density

• from warm to cold sides (as heat moves)

• permeates certain materials, some more than others

• transported through the air and through materials

• can be slowed or nearly stopped by some materials

Moisture entrained or entrapped in a building can take up to 3 years to dry following construction, damage wall coverings, breakdown floor tile adhesives, rot wood, leave standing water on concrete and may cause mold. All of these problems will appear over time and will result in unnecessary and expensive repairs.

One of the complaints about today’s airtight buildings is that they don’t breathe.

In many cases, most of the attention about trapped moisture has focused on walls and ceilings, and while that’s important, new advice is being directed toward the ground.

In the past few years, we have seen an increasing number of problems with floor coverings related to moisture vapor. One concern is that some of today’s modern floor materials are being created with less permeable surfaces. This increases the surface’s resistance to stains and improves its ability to be cleaned, but the lack of permeability makes it susceptible to moisture and alkalinity damage. These materials are less likely to allow moisture to pass through and escape to the atmosphere.

Engineers and designers typically follow slab construction and design guidelines for vapor barriers and retarders. Many of the model code agencies and technical organizations have found the guidelines to be outdated and are raising new issues that demand attention. Among those issues are, what level of below-slab protection is needed? And, where should the vapor barrier or retarder be placed?

Some years ago in an attempt to reduce the effects of concrete slab curl, the idea of placing fill above the vapor barrier or retarder gained popularity. Placing 2 inches of sand over the vapor-proofing material proved to be a mistake because there was no way to keep the sand stable when placing the concrete. Soon, it was recommended to increase the sand to 4 inches, which worked in most cases, especially in industrial projects where the floors would not receive a floor covering or coating. However, when used in hospitals, schools and offices, the fill method was less effective because it took on water from rain, curing, sawcutting and cleaning, and bleedwater from the concrete.

The fill below the slab can become saturated and hold a considerable additional volume of moisture that would always be present in the cured concrete itself. This additional water supply makes it very difficult to dry the slab to a level that will meet the requirements of the floor covering manufacturers.  To remedy this problem, the industry is returning to a method abandoned years ago of placing the vapor barrier directly below the concrete.

To avoid or reduce concrete slab curl, adjustments can be made to the concrete mix designs, including the amount of water, size of aggregates and admixtures used. Furthermore, these changes to the mixture also improve the durability qualities of the concrete while making it less permeable to vapor penetration.

The most effective vapor-retarding material should consist of extremely low permeance materials, or moisture within the concrete will increase and condense near the slab-top surface over time. That moisture will damage the flooring or will liquefy the adhesives when entrapped by an impermeable floor covering. (As this occurs, soluble salts within the concrete can raise pH levels below the flooring to levels that can attach or destroy the adhesive — pH of 9.0 or above.)

Not only should the vapor retarder be evaluated on the perm rating (grams of water penetrating a given area for a given temperature) but consideration should be given to its puncture resistance and ability to be sealed. Today’s standards recommend that vapor retarders be placed directly below the concrete when making the surface subject to punctures and physical harm. But damage can be prevented by using a heavier and more durable membrane. Installation also should include an evaluation of how the construction joints between pours will be placed to allow proper overlap and sealing of the membrane.

Also, consider existing structures where vapor has moved out of the floor slab into the building environment. Test the vapor transmission levels through the floor to determine the rate that is anticipated. (This is done using a calcium chloride moisture test that will measure the number of pounds of water passing out of the slab over 1,000 square feet in a 24-hour period.) 

In offices, plastic chair mats on top of carpets also create problems with mold growth. These mats trap vapor in the center of the pad and create mold in the organic materials. New types of mats are available with holes drilled about every inch allowing vapor to escape. To prevent problems in older buildings, owners might add provisions to the lease that would require these types of mats as well as a limitation of impermeable flooring.

Finally, as new finishes are installed by various tenants or as remodeling occurs, vapor problems can cause the new finishes to fail. New adhesives are more susceptible to moisture and breakdown, resulting in loose tile or causing stains to rise through the joints in the material. New carpet tiles and heavy urethane-backed carpets would allow less vapor through, and action-backed carpet would breathe better.

Most of the time, older buildings without a history of water problems do not have issues with moisture in the floor finishes. However, 1 to 2 years after remodeling and installation of modern floor finishes, a potential leak may arise. The vapor takes some time to build up and create problems with sticky adhesive on the tile joints, which results in the blistering of epoxy terrazzo and breakdown of carpet adhesives. The problem is not a new leak, but the inability of the building to transpire the moisture the way it has in the past.

As commercial and consumer complaints about moisture vapor continue to rise, builders, contractors, architects and engineers will pay more attention. And as is generally the case, it is always wise and usually quite possible to prevent problems — and possible liability — from occurring in the first place.

Steve Flaten is an architect of building sciences with Minneapolis-based Braun Intertec.



©2005 France Publications, Inc. Duplication or reproduction of this article not permitted without authorization from France Publications, Inc. For information on reprints of this article contact Barbara Sherer at (630) 554-6054.