Avoiding the Moisture Sandwich: Vapor Control for Floor Systems

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Advancements in framing and insulation coupled with a desire to build tighter, more energy-efficient homes have builders looking carefully at evolving wall assemblies. But as finishing materials become more sophisticated, there may be a need to look down — to the flooring system. Trapping moisture that can lead to rot is not just a problem to avoid in wall assemblies.

“If I asked you to think about vapor control, it’s likely you would immediately think of the wall assembly,” said Chris Vegas, AdvanTech® technology director at . “It can be equally important to think about moisture and vapor control layers with the floor assembly.”

In coastal areas, moisture should be an even greater consideration due to the field service conditions of these assemblies. For example, if you are designing or building a home on stilts or with a crawl space, you’ll want to pay extra attention to moisture concerns for the floor system. As with the wall system, this means managing bulk water, airflow and vapor.

Has the bulk water been managed?

First, make sure there isn’t an opportunity for bulk water to reach or build up on the floor system. In coastal areas, this typically means raising the floor system above the flood plain to account for potential storms and flooding. For houses with crawl spaces, bulk water is managed by proper grading to allow for drainage away from the foundation. In the case of vented crawl spaces, cross ventilation will help to maintain acceptable relative humidity levels.

How is air flowing in and around the structure?

The summer months in the southeast and other coastal areas can be brutal when it comes to humidity. In these areas where hot temperatures are paired with prolonged periods of high relative humidity, builders should consider the flow of air into crawl spaces, floor cavities of stilt-built homes and even unfinished basements.

“The warmer the air, the more moisture it can hold. So, when air flows into a cooler place like a shaded crawl space adjacent to air conditioning, it has the potential to condense on the underside of the subflooring, especially if the home is kept at lower temperatures,” Vegas said.

This inward vapor drive can cause issues such as mold growth or rotting of the floor assembly if not properly managed. It’s important to take all components of the flooring system into consideration when determining the most appropriate, and moisture-resistant, flooring assembly for your build.

How can vapor concerns be addressed? And what role does finished flooring play?

Luxury flooring alternatives such as vinyl tile and vinyl planks have gained popularity. However, vinyl products are vapor barriers.

One method for limiting condensation buildup in a home with a vented crawl space is to add insulation under the subfloor. Consider the permeance of the finished floor when designing the system to allow for proper drying. A home in the Southeast built over a vented crawl space would need insulation that is less permeable than the finished floor, such as closed-cell foam used with hardwood flooring.

Overall, it’s important to keep a few things in mind when it comes to floor systems.

Take relative humidity and large temperature swings into consideration.

Think of the floor as a full assembly. Know the vapor permeance of all products in the assembly to avoid a double vapor barrier.

Control the movement of air, and you control the movement of moist air, which can cause condensation buildup in the first place. If crawl spaces are necessary, consider building over a closed and conditioned space.

Make sure you’re installing subflooring that will give you the best moisture resistance during construction exposure. “One of the reasons builders have chosen AdvanTech® subflooring for decades in humid or storm-prone areas is its ability to maintain its strength, stiffness and fastener-holding properties even through wetting and drying cycles,” Vegas said.

For more information on AdvanTech® products, visit .

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