Crawl Space Moisture and Hardwood Flooring

To understand the cause of wood floors cupping or buckling (when you’re sure there is no water damage due to a flood or leak), you must understand the relationship between relative humidity, your wood floor and your crawl space.

During the summer months in the northeast region, wood subfloors and floor joists often measure 19

-25% moisture content in a VENTED crawl space. In an air conditioned house the wood floor

moisture will typically measure 7-10%. The difference between the wood subfloor and the hardwood flooring shouldn’t exceed 4% for solid strip hardwood and 2% for solid plank. The maximum subfloor moisture should not exceed 13%. Greater differences in moisture may lead to buckling or cupping of the hardwood. A vapor retarder such as 15 lb roofing felt installed under the wood flooring will not prevent cupping or buckling. It slows vapor emission, but doesn’t stop it.

As a flooring contractor, it is your responsibility to check the subfloor and hardwood for moisture content. In today’s world of blaming everyone, you better document it. If your readings at the time of installation were within the acceptable range and now there is a problem and the readings are high, it gives you some ammunition to support something has changed and it is not faulty installation. When the subfloor is at acceptable levels, use a vapor retarder such as Aquabar B. Aquabar B has better moisture retardation than 15 lb. felt.

Most homeowners believe they have no moisture in their crawlspace- after all, they have a vapor barrier installed, the crawlspace vents are open and the space looks dry. So why did the wood floors cup and buckle?

Crawl space ventilation was meant to keep a constant supply of fresh air circulating throughout the crawl space environment. It was thought that this would help purge the space of the heavy, damp air that settles in the lower levels of a house. This belief had become so widespread that crawl space vents were written into the International Building Code. As a result, many have crawlspace vents which are counterproductive in keeping your crawl space dry, mold and insect free.

Water surfacing from the damp earth, humidity entering from crawlspace vents, and moisture seeping through porous concrete creates a perfect environment for condensation, mold and mildew.

As air warms in a home, it rises and leaves through the upper regions of the house, much in the same way that air moves through a chimney. This phenomenon, called the “stack effect,” causes the home to suck air up from the crawlspace into the main area of the home. Moisture from the crawl space, mold spores and fecal material from dust mites enter the home.

Installation of a minimum 6 mil Black (not clear) plastic vapor barrier that will not support mold growth or allow humidity from the earth into the crawlspace is recommended. This helps to insulate

the crawlspace and discourages insects and vermin.

Winter air fills your crawl space, making the floor above cold. It cools your hot water pipes and heat vents, and in some instances, your furnace and hot water heater, your heating bill increases and the house is drafty and uncomfortable.

Summer air is hot and humid; when it enters the crawl space it instantly cools and drastically increases the relative humidity. When the relative humidity goes over 100 percent, the moisture is released into the crawl space atmosphere leading to condensation on your wood, pipes, mechanicals and other cool surfaces. It also leads to mold, rot and insect infestation. This damp

air rises upward into the house with mold spores and other contaminants and you spend more on utilities. Over time, the crawlspace insulation gets wet and loses its insulation value, mold begins to grow, moisture drips to the floor and wood begins to rot. Your crawlspace is now the perfect haven for insects such as termites, ants, spiders, centipedes, camelback crickets, cockroaches and rodents.

Molds cannot grow when humidity is maintained between 30 and 50 percent. By maintaining humidity levels -wood floors won’t cup or buckle, mold and contaminants will not enter the house, and heating/air conditioning bills will be lower.

SIGNS OF MOISTURE PROBLEMS IN A VENTED CRAWLSPACE:

• Damp, musty odor
• Water stains on foundation
  • Efflorescence on block/concrete walls (white crystalline or powdery deposit on the surface of the block)
• Rust on exposed metal parts.
• Black discoloration on wood.
• Dirty, dripping fiberglass insulation
• Water dripping from duct work or insulation
  • Standing water-puddles or wet marks on the concrete floor or on top of the plastic vapor barrier.
• Cold damp sand/dirt,

Replacing a cupped floor is usually the worst choice, especially if replacement is made before the moisture condition has been corrected. A replacement floor is almost certain to react to the moisture situation by cupping, like the first floor, and the whole process must be repeated.

Fix the moisture condition and allow the floor to dry to normal levels. Spot repair/replacement, reinstallation, re-nailing and refinishing are some options. In most cases where the flooring has loosened from the sub-floor throughout the installation, reinstallation or replacement may be necessary.

Requirements for a Vented Crawl Space per the NWFA

• Crawl space should be a minimum of 18” from ground to underside of joists.
• Crawl space earth (or thin concrete slab) should be covered 100 percent by a vapor

retarder of black polyethylene (minimum 6 mil) or any recommended puncture-resistant membrane, such as Class C, meeting ASTM D-1745.

• Where a proper ground covering is in place and when venting is required by local building codes, the crawl space should have perimeter venting equal to a minimum of 1.5 square feet per 100 square feet of crawl space square footage, unless local building codes differ from this specification. Note: Local-building codes may differ. Follow local building codes.

Requirements for a Sealed Crawl Space (no vents) per the NWFA

• For crawl spaces without ventilation openings, vapor retarder joints must overlap a minimum of 6 inches and be sealed or taped.
• The vapor retarder should also extend at least 6 inches up the stem wall and be attached and sealed to the stem wall.
• Continuously operated mechanical exhaust and perimeter wall insulation or conditioned air supply and insulation must be provided.

If you are installing any flooring over a crawl space, investigate it to make sure it meets the requirements outlined above. This is your responsibility as a flooring contractor to know what you are installing over.