Moisture & Solid Hardwood Flooring

Solid or engineered? Which flooring is right for your home?

"Cupping and crowning" are common complaints caused by moisture. In our Tampa Bay climate cupping can develop with high humidity. Both problems occur across the width of the flooring material.

Cupping is a condition in which the edges of the individual boards are slightly higher than the center. Cupped boards appear to be curled slightly. Increases in moisture or humidity cause a slight expansion of wood flooring, (in fact, any wood) and a decrease causes a slight shrinking. You want to keep water off your floors and follow our instructions on cleaning and maintaining your floors. Also, in building conditions where there is more moisture on one side of the wood flooring than the other, the solid wood flooring can cup. This will be more fully explained below.

Crowning is the opposite of cupping. Here, the center of a board is higher than the edges. Moisture imbalance is sometimes the cause of crowning if excessive moisture is introduced on the top of the floor, perhaps from water used in maintenance or plumbing leaks from an overhead sprinkler system. However, a common cause is that the floor was previously cupped, but was sanded too soon-before the moisture content returned to normal, and the board had the chance to flatten on its own.

Preventing Cupping in a New Solid Wood Floor

The most important factor in preventing cupping or crowning is proper acclimation of solid wood flooring in your home or job site. Acclimation of wood flooring means allowing the flooring to adjust to the humidity in your home. This is important because wood expands and contracts based on the amount of moisture in the air.

In new construction or major remodeling the wood flooring should not be delivered until all “ wet work “ is done. Concrete must be fully cured, which normally takes about 8 weeks. Drywall and plaster work should be completed and the moisture created by this work allowed to dissipate.

The wood flooring industry requires that the HVAC system be operational when the flooring is delivered. If it is during a time of year where heating or cooling would normally be run, it should be running when the flooring is delivered. Minimum acclimation for solid wood flooring is one week. In our humid climate, two to four weeks of acclimating is safer, especially with wider floor boards. If your flooring material is to be delivered unfinished and will be sanded and finished on site, a second acclimation period of at least one week is required between installation and the sanding.

Wood flooring is delivered from the mills at a moisture content of between 6% to 9%. In Tampa Bay, it can often end up acclimating at between 10% to over 13%, depending on conditions in your home. Wood installation on the first floor of your home will take longer to acclimate than wood flooring going upstairs. This is especially true in raised foundation homes with crawl spaces.

Through the Woods will take moisture readings of both your flooring material and your wood or concrete subfloor and advise you of when we feel the floor material is properly acclimated and ready for installation. When this acclimation period is dispensed with or shortened, some cupping will usually occur.

Crawl Spaces

Crawl spaces, a very common feature in older homes, tend to have more moisture in the subfloor than homes with a concrete subfloor. For the last 40 years, the construction industry has required that there be a vapor barrier under concrete foundations, but there is no vapor barrier required in a raised foundation.

During warm weather, the use of air conditioning removes moisture from the interior air and causes the surface of the hardwood floors to have a relatively low moisture content. At the same time, the hot and humid summer air we experience here in Tampa Bay, enters in the crawlspace under the home. This humidity causes the joists, and wood subfloor to have a high moisture content. Additionally, at night the humid air cools and moisture condenses out of the air forming dew which is absorbed by the wood joists and subfloor. The moisture then passes through the subfloor and into the bottom of the flooring. With this uneven moisture in the boards, the tendency of wood floors to cup is greatly increased.

A good solution that has been recommended both generally in the construction industry and specifically in the wood flooring industry is to add a 6-mil or thicker layer of polyethylene plastic over the soil in the crawl space. This is not a service that Through the Woods provides, but the plastic can be installed easily by a handyman, or contractor. In addition, it is recommended that you should place dehumidifiers or ventilation fans in your crawl space. These are normally installed by a HVAC contractor.

Another important factor in reducing crawlspace humidity is to ensure that you have adequate ventilation in your crawl space. Most building codes require that 1.5% of your floor space over a crawlspace should have vent openings. So, if you should have 2,000 sq. feet of floor space directly over the crawl space, you would need at least 30 square feet of crawl space vent openings. Older homes with pier construction are normally well ventilated, however, sometimes when porches or patios are constructed, vents are omitted or closed. We recommend the combination of a plastic vapor barrier with proper ventilation to help keep moisture under control.

It is not normally recommended in crawl space homes, to add insulation between the floor joists under the house. The vapor barrier on the insulation can trap moisture between the insulation and the bottom of the subfloor causing cupping and other moisture related conditions.


Before we install your new wood, we will check the moisture content of your concrete subfloor with a concrete moisture meter. If the moisture should be too high, we will determine the cause of the condition and let you know how it can be remedied.

It is unusual to have high moisture readings in a home with a concrete slab subfloor unless the slab is under 8 weeks old because, according to current building codes, there must be a vapor barrier (usually 6-mil plastic) over the soil and under the concrete.

Solid hardwood floors require that a wood subfloor and an additional vapor barrier be installed on top of the concrete, under the subfloor, before your hardwood floor goes in on top.

We normally use a three layer vapor retarder consisting of a layer of cold process asphalt mastic, a layer of 15 lb roofing feltpaper, and a layer of 6-mil plastic. We then install either 5/8” or ¾” plywood. This subfloor is then nailed to the concrete using special concrete nails. Moisture problems are unusual with this type of installation assuming that the wood was acclimated properly and there are no plumbing leaks or major drainage issues.

After installation there are a couple of things that can cause cupping, both with concrete or raised foundation homes. If you do not run your air conditioning when the weather is very humid you can cause your floors to pick up moisture due to increased humidity and some cupping normally occurs. If you leave for vacations you should leave your HVAC system on for this same reason. Many people have turned off their HVAC during summer vacations and come home to cupped floors. The temperature can be set slightly higher than normal but the AC should remain on. Both heat and air-conditioning dry out air and lower humidity.

All wood floors, both solid and engineered, will cup due to large spills, plumbing leaks, or water saturating a foundation wall and getting into the wood flooring laterally through a wall. Make sure that your plumbing is in good condition and that your drainage is good allowing your foundation and crawl space to stay dry.

Handling of moisture issues is important beyond the affect it can have on hardwood floors. Excess moisture damages wood and other building materials and can allow mold or mildew to grow. Moisture control is a very important consideration if you are building or own a home.

As a final note, there is a movement in the Southeast US toward enclosed or non-ventilated crawl spaces in newly constructed homes. These crawls spaces are then insulated and kept at a lower humidity than if the hot and humid outside air is allowed in. If you are interested please look at This website gives a simple and non-technical overview of this type of construction. There two publications from the National Oak Flooring Manufacturer’s Association which provide more information about what cupping is and how to prevent or minimize it, and about proper vapor barriers. These are “Cupping and Crowning“ which can be downloaded here and “”Vapor Retarders: Concrete Slab and Wood Joist Construction“ which you can find here.

Although the information is primarily written for homes with solid wood flooring it still can be pertinent with engineered wood floors if moisture conditions should be severe. Engineered floors are manufactured on a plywood base and therefore are less subject to expansion contraction issues. However excessively damp crawl spaces or concrete subfloors can cause engineered floors to cup.