Check the subflooring not only for proper moisture content, within 4% rule, but also check the ongoing conditions from the EMC chart.


In the beginning, the only environmental requirement for wood flooring installation was that the house be “dried in”. In other words, the roof was on and windows were in place to prevent rain from getting on the floor. Many inner city homes, mid-town homes and country homes built in the 1920s, 1930s and 1940s are still standing with wood flooring in place and performing to the satisfaction of the owners. This is in spite of the lack of environmental controls at installation and minimal control before the advent of air conditioning and central heating systems. These older homes do have some issues with the floors such as gaps and noises, but these can be minimal.

For 18 years I lived in a 2,100 sq-ft. mid-town home that was built in 1933. The original 2 ¼” strip flooring was installed throughout the main floor except for the kitchen, back entry, and bath.  I replaced areas totaling about 300 sq-ft. of the flooring before moving in because the previous owner had many cats and there were deep stains in a bedroom, hallway, and dining room. The floors were refinished with a seal-and-wax system. The living room and three bedrooms were in good shape and there were minimal gaps and only the occasional noise. Before I moved in the house had radiator heat and window air-conditioners that were replaced with a central system. The house was mostly over a crawl space without ground cover. A small basement was under the kitchen and back entry where the boiler and water heater were. So for 50 years the home was subject to the environment dictated by weather without whole-house control, and the flooring performed admirably.

Equilibrium Moisture Content of wood flooring is determined by relative humidity and temperature. The reverse calculation can determine the ongoing environment. If the wood moisture content is consistent at 7% then the related environmental conditions are near 35% to 36% relative humidity at a temperature of 70 degrees.

Today many installation instructions state that the interior environment must be established and continually maintained at 35% r.h. to 55% r.h. (others require 30% to 50% r.h.) and at normal living temperatures in order to install flooring. The instructions further state that in order for the flooring to perform and not fail; these conditions (as a warranty issue) must be maintained after installation. I have seen claims denied simply because the relative humidity was just less than the 35% (or 30%) on the day of the inspection. I contend that these requirements are unreasonable and in many areas of the country cannot be maintained continually without near museum quality environmental controls that would result in costs that would price wood flooring out of the market. Also, high levels of humidification in winter where heavy heat loads are required can result in condensation issues and possible rot of framing members if insulation and related vapor/air infiltration barriers are not perfectly installed.

So what are the environmental requirements for a wood floor? The mantra that the space must be “clean, dry and flat” is the requirement.  We know that clean and flat are conditions generally not related to environment. They can be addressed by simply scraping, sweeping, repairing, leveling or grinding. The environment is different; weather, even the season, can affect the site. Also, whether HVAC is operational, certainly affects the condition. Not only these immediate conditions but the conditions say a month before can influence the conditions of the day or if the system is not operational future conditions can affect the site.

The basic requirement is that the environment is near the expected occupied condition. Yes, it can be somewhat different, either wetter or drier, but the trend should be toward the occupied condition. A previously wetted site from such causes as: rain during the framing stage, a basement full of water, water standing in the crawl space; increases the moisture content of all wood components and should be dried out. The best way to check is to use a moisture meter to check the subflooring and any exposed studs or framing members and record the readings on the wood members. Their moisture content will reflect the on-going environmental condition. As a professional you should know what the average area moisture content should be for an occupied environment. In many areas of the country this will be around 7 to 7 1/2% moisture content. So the subflooring should be no more than 11 to 11 1/2% average moisture content (the 4% difference rule) and decreasing in most cases especially for new construction. This means with higher moisture conditions, drying should be initiated. If drying is required, the builder or other stake holders can be informed this may involve removing any noticeable water, dehumidification, running the HVAC system, and or simple air movement with fans. Of course a moisture re-check of the same areas previously checked will be necessary to confirm the reduction. 

Use your moisture meter to check framing to help determine ongoing conditions referenced by the EMC chart.

Basic requirements can certainly be different for the different areas of the country. For instance in the mountain states and southwest average moisture content when occupied may be 6% or even less. And for very humid environments the average m.c. can be near 10%. For both these areas the conditions remain mostly drier or wetter throughout the year; so once the occupied environment is established, there is little seasonal change. The target for wood flooring in these areas is to focus on acclimation to the average area condition and install.

Another important directive is to inform the stake holders that the instructions and warranties may call for unusual environmental controls that can’t be maintained with standard equipment. In today’s world, residential HVAC systems operate continually 365 days a year with outside air only supplied as doors are opened. The interior environment is more stable than in pre-air conditioning days. In addition today’s houses are more insulated and have less air changes than the older homes, further reducing the environmental range. Before central systems, during the summer, windows were often opened, particularly at night, which added extra humidity and resulting seasonal expansion to the flooring. In winter the heating systems were supplemented with fire places with either wood or coal as fuel, and with significant outside air infiltration the environment was very dry with resulting shrinkage, and still the wood flooring has performed.  

The environmental requirement is to have as a target the normal or average condition for your area. The site if different should be adjusted to approach these normal conditions. The flooring should be assessed and acclimated to normal conditions. For dry conditions, spacing of flooring may be necessary for any future expected expansion. For wetter conditions or wetter flooring identified before installation, drying may be the only recourse as shrinkage will ultimately occur with related gapping. The 35% to 55% relative humidity installation and use requirement averages 8.5% EMC which would be high for most of the country. Even the 30% to 50% relative humidity requirement averages 7.7% EMC which is higher than the 7 to 7 1/2% stated earlier.

As a final note, consistency of manufactured moisture content is critical for good flooring performance. This gives a starting point with most of the flooring pieces in a tight range for adjustment when necessary. For solid wood flooring, the 6% to 9% manufactured range averages 7 1/2% which likely requires little acclimation in most areas. For engineered flooring, as stated in the last issue, the required 5% to 9% m.c. at time of shipment averages 7% which is in line for most of the locations in the country. Also, some manufacturers may produce products that average lower or higher than these averages so extra acclimation may be necessary. I have recently been involved with testing solid wood products that averaged 6% and below at manufacture. As flooring professionals we must determine the pre-installation condition of the flooring by taking appropriate moisture readings in order to adjust the flooring accordingly. And finally, if the site is approaching occupied conditions, again as determined by our moisture meter, then installation can proceed.