*From the Wood Handbook: Wood as an Engineering Material, 1999, the Forest Products Laboratory, USDA Forest Service. At 70 degrees F a change in relative humidity from 30% to 65% results in an overall EMC change of nearly 6%. Moisture changes in wood flooring this great will significantly impact the performance of flooring associated with gaps and cupping. Typically, a 20% RH change results in an EMC change of near 3%. This is well within most customer expectations.

  “Proper relative humidity” is a moving target in the wood flooring industry. Some say best wood flooring performance is obtained when the RH is controlled between 35-65% RH; others state the RH should be maintained between 35-55%. Still others say the general recommended range is 30-50% RH. Other statements use the 35-65% range and warn that RH falling below this range can result in cupping, gaps, and checks in the flooring. These statements suggest that flooring does not perform well when humidity levels are outside these ranges.

But that is simply not the case. Experience shows you can certainly get very good and even best performance when humidity levels regularly fall outside these ranges. For instance low humidity associated with Arizona or Colorado environments can fall well below 35% RH regularly. There are many floors that perform to the highest expectations in these areas. However, flooring subjected to regular seasonal swings of humidity from 35 to 65% won’t perform on the par of flooring subjected to the narrower humidity range of 35 to 55% RH.

So who is right?

NOFMA instructs that the average relative humidity of the area is the target for best performance of wood flooring in that area. What is good for Miami, FL is not necessarily good for Memphis, TN or Denver, CO. An overall humidity range of 10% more or less than this target will certainly give very good to best performance. A wider seasonal range say +/– 15% (30% overall change) or +/- 20% (40% overall change) can result in noticeable gapping and or cupping. The customer must be informed of these associated  extremes of expected performance so they can adjust their customer expectations or may even chose a different product.

NOFMA promotes the use of wood flooring in all areas of the country and wants consumers to know that excellent performance of solid wood flooring can be obtained with most any normal interior seasonal environment. We do not discriminate against Bar Harbor, ME and its wide seasonal humidity range of 25 to 60% +. We do caution that wide plank flooring, 5” and wider, can and likely will exhibit significant gapping in the winter and may noticeably cup, particularly if not back sealed, in the summer months when windows are regularly left open. We also do not fault the Desert Southwest for having consistently low relative humidity between 15 and 30% +.  Wood flooring can perform at its best in this narrow 15% range with little seasonal gapping and no cupping when properly acclimated to the dry overall environment before installation.

In fact, the best performance of flooring will occur when relative humidity is static with little change. A constant relative humidity means there is no expected expansion or shrinkage after being acclimated to the particular environmental condition. Install the flooring tight, finish it, and it will remain stable. We know a static environment is not a realistic expectation. However, flooring placed in those areas with the least seasonal change will perform better than flooring placed in areas with wide seasonal differences.

Thus far, we have been talking about solid wood flooring for the most part; so what about engineered flooring?

Overall, properly manufactured engineered flooring by its engineered nature should be less reactive to relative humidity change than solid wood flooring. That is, much less related expansion and shrinkage in width due to environmental change than with solid wood. However, engineered flooring does change more in length than solid wood, but under normal conditions not enough to be an issue. Engineered flooring should therefore perform better than solid wood where environmental conditions have the widest range. In fact many engineered flooring products are allowed below grade where moisture conditions are generally higher than on or above grade. Solid wood is not recommended in these cases.

However, there are engineered manufacturers that recommend the use of their product only where the environmental conditions are within a specified relative humidity range, typically from 30 – 35% to 50 – 65%. You note that these are the same as with solid wood. When issues arise, particularly those associated with delamination, claims have been denied because the recorded humidity was above or below the recommended range. Both NOFMA and the HPVA (Hardwood Plywood Veneer Association) feel that properly manufactured engineered flooring should not delaminate under normal environmental conditions associated with any area in the USA. This includes the desert southwest with typically low humidity and the gulf coast and southern coast with typically high relative humidity.  

So what is the "Proper Relative Humidity"?

The related environmental relative humidity of your area is the proper humidity according to NOFMA. Overall performance will be modified by the conditions. The wider the range the greater the expected movement and associated performance.

If the range is wide you can supplement low humidity with humidification. This can help in those areas with long seasonal heating that tends to over-dry the air, but a consistent relative humidity above 30 or 35% is likely unattainable. Also, on those very cold days of low outside RH, the interior heated air will often be 10 to 15% RH and supplemental humidification set to 30% and above can create condensation on windows and any area where air leaks occur.

The same is true for seasonal high relative humidity associated with the warm summer season. Warm air at high humidity cooled by air conditioning can result in condensation on surfaces where the temperature drops to or below dew point. This can result in actual moisture collecting on the related surface. If the subfloor gets wet the increase in moisture can migrate to the flooring and result in cupping. In areas where this potential exists remediation actions, such as keeping the warm moist air away from the cool surface by use of vapor retarders; actively dehumidifying the at-risk space (such as a crawl space or basement); or even raising the interior air temperature above the dew point may prevent the condition. The recommended actions are best done after review by an engineer familiar with HVAC systems. 

Again, NOFMA believes solid wood flooring and engineered wood flooring should perform according to realistic customer expectations in any normal interior environment. A full bath, over or around an indoor pool, directly above a wood fired heater, are examples of unusual conditions and do not fall in the normal environmental range. Very low humidity can impact solid wood by causing shrinkage gaps and some minor checking of the face associated with season checks already in the wood. It can also impact engineered flooring the same way, but the shrinkage will be less. The checking can be somewhat more numerous since the manufacturing process of creating the veneers typically fractures the wood more than with solid wood. But the checks should be small without wide splits. High seasonal humidity can create expansion and associated cupping with both products. But proper acclimation or spacing will minimize seasonal cupping. The stresses associated with cupping may also result in minor checks but again properly acclimated and/or spaced flooring will minimize the condition.

Despite the ability of properly manufactured wood flooring to perform well in conditions outside the commonly stated ranges (30-35% RH to 50-65% RH), there are some manufacturers who recommend using their product only in certain RH ranges. Typically these recommended ranges are given as a means for the manufacturer to avoid being liable for claims. NOFMA recommends taking those manufacturers on their word and avoiding use of their products if the stated environmental conditions cannot be maintained over time.

Instead use a properly manufactured wood flooring product that does not make these restrictive recommendations. Then, properly acclimate it, install it, and enjoy it.