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Wood flooring and other wood products can give excellent performance over a wide range of environmental conditions. To find examples we only have to look to older homes built 50 to 100 years ago. Even some of the castles and palaces in Europe built in the 16th through the 18th centuries have functioning wood floors that date before any environmental control was in place. Other wood products such as antique wooden furniture show that through the decades, even centuries, wood materials can perform as intended in many environments.

So, why all the hoopla about environmental conditions and wood flooring? In today’s homes and commercial installations the interior heating and air conditioning units are continually operating.  Wood products react to the surrounding environmental conditions. This is the nature of wood as a hygroscopic material. When the environment is dryer than a previous condition, wood flooring will be influenced to shrink. The opposite is also true; wood flooring will be influenced to expand when the conditions are wetter than previous conditions. The beauty of wood flooring is that it is generally protected by a finish that can slow the imposed changes. In addition, most seasonal conditions are variable so extremes are not in place continually and when coupled with the slow reaction, unacceptable results are not common. This is particularly true if customer expectations are properly set for the reality of a wood floor.



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A few examples can illustrate how environmental conditions affect wood flooring. Just recently, I was involved in an installation where cupping was the complaint for 1,500 feet of site-finished 5” plank flooring. The floors had been flooded by a burst pipe at the beginning of winter. The flooring cupped and buckled in some areas. Insurance authorized removing the flooring and drying the system out. This was an existing home with a well established environment. The goal should have been to reestablish the original environmental conditions. They began by placing desiccant dehumidifiers in the affected rooms which ran for three weeks with concrete readings remaining elevated. The flooring was delivered and placed in an adjoining room for acclimation. The rooms with high readings were isolated with plastic at all doorways for an additional three weeks. The flooring was installed over new plywood, sanded and finished within the next 10 days.  Cupping was observed within the next six months and progressed for another 5 or 6 months.  The overall review showed the flooring, which had been in place more than a year, was at normal moisture content for the area at 7 1/2% to 10% with the subflooring 10 to 11%. The flooring had passed through more than a complete year of seasonal change and remained cupped.  Overall, the determination was the flooring at installation and sanding was drier than the normal environment of the home and later gained moisture.  Whose fault? The flooring contractor installed flooring that was 6 to 8%, within the range of area conditions though on average somewhat low. The remediation contractor created a dry condition as the flooding had required. The two conditions created an unusual dry environment for the site. (Photos 1 and 2)



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I also was called on to consult on cupped flooring installed in the tallest building in the world, the Burj Tower in Dubai, UAE. The building is reported to be one half mile tall. The building specification calls for maintaining relative humidity between 45 and 55%. The flooring manufacturer calls for maintaining the relative humidity between 50 and 60%. The building contractor required the flooring installation begin even before environmental controls were in place to provide for either of the conditions. The flooring as installed in many apartment units was cupped. The interior environment approached the building specifications closer then the manufacturer’s specs. The determination was the flooring was drying which resulted in the face veneer shrinking abnormally, resulting in the cupping. As we know, with $500K to $1 million flats, owners will not accept cupped flooring as normal. So, which environmental condition is correct? I left that determination to the G.C., the flooring contractor, and the flooring manufacturer. (Photos 3, 4, and 5)



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Or, what about the call to inspect numerous cupped floors in different stages of construction at a residential development. None of the homes was occupied so normal environmental conditions had not been established. Unfinished flooring illustrated the lack of environmental control present at installation which resulted in moisture gain and expansion. The finished floors showed the site environmental conditions continue to add moisture to the system resulting in the cupping. (Photos 6 and 7)



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What are the requirements for wood flooring?

The environmental goal is to create conditions that are normally expected for the geographical area and the site involved. If the area is arid it makes no sense to require restrictive high relative humidity conditions. This creates an extra cost that changes a normally quite stable, dry (relative humidity consistently 20 to 25%) environment. A consumer will likely resist the extra costs for the equipment to maintain such conditions and ultimately choose another floor covering that doesn’t require the added burden.  Another caution is when malfunction occurs the moisture drop is much higher than normal environmental swings and the flooring can be adversely affected with associated gapping and cupping.

Environmental conditions for those northern areas with extended heating are also different, particularly during the winter. From the Washington D. C. area across to St. Louis and northward the heating season is significant. During December, January, and February outside temps often average below freezing. Some wood flooring manufacturers/dealers support the recommendation that the relative should be maintained at 35% to 55%, or 40% to 60%, or 30% to 50% for the life of the flooring. In these areas during high heating degree day requirements the related indoor relative humidity can often be from below 10% to 15% RH. Humidification units on heating units would normally struggle to maintain humidity at 30% much less above 30% as suggested by many. At 30% RH and above, on those below-freezing days condensation would likely form on the cold surfaces like windows. After 3 or 4 days any air leak in the building envelope would likely result in some form of condensation if the relative humidity is maintained above 30 to 35%, thereby putting the structure of the home at risk. Another choice the homeowner doesn’t want to make.



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The realistic requirement is for the interior environment to be comfortable for the inhabitants. This would normally require some form of humidification in the winter.  But the goal of maintaining it above 30 to 35% is not realistic. On very cold days humidity near 20% to 25% would be a more realistic target.  The directive is to maintain the interior environmental conditions within an overall 20% range if possible. These conditions will yield minimal gapping in the winter and little to no cupping in the summer where the flooring was properly acclimated or spaced during installation. Wider ranges will result in greater movement of the flooring.  For ranges with than a 20% average dry to humid swing requires us to make a choice when installing the flooring. Do we accommodate for gapping or cupping? It is my experience that cupping will result in the most call backs and complaints. The examples show this is likely the case. So, particularly in winter, when installing flooring add field spacing to accommodate the expected summer moisture gain. When installing in the summer the higher seasonal conditions often result in flooring with extra moisture and a slightly expanded state so that seasonal gapping is the primary issue and cupping is not a problem. 

What about the humid areas with normally higher relative humidity? The objective is the same so check that near occupied conditions are present. The moisture content of building components again shows what the ongoing environmental conditions have been and what adjustment is needed. In most cases the conditions are wetter than normal and drying by either time, ventilation, or heating is the requirement. These areas can also be problematic when the HVAC systems are not operating during the more pleasant seasons. In these cases, windows are opened letting the humid exterior air into the home. Depending on the length of the pleasant weather, the associated gain in moisture may or may not result in slightly cupped floors. Customer education is critical in these areas in that the cupping is generally not permanent and will flatten with heating and or seasonal air conditioning. Again the upper range of RH recommendations-50%, 55%, or 60%--can be exceeded without causing the flooring to fail.

Of course if site environmental conditions at installation are not anywhere near the normal occupied range, problems will result.  The most often situation is with the extra or excessive moisture of the building process. The home was rained on extensively during framing, the basement has standing water, the crawl space is wet and muddy, the slab is too new, the slab has been repeatedly wetted-are all situations that are not normal and require drying before any wood flooring is even delivered to the site.  These situations require the use of a moisture meter. Checking wood building components determines if the environment is within limits. If normal moisture content for flooring in the area is 6% winter and 9% summer then subflooring at 15% tells us that the environment has been adverse and conditions are too wet.  The 6% to 9% average is 71/2% so when other wood building components check at 111/2% and less the conditions are good for installation. We can even get by if some of the readings are somewhat higher at 12% and 13% as long as there are still the lower readings and the overall condition is drying out. As reported before the wood does not react immediately and with the felt paper over the subfloor it is initially protected while additional drying occurs.

Know your area’s normal environmental conditions; use your moisture meter to check that conditions are near these normal conditions. When situations are on the drier side add spacing to prevent later cupping. And, as always, set proper customer expectations, particularly where the overall humidity range is greater than 20%.