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Acclimation Hardwood Floors - How Long?

Acclimation to the installation environment for solid Acclimating hardwood floorswood floors should never be overlooked. While it's not as important for engineered products due to the stability in the way they are constructed, solids will expand and contract considerably, given the right conditions with wide changes in relative humidity levels.

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It's Not About How Long

Contrary to what the common misconception is, acclimation is not about time only. The focus should be accommodating any new material to what the average living conditions will be in your home. It does get a bit tricky, but if you’re patient enough, a thorough understanding is included below.

Tips To Avoid Problems

Far too many times I hear of separating or gapping months after the installation, or the floors have buckled. By following some simple guidelines these events should not occur. Unfortunately, many run of the mill installers and builders for that matter, don't know about proper jobsite conditions. In some cases it's often...."get it in and we'll worry about problems later."

Basic Guidelines

Inevitably they learn their lesson with costly repair bills and replacements, not to mention disgruntled customers. Following are some basic guidelines for a problem free installation; before during, and after.

• Solid wood flooring should be delivered to the jobsite and allowed to acclimate to the conditions in which it will be installed or near occupancy levels. Temperature and humidity should be monitored.
• Delivery should be after major projects have been completed and thoroughly dry, such as dry wall and masonry work. Doors and windows must be in. Testing of wood subfloors should be performed and moisture levels should not exceed 12-14%¹
• Drainage- It is vital the grading should be directed away from the home to prevent unnecessary moisture problems.
• Basements and crawl spaces need proper ventilation or environmental control. Dehumidification systems have become the ideal solution in both situations. Polyethylene film of 6 mil (minimum) in thickness must be used as a proper vapor retarder in any crawl spaces or where the soil line lies underneath.
• All areas of the home should be in harmony with relative humidity levels; before, during and after the installation. This also includes basements. See case studies below.

Species Stability Can Play A Role

The stability of hardwood flooring relates to how much change can take place with moisture changes in the environment. With lower stability ratings expect Gapped maple floormore shrinkage (gapping) during extended dry conditions. Higher moisture conditions will cause more expansion.

These characteristics are more common with solid and not engineered. It should be noted, products installed with a high moisture content will shrink regardless of the stability factor.² The opposite will be true of those with low moisture content.

There are differences in stability from one hardwood to the next, although most are suitable without much need to make adjustments. Two common species in particular that should be addressed are Hickory and Maple (shown). The two are not suitable for climates or conditions that present wide swings in the environment's relative humidity during a calendar year. Adding systems that regulate the relative humidity within the home (including basements and crawlspaces) is the only real answer to maintaining these two species without complications.

Related to stability we could throw in wide plank products. With all species, the wider the plank (exception engineered and vertical grain) the more movement the floor will take on with excessive movements in relative humidity within the environment. For a better explanation, wider boards show more gapping or swelling during these periods opposed to narrower boards.

An Interesting Story. Effects of No Acclimation

From: Jay at J. D. Reynolds Home Improvements
Westchester County NY

Regardless of whether you prefer a manual flooring nailer or pneumatic, cleats or staples, be sure to allow your natural flooring to acclimatize! This is not merely a suggestion, this may very well be the most important rule of the hardwood floor installation process.

It's too easy to allow schedules to dictate how and when we approach various portions of individual projects, but unlike priming one room when there are four others still awaiting a first coat of mud. This one cannot be rushed for the sake of expediency.

Quick nightmare tale. A buddy of mine was building his home at the same time I was renovating my own. It turned out that we were going to be ready for flooring at approximately the same time. He, a bit sooner than I, but close enough that it made sense to purchase our flooring (through another friend) in a great enough quantity that we'd benefit from an even greater discount.

Against the advice of both myself and our friend (the flooring distributor), he had his installed virtually immediately. I, on the other hand, having waited ten years to attack my house was in no rush to get the floor installed. We had the flooring delivered about mid August.....a time of year I try and avoid installations to begin with, due to the high humidity levels in this neck of the woods. I allowed mine to sit for nearly a month or mid September, before bangin` em down. We're now in the beginning portion of the heating season. His floors have begun to shrink drastically, even cupping and raising to a degree. Mine, on the other hand have yet to show any signs of shrinkage at all.

I'm not an "I told you so" kind of guy, but this is a lesson he won't soon forget and one I hope some of you may benefit from.

Acclimation With Engineered Hardwoods

Always follow individual manufacturer specifications that are available on their respective websites or contained within the cartons of flooring. Pulling planks from cartons days or weeks prior to installation can cause installation problems. Why? Moisture from the environment can actually swell the material ever so slightly (depending on the relative humidity) creating problems with fit of some engineered products that are milled with precision. Conversely, in extreme dry environments shrinkage may occur. Click type floating floors are very susceptible to these potential problems.

It’s Not Just About Acclimation Either

As today’s homes are insulated to the teeth, imbalances in moisture can happen after you move in which is not an installation failure. Unfortunately some inspectors that have nary a clue what may cause issues after the fact, have a bad habit of blaming the wrong party.

Basements and crawlspaces are the biggest culprits. Too often we put far too much emphasis on simple acclimation and having the hardwood floor near occupancy levels. Yea, makes sense doesn’t it? But the real issue is what kind of moisture levels are maintained in the areas below and above the new floor areas.

Case Study - Hardwood Floors Cup

A new home is being built in Atlanta that has a walkout basement. Construction is typical with hardwood floors installed above on a wood sub floor. The basement is not built out as the younger homeowner needs to save more money for their sports zone party area a few years down the road.

Floors are installed with all the acclimation specs followed. The builder also managed to get the HVAC system running a month prior as well. Sounds perfect huh? But what about that basement? The new owners don’t want to pay the added costs after they move in. A dehumidifier can work, but it’s important that it maintain a relative humidity level close to what’s seen above. It’s also important to find the right dehumidification system, or one that can keep up.

Four months going forward into September and the owners are horrified that their three inch wide red oak floors are cupping. They had the upstairs areas climate controlled and kept the humidity in the range where wood floor manufacturers recommend. However, the higher moisture from below without any controls was affecting the actual wood floor. Sure it had the typical moisture retarder requirements under it, but the overwhelming force of higher humidity from the basement was causing the floor to expand which causes cupping.

Case Study - Hardwood Floors Crown

Crowning is where the edges of the boards are lower than the center or opposite of cupping. This effect happens far less than ordinary cupping.

This new home is actually across the street from the first one Panelization and crowned hardwood floordiscussed. Very similar design and exact time period it was built. Same Red Oak floors too. However this single bachelor had his man cave built out at the same time. He also upgraded to a high end whole house air system. This floor was also installed by the same crew, but something went wrong here too.

The second floor began to crown and separate in some areas. In essence the areas below the new hardwood floor were much drier than where the hardwood was installed. Separation or what experts call penalization was caused by the contraction of the plywood sub floor, opening gaps and forcing more dry air into the sub floor. This condition can also create popping sounds as fasteners lose their grip in the sub floor.

Gaps? We Don't Want Any Gaps!

We have an entire page on this subject.

Best Time To Install Floors?

Often a myriad of opinions can be found on this subject, but professionals agree finding what the average humidity level in the home year round will reduce contraction and minimizing gapping. It's also vital the actual installation and acclimation period be close to this level to achieve the best results. Products that test humidity, called hygrometers are best used to monitor jobsite conditions.

Hygrometer readings from one brand to the next can vary. I would suggest using several and move them around from time to time. Relative humidity levels may also be different from one area to another in the case of using standard portable dehumidification systems. One may not get the job done, requiring two for effective control. Always test and monitor conditions for best results.

What Filler Will Fix My Gapped Floors?

Unfortunately no filler will work effectively unless the humidity level is kept constant. During the expansion process when warmer days allow for houses to be open to outside air, gapping will close up as moisture is taken on and the filler eventually gets squeezed out.

For very wide more permanent gapping, gluing rope with a urethane adhesive between boards has been successful. Caution should be exercised here as glues can stain and become rather noticeable once the floor is finished. Wiping clean with a rag and mineral spirits is suggested prior to the application of any finish.

Maintaining Relative Humidity - A Desert Climate Example

A common complaint about wood flooring in arid zones is checking and splitting due to low humidity. Many people believe running a humidifier in arid zones in summer is futile. This is largely due to the fact that an AC unit is a dehumidifier. Humidifiers and HVAC units will counter act each other but not totally as your AC does not run continually and this counter reaction by the AC unit can be neutralized to a point. However, some very minor checking, gapping should be expected as nothing can be 100% effective. Hey, it's wood not steel. I expect to see some verbiage to this effect in all manufactures requirements like this for arid zones soon.

Humidification and HVAC Systems

In my zone (Southern Arizona) it is still very dry during the summer months or down into the single In home humidifierdigits until the rains (monsoons) come in mid July to August. Even though one may think it is useless, a humidifying system attached to your HVAC system is still working to keep up the Rh in your home, lessoning the chance your floor will check (show minor surface splits).

Stand Alone Humidifier

Other options exist such as using a stand alone humidifier which I think is more effective than the built in humidifier as it runs all the time and does not shut off with the HVAC unit. Recently one of my clients found a stand alone humidifier that holds 10.5 gallons at Home Depot. So far it has kept the Rh in the house up to 35% as compared to the 19% Rh level it was at previously! Any qualified HVAC salesman can assist in your needs.

Dual Systems

Also, quite a few houses like mine have dual cooling systems. I am able to run an evaporative cooling system (MasterCool II) which easily raises the Rh of the home to any manufacturer requirements. Then when the rains come I switch to AC. Humidification of your house in arid zones like Arizona will also help your wood furniture as well as your sinuses. In fact it is very healthy and typically called a healthy relative zone of 35 to 55 % for wood and people too!

After a discussion I had with a consumer who had a bad experience with their wood floor checking I called a Steinway Piano dealer I put a floor in for last fall. He humidifies his entire store, winter through summer. In fact I got a good chewing out when I was installing solid maple in his showroom as I left the door open when transporting tools in and out. His $100,000 Steinway he had near the entrance needs a constant ambient temp and Rh or the wood will__________ you fill in the blank.

Needless to say I don't have to worry about that maple floor. And there really is no excuse for not at least trying to maintain your floor close to manufactures ambient zone requirements. Even though it might not be attainable. So go out and buy a hygrometer for your house if you do not have one and check the relative humidity. Your floor will love you for it and so will your wood furniture.


The house is around 2200 square feet. Filling three separate plastic bottles in the unit is done in a facet. Initially the unit needed filling every day until the house absorbed the moisture. Then leveled off to 4 days. When I left the owner could turn it down to run less and the Rh was right at 30 to 31 %. Remember her inside Rh was 19%, Outside it was running 8%.

¹ Will depend on the average moisture levels from one region to another.

² The reference pertains to climates that have seasons, or those homeowners that at times utilize heating or air conditioning, while opening the home during pleasant outdoor weather conditions. As always, be aware of the conditions in which the floor will be installed and maintaining a committed level of relative humidity within the home environment afterwards.

Contributor: Stephen Perrera. Top Floor Installation Co.
Specialty: Jobsite Moisture Testing
Location: Tucson, Arizona

Stephen Perrera Top Floor Installation Co

Contact: (520) 444-1885