Acclimation, sometimes called conditioning, is the process of allowing wood to reach its equilibrium moisture content (EMC) within “normal living conditions.” It is also one of the most important steps of hardwood floor installation. Not properly acclimating or conditioning wood flooring may cause excessive expansion, shrinkage, dimensional distortion, or even structural damage.
If the flooring material being installed does not have specific acclimation and conditioning instructions, here are the steps to follow:
Step 1: Make sure that the heating and air conditioning units are in operation at least five days before delivery of the flooring, during installation and after the flooring is installed. If it is not possible for permanent HVAC to be operating before, during and after installation, a temporary system that mimics normal living conditions may enable installation to proceed.
Step 2: Once the facility has been confirmed to be at the expected living condition, proceed with delivery of flooring material. Check the moisture content of the wood flooring as soon as it is received at the jobsite.
Step 3: Check the moisture content of the subfloor. The moisture content of the subfloor should coincide with the temperature and relative humidity of the jobsite, based on the temperature, relative humidity and average moisture content chart shown below. This moisture content reading will give you a good idea of where the conditions in the facility are being maintained and allow you to compare to the expected “in-use” conditions.
Step 4: Ensure the flooring material is exposed to the “normal” conditions of the environment in which it is being installed. To accomplish this, break the flooring units into small lots and/or open the flooring packages. Cross-stack the material with spacers between each layer to allow air circulation on all sides of all boards. Start stacking elevated from the subfloor. Acclimate to equilibrium moisture content for as long as it takes. Some species will take much longer to reach equilibrium moisture content than others. It is never a good idea to base acclimation on time alone, but rather on actual moisture content. Check with the manufacturer before beginning this stage, in case they have different acclimation instructions.
Step 5: If the flooring material cannot be delivered to an adequate jobsite, pre-acclimate the material in an off-site location set to mimic the expected conditions of the jobsite. Then deliver pre-acclimated material to the jobsite once “normal conditions” can be established. Again, refer to the temperature, relative humidity and moisture content chart to determine ideal conditions.
Step 6: Finally, make sure the flooring and wood subfloor moisture content is within the acceptable range for the jobsite. The subfloor should be within 4 percent for strip and 2 percent for plank wood flooring.
Wood is only acclimated or conditioned once it reaches its equilibrium moisture content for the space in which it is expected to perform. Equilibrium moisture content is based on an “unchanging” environment. After a wood floor has been installed, changing conditions within the environment will change the equilibrium moisture content of the wood floor, ultimately resulting in dimensional change.
And therein lies the rub. Changing environmental conditions. Especially in New construction. This is where we get into trouble. You cannot predict the weather. You cannot predict that the other subs will leave doors open and turn the heating and cooling system up or down. You cannot predict the ambient humidity dropping 30% for 2 weeks straight during your acclimation time. You cannot predict that when you make you moisture readings on the subfloor and it reads 3% because it’s new Osb and you’re wood flooring is measuring 6 to 8%. Tell me why no studies have ever been done detailing what the effects are when the subfloor is more than 4% dryer than the wood floor your putting down. Believe me I have done much research on the matter. Apparently I am the only one in the country that has this problem. Granted it is not common but in 30 years of business I have seen it at least a dozen times. When you have a $50.000 floor at risk it would be nice to know that NWFA has my back. I guess I’ll just wait as long as it takes for equilibrium while the bank refuses to pay the general contractor for lack of progress and thank my lucky stars if I ever get another job. Mike Imel Sr. Imel and Sons Hardwood Floors Grants Pass Oregon
Hi Mike,
You make some great points.
What you Can do as the installer is test the facility to ensure its ready to receive the flooring that is being installed in it.
If you are concerned with the conditions of the facility while the wood is sitting on-site acclimating you can condition the wood in an off-site location mimicking the expected conditions of the facility its being installed in (and again make sure the facility is ready to receive your flooring when the builder has you scheduled for install).
Regarding your question about MC of subfloors being >4% lower than the wood floor at installation- we are currently researching this scenario as well as other variations of lower and higher subfloor MC at install with some well know labs. Although we’ll never be able to replicate every scenario, we’re hoping to help the installer best approach different scenarios with the results of this testing. I’d love to talk more in depth with you about the results of your research.
Lastly- look up “Spearin Doctrin” – this dates way back to a 1918 US Supreme Court case that, in short plays an important role in deciding who is responsible for construction related failures. Specifically failures caused by circumstances out of your control, like many of which you refer to above.
Thank you Brett. I eagerly look forward to the new recommendations set forth from the testing of lower than normal subfloor scenario. I got the name of someone in the lab that does all the testing for you or for NWFA from technical support . I called him. It was a fascinating conversation. He recommended I submit an article to your magazine to draw attention to the need. Alas I am not a writer. In the end at that particular time no funding was available for further research. Thanks for bringing awareness. Mike
If the floors are not acclimated properly will they curl up and appear like they got wet? I am having this issue on my new hardwood floors. Thank you, in advance, a response to my question.