Relative Humidity and Hardwood Flooring

As we try and escape the grip of one of the hardest winters ever in the Mid Atlantic region, gapping of hardwood continues to be an on- going issue for hardwood installations.

I continue to receive calls asking for the tolerance of gaps in engineered and solid hardwood flooring. I ask four simple questions;

What was the subfloor and hardwood moisture content at the time of installation? Was the hardwood tight together at the time of installation?
Does the home have a humidification system installed?
What is the relative humidity in the room where the hardwood is installed?

Generally, I get one question answered, question #2 and the hardwood was tight at the time of installation.

It still shocks me how many stores installing a sensitive product like hardwood flooring are still not checking and recording subfloor and moisture content, and talking about relative humidity requirements to the home owner.

If you are selling hardwood flooring, one area you want to address with your customer is maintaining the relative humidity in the home where the hardwood flooring is installed. If not, expect to have questions about why the hardwood flooring is gapping in the winter time. Without additional humidification, in-home relative humidity can drop below 20% during the heating season. I have had some instances, where the relative humidity has been as low as 15% relative humidity. Generally, the recommended range for maintaining in home relative humidity is 35% – 55%. Some exotic flooring manufactures may have a minimum requirement as high as 40%. Consumers have unrealistic expectations for their flooring and feel it should always be nice and tight throughout its life. Again, not having the conversation about maintaining relative humidity with a picky customer can only lead to frustration with your customer.

In line Furnace Humidifier

Wood is a hygroscopic material, which absorbs or looses moisture until equilibrium with the surrounding air has been reached. Wood flooring reacts to the environment it is in. Wood gains or loses moisture and correspondingly gets bigger or smaller based on the moisture content and temperature of its surroundings. When the moisture content of the environment changes, shrinking, cupping or even crowning of hardwood flooring can occur. Wood is stable when no more changes occur; it has reached the equilibrium moisture content (EMC). Finishes slow this movement of water in and out of the hardwood, but do not stop it.

As flooring professionals, you need to know what determines or affects humidity and temperature levels in a house, and how those interactions affect wood floors. Selling a hardwood floor to a consumer with a hardwood stove is probably not the best scenario. The hardwood stove will dry out the air in the home severely, causing all kinds of flooring issues. Customers with an in room or table top humidifier are wasting their time with these products as the remaining dry house will be stealing the humidity from these products. On possible solution is to leave behind a hygrometer at the installation to alert the customer about maintaining the relative humidity.


Hygrometer measuring temperature and relative humidity

With solid hardwood flooring, there will probably be some normal seasonal gapping. The size of the gaps will depend upon the size of the planks. The wider the plank, the wider the gap. The seasonal gap for a solid red oak strip floor is 1/32 of an inch. With that, the expected seasonal gap for a solid 5′′ wide red oak floor would be 0.070′′, or about the thickness of two dimes. Extremely dry conditions (those below 25%) may also result in cracks and checking in the surface of both engineered and solid planks.

These are not considered defects and are not covered under manufacturer’s warranties. Again, a more stable environment results in a more stable floor.

Back to question #1 above, why is record keeping important?

In case of a flooring failure, the question always comes up, who is to blame and who pays for any necessary repairs. The problem may be very obvious but the cause for the problem is often hard to find. Keeping reliable records gives you a reference point to go back to and can help protect you from un- warranted claims. Write down or photograph the moisture readings with your phone and record your readings on the job tickets or reports that get filed away. This should be done with any type of flooring. If you’re not going to take the time to cover yourself, who is?



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