Humidity and cracks forming on the wood boards

Q: I installed pre-finished floors in my kitchen, family room, and entry way about a year and a half ago. They were installed over plywood which was over concrete. There was a moisture barrier put down as well. I have been noticing cracks slowly forming on the wood boards, especially now as we are transitioning from winter into spring.

It seems like the cracks start somewhere within the board and somehow get to the surface and bubble up the finish. The cracks go with the grain. In all your answers, you say that the cracks are due to lack of moisture. Could the cracks also be due to too much moisture? I bought a hygrometer and put it on the floor and the humidity in those rooms run between 55% and 65%. I think in your answers, you said the relative humidity in the air should be between 30% and 45%. I’m just not sure if I should add more moisture in the air or take out moisture.

A: This humid question is a sticky topic. If you read the advice of floor manufacturers, they will give the figure, likely of 45% relative humidity. However, where you live can dictate what to do. If I kept the humidity in my house at 45% in the cold winter, with furnace running, I would have condensation running down the windows.

I think the most important thing one can do is this: you know your environment and how humid it will be throughout the year. Install the flooring at the average and try to keep it close to that in your home throughout the year if possible. Also, make sure the flooring is in the home at least 5 days before installing so it can have a chance to adapt to the environment. If flooring was brought into a home in a high humidity environment, measuring, say 9% relative moisture content and you had high humidity outside, say 70% and the floor was installed immediately, it could expand after installation and cup, especially if no gap was left on the outside walls. If the humid condition dropped significantly, the floors could then shrink, leaving gaps.

In the above case, excessive moisture caused gaps because it caused the boards to swell, pushing against each other and moving each board just a little, then shrinking. This however, usually includes cupping of the boards.