Generally when we talk about humidity we generally talk about the RH% (relative humidity) in a house, but the humidity and temperature outside play a big role in how high or low your RH is inside. Here are a few things to keep in mind on how humidity affects your hardwood floors.
Hardwood Floors are Living
We all know that real hardwood floors come from living plants--trees. But once a tree is cut to be used as flooring, although the wood is no longer alive and growing, it still behaves like a living thing, meaning that it is affected by the climate of your home, which is unlike that of man-made materials. What this means is you should try and keep a consistent temperature and humidity level in your home year round. Keeping your floor flat all starts before its installed. Before the hardwood flooring is installed it needs time to acclimate to the surroundings its going to spend the rest of its life in. What this does is allow it to stabilize according to its surroundings, making a happy floor! Once it is within a 4% moisture content of the sub-floor its ready to be installed.
HVAC is Heating Ventilation and Air Conditioning. This plays a big part in keeping your hardwood floor happy too. Keeping a consistent temperature and humidity in your house is a big part in keeping your floor flat and fasteners sound. You generally want to keep your house between 65-75 degrees all year round and RH in the house between 35-45%. Now keeping the temperature consistent in your house isn't near as hard as is keeping the RH between where it needs to be, but with a few additions to your furnace it can be quite simple to achieve the desired relative humidity. An easy way to maintain the RH in your house is to have a whole house humidifier installed on your furnace to run during the winter, and during the summer run the AC. In some cases, you may need to also invest in a dehumidifier. This will help keep your floor flat and reduce the movement in your floor.
Cupping and Crowning
Cupping takes place when wood takes on too much moisture, can't expand any farther, and gets pushed up against the adjoining boards, so it has no where to go but to curl up. There are many different degrees of cupping. Sometimes the floor will pull on moisture during the summer causing it to cup slightly which isn't too much of a concern as long as it isn't severe. As long as you don't let the flooring to cup too much, it will return back to how it was once you get the RH in your home back to a normal range.
Crowning happens when the wood dries out and shrinks causing the edges to go down and the center to slightly rise. This generally occurs when your RH% becomes too low inside the house causing the floor to crown.
Cupped or crowned floors shouldn't be refinished until the moisture content in the hardwood floor returns to a normal state, the reason this is so important is that if you sand a cupped floor you'll be making it flat which is what you want, but once that hardwood returns to a normal moisture range the edges which where high get sanded off, so once the floor goes back to normal state you'll have large gaps because the edges where sanded off.
Gaps between floor boards occur naturally and are not deemed an imperfection. No matter how well you keep the RH% in your house gaps are still very possible. These gaps are usually no more than a few pieces of paper thick. By keeping your RH% in check, you greatly reduce gaps which in turn helps keep the fasteners intact better. What happens when the floor expands is it pulls at the fastener and the fasteners are designed to move a little with the wood but the more you let your hardwood floor take on moisture the more it will pull at the fasteners. When this happens, the floor pulls them away from the hardwood and then when it shrinks the fastener stays where it was when the floor was expanded loosing up the fastener. The more times your floor drastically moves the more it won't snap back together properly and you will start to see more gaps.
Buckling happens when your hardwood floor takes on more moisture than it should and expands until it has no where else to go except up. I have actually seen a floor where the buckling was so bad that it lifted a piano right off the ground! Once a floor buckles you have to let the floor return to a normal range, then the pieces that are loose from the floor buckling need to be removed and and new wood installed in its place.
So keeping the RH% in your house is very important for keeping your floor flat, and minimizing the gaps and potential gaps. A couple of things you can do is to have a hygrometer sitting put in a central place, making sure your HVAC is working properly and if you don't have a whole house humidifier I would suggest having one installed. Doing all of these things will ensure your floor stay very happy!