Relative humidity for a house

Q: What is the recommended relative humidity for a house in mixed climate (heat & air condition)? The house is equipped with a humidifier to regulate the RH during the heating season.

The house is equipped with forced air heat but, at night, the owners shut down the forced air and heat with wall radiators with hot water circulated from a gas fired boiler. The *** pine flooring is shrinking, leaving unsightly cracks between the boards (some, but not all).

A: The issue of relative humidity is an interesting one. Pre finished manufacturers tell us to keep the humidity level at about 45-50%. Quite frankly, that is too high in the cold winter months. You would have water running down your windows.

If you keep it above 30% in winter, say best is 35-40% and in summer not more than 65% you should be fine. I have been told that a big factor in this: try to keep the floor near to the relative humidity at which it was installed, but keeping the above facts in mind. If you can avoid huge swings in humidity there shouldn’t be a lot of floor movement, although pine seems famous for major shrinkage in the first season or so after it has been installed.

Related Q: I installed a new hardwood floor and everything was good until I put in a propane stove (it is vented). It’s in the finished basement. Now there are gaps and cracks in the floor and hardwood stairs. Is the stove causing this problem? What can I do?

A: It seems clear that now with the heating season your floor is shrinking because of dry conditions. Was the floor and sub floor checked with a moisture meter and acclimated to the climate in the house before installation? It may help to increase the RH in the home. In a cold climate I wouldn’t raise it above 40%. You may find some of those gaps will lessen in summer. I heat this little place with a wood stove. In winter it isn’t unusual for the relative humidity to drop to 20% which is quite low and dry. Generally you don’t want a hardwood floor in this environment unless it was acclimated to that before installing.