What’s the Proper Humidity for Wood Floors? 

The question is often asked “What is the proper humidity to keep my wood floors looking good?” The truth is there is no set limit for a “one setting fits all” approach. Much depends on building envelope, window construction style, window treatments, outdoor temperature, etc. Following some very simple rules will help minimize the possibility of window condensation and gapping of your wood floors.

Indoor relative humidity is based off what the current outdoor temperature is and plays a direct relationship on glass condensation. For example, in Minneapolis one may experience an outdoor temperature during the day of 20°F and -10°F at night. This means, based upon the chart below, the interior humidity should fluctuate between 20% and 35% and provide minimal to no window condensation.

Window Condensation

Many factors come in to play that we must pay attention to like glass temperature. To reduce condensation/frost we must always keep in mind the window dewpoint temperature and household life styles. If outdoor temperature is +10° F and indoor humidity is at 30% and indoor room temperature is at 70°F, the glass dewpoint temperature is at 36°F. This simply means if glass temperature is 36°F or below condensation and/or frost will occur. Many things can influence glass temperature. Leaving inside window screens on during the winter or pulling the window treatments closed at night keeps warm air from keeping the glass warm. A set back programmable thermostat and programmed for 60°F at night will cause moisture to condense on windows. If we look at the humidity chart above, the key is to have the indoor humidity follow the outdoor temperature. This can be done manually by setting the humidistat in the morning and the evening or install an outdoor sensor that tells the humidistat the appropriate humidity setting and control it automatically.

Wood Flooring Requirements

The National Wood Flooring Association states the optimal performance for solid wood flooring is when inside relative humidity is kept between 30 to 50%. This doesn’t mean if we keep humidity at 30% shrinkage won’t occur as gapping up the thickness of a dime is likely. These numbers would be considered normal seasonal shrinkage for wood floors. If the relative humidity drops to the high teens (long term) one would expect greater than normal gapping due to wood flooring moisture loss.

How to Minimize Shrinkage and Condensation

The first thing we must understand is the wood requirement (30 TO 50% RH) is more a seasonal concern rather than a daily concern or even hourly. If the indoor humidity drops to 12% for the day or even a few days is not a huge issue as long as the HVAC system has the ability to raise humidity to recommended limits by outdoor temperature. In many cases we are on a 12 hour cycle, humidity ranging from 20 to 35% during January where other months allow for more stable humidity levels without condensation concerns. The greatest problem we see in the wood floor industry is where the homeowner dries out their home during early winter (which is preventable) and when January hits their floor goes under heavy stress with boards gaping splitting, cracking, etc. Think of a rubber band, it can be pulled and released several times without breaking. However, you can stretch and hold the rubber band, then stretch some more until it breaks. Wood fiber reacts the very same way, expanding and contracting to the conditions they are exposed to. When the fibers are stretched due to low humidity conditions they will stretch to a point of fracture. The key is proper humidity control to prevent window condensation and wood shrinkage. There are many new humidification systems that have outdoor sensors to control the amount of indoor humidity based on outdoor temperature along with off-site monitoring.