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.
