CFCRA Technical Bulletin
The CFCRA has observed the flooring industry and flooring installations that fail for a few decades now. We feel it is important that the CFCRA helps promote the requirements of flooring before, during and after installation so that you have a problem free, wonderful looking floor for decades.
There are basically two categories of wood flooring. There are solid woods of various species and there are engineered wood products consisting of multiple layers of wood or that have a core made of another species of wood, MDF/HDF, or something else.
Solid physically shrinks or expands as it loses or gains moisture. As the RH increases, wood's moisture content (MC) increases, and because of that, its physical size increases. It expands. As the RH goes down though, wood's MC decreases, and its physical size decreases.
To determine how much a floor can contract or expand you will use the following sequence (assuming it is a plain sawn wood)
Multiply the width of the board (in inches) by the annual change in MC (highest annual MC minus lowest). Then multiply the result by the number from the chart below:
This form of wood is hard to discern from a solid wood product once it has been installed.
Engineered wood is actually plywood and is “engineered” so that the expansion and contraction of the product is minimalized.
The real life implications: The chart below shows the ideal requirements for solid wood temperature and relative humidity wise.
It is unrealistic to believe that we can maintain 30-50% relative humidity levels in a home in Canada during the winter months without some kind of additional water vapour being introduced to the environment. In other words, the planks shrink and there are gaps now between the planks.
It is not uncommon to see the relative humidity drop down as low as 15-20% in many areas of this country. This causes all hygroscopic items to lose moisture into the ambient air and, after a month or so of winter weather, the site’s building materials have lost a lot of their moisture content.
If Canadians are not proactive in keeping the relative humidity up, in other words, if you let the relative humidity stay below 30% for a long period of time, your wood will shrink and there is almost no way to get the humidity back up to 30% as all the materials and contents are absorbing the extra moisture put into the air by your humidifier.
The first section of this release outlines that the width of the floor boards is a key factor in the function of wood’s expansion/contraction. Let’s take a 2” strip of wood and compare the contraction rate of a floor made of this kind of flooring versus a 6” plank. If you follow the formula:
Moisture change x width of plank x co-efficient from chart= change in size.
The six inch plank will have gaps 3x as wide as a two inch strip floor over the winter under the same conditions. This size of gap is almost always considered unacceptable by homeowners.
Start your humidifier now, at the very start of the cold season. Set your humistat at 40% at the beginning of October. Buy a small thermos-hygrometer at Canadian Tire and monitor the relative humidity in the house to ensure it is not dipping below 30%.
If you are reactive and start the humidification process too late in the year, you will not be able to get the RH up to where you need it.
**Be forewarned that some wood products require a relative humidity of between 40-60% which is almost impossible to maintain in the winter anywhere in Canada.