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Moisture And Humidity - Important Factors Before Floor Fitting
The majority of the problems plaguing a wooden floor are related to moisture. Wood is a hygroscopic material. During the different seasons, when exposed to certain atmospheric conditions it will shrink or expand regularly. This constant movement can lead to buckling, curling, cupping, gapping and crowning. Moisture also creates the necessary environment for the growth of mould. As you can see, many of the issues your floor can face can be traced directly to moisture. This means that moisture and humidity have to be controlled. The question is how?
Air contains a certain amount of moisture. When it gets hotter, water vapours everywhere begin to evaporate and goes where? That’s right, in the air. So the amount of moisture in the air depends on the temperature changes and the atmospheric conditions.
So, relative humidity or RH is used to measure the number of water vapours in the air compared to the maximum amount it can hold at a given temperature and is expressed in percentages.
Wood Moisture Content
Generally, moisture content or MC is a measure of the amount of water or water vapours within a given substance. In the flooring industry, it is the weight of the water contained in the wood compared to a sample of the same species dried at a specific temperature for a certain amount of time.
Of course, every living tree contains moisture. It has pores and cells dedicated to the transportation of nutrients across the trunk. When you cut the tree, those juices remain there and need some time to evaporate naturally to make the material useful. However, time is money so the lumber mills use kilns to remove most of the moisture. The cellular structure of the wood remains intact, though, and can still transport vapour. Most flooring products come with a moisture content between 6 to 9%.
The water vapours always seek a balance with their surroundings. If the air has higher RH than the floor has MC, some of its humidity will be transferred to the wood. If the air is drier, it will absorb some of the vapours from the floor instead. Once the wood no longer gains nor loses water vapours it has achieved equilibrium moisture content (EMC).
Controlling Moisture before Installation
Essentially, if there is a disparity between humidity in air and moisture in the floor, the wood will gain or lose moisture until a balance is reached. This can lead to dimensional changes in the floorboards. Not along their length but in their width and thickness. To prevent movement, floorboards are left to acclimatise for two weeks before floor installation in the place they are about to be laid. If this rule is not followed the wood will probably warp unpleasantly. This is especially true for exotic timber from countries with vastly different climates.
Before fitting your new floor, you must also ensure that the subfloor has appropriate moisture levels. To determine this, you need a moisture meter. If the subfloor is timber, you should ideally take around 30 readings in the area and check if the results are consistent. 2-3% of moisture content is a good average. Even if you have a single higher reading, most of the time it should present an issue. The perfect amount of moisture on a timber subfloor is 12%. The less, the better.
A concrete subfloor is usually a bit more troublesome because those typically have moisture problems. 35-40% humidity is ideal but since this is rarely the case you’ll need an underlay with a moisture barrier or even better - a damp proof membrane (DPM).