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Mystery Moisture

Rob Winstone on Equilibrium Moisture Content — Part 1

EQUILIBRIUM Moisture Content (EMC) is a subject that has been presented numerous times in the past in CFJ. However from the inspections of floors which I have carried out in recent years, there still seem to be a lack of understanding on how important EMC is when it comes to installing a timber floor and the effect it will have on the finished project.

We should all understand that timber is hydroscopic which means that it has the ability to absorb or lose moisture until it is in equilibrium with its surrounding environment. This is a process that results in the changes in vapour pressure within the material and the surrounding environment.

If there is an imbalance in pressure between the surrounding environment and the moisture in the timber, the moisture vapour will move one way or the other until the two are in balance, in equilibrium.

At this stage we need to take a step back and look at water in the freshly cut timber. The timber is made up of cells which, in simple terms, comprise a wall and a cavity. These both hold water when the tree is felled. The water in the cell cavity is known as free water and the water in the cell walls is known as bound water. As the timber dries, be it naturally in the air or in a controlled Kiln environment, the water is lost first from the cell cavities, the free water, and then from the cell walls.

Generally the timber will be in the region of 28% moisture content give or take a couple of percentage points depending on the species of the timber. This is known as fibre saturation point (FSP). As it is free water that is lost the section of timber remains the same dimension.

When we continue to dry the timber below FSP the water is then lost is from the cell walls. Because of the way the fibres align in the cell wall, the cell wall reduces in dimension and the timber starts to reduce in dimension. This is known as movement.

The more water that is lost from the walls the more the timber moves and reduces in dimension. The amount of movement is very dependent on the species of timber and is categorised as small, medium and large movement timber. In flooring the most widely used species is currently oak and this is a medium movement timber.

A small movement timber sometimes used is teak. Movement is most noticeable in the width and thickness of the timber, depending on where the section has been cut from the log. Solid timber moves very little in its length and this amount of movement is disregarded.

However over the years I have seen very small and random open heading joints and have not been able to identify a reason for this other than poor installation or some minor shrinkage.

Next month I will discuss the effects of relative humidity and temperature.

Rob Winstone is a flooring consultant, an expert witness for timber flooring and timber issues and an accredited civil and commercial mediator.

T: 07831 443088

This article has been reproduced from the Contract Flooring Journal website. You can find them at