Richard Renouf explains that while there are many ways to measure moisture in the air and in building materials, a surface-mounted hygrometer is the best, most accurate way of taking moisture readings.

RELATIVE humidity, % MC, % CM, Aquant, WME… there seem to be many ways of measuring moisture in the air and in building materials. Most flooring installers know that British Standards recommends that the moisture content is below 75% relative humidity when measured using a hygrometer, but what does this mean? And how does it relate to the other methods and units of measuring moisture?

Relative humidity is a measurement of the amount of moisture in air. It’s different from the other measurements listed above which are measurements of the amount of moisture in building materials such as concrete, gypsum and wood. It seems odd to take an air measurement, but it’s very important for several reasons.

Moisture is only a problem if it’s going to affect the flooring, adhesives, or other products a floorlayer uses. If the moisture is held under a damp-proof membrane so it cannot escape from the subfloor it won’t be a problem. A moisture reading of the subfloor would give us a high figure, but a reading taken from the air above the subfloor would not, because the moisture is being held safely. That’s why a surface mounted hygrometer is the best and most accurate way of taking moisture readings.

But what is ‘relative humidity’? And what effect does it have on building materials?
Air can only hold a limited amount of moisture, but this varies with temperature. For example, the 44 cubic metres of air in a room 4m wide, 5m long and 2.5m high at 20deg C can hold a maximum of 748 grams of water vapour – that’s almost three quarters of a litre, a pint and a third in imperial units. But if the temperature dropped to 10deg C the same volume of air could only hold 396 grams. And if the temperature went up to 30deg C the air could hold more than 1.3kg of moisture.

However, the actual weight of the moisture isn’t the critical factor: what matters is how ‘full’ the air is – we use the term ‘saturated’ for air which has the maximum possible amount of moisture in it.

A cubic metre of air at 20deg C can hold a maximum 17g of moisture. If, however, it only has 8.5g of moisture, the air is said to have a relative humidity of 50% because it only has half the amount of moisture in it that it could have. If the temperature dropped the amount of moisture the air could hold would go down, so the relative humidity would go up.

If the temperature dropped enough to increase the relative humidity to 100% then any further drop in temperature would result in the air being over-saturated and so the excess moisture would form condensation. This can happen even if only a small area drops in temperature – which is why condensation will form on your G&T even on a hot summer’s day.

Condensation can, of course, affect floorlaying and this is why British Standards, and manufacturers’ instructions, insist on a stable ambient temperature above 18deg C for flooring installation.

But it’s not just condensation we need to be aware of. The moisture content of building materials will change in direct proportion to the relative humidity of the air.

When the relative humidity is high the materials will absorb moisture until the moisture in the material and the moisture in the air are in balance. This is known as ‘equilibrium’. When the relative humidity is low, the materials will give off moisture into the air until the materials and the air are in balance – at equilibrium – again. Of course, relative humidity in the air is changing all the time, but as the changes in building materials take longer, the materials usually settle at an average moisture content which is moisture content it would have if the air temperature was 20deg C and the relative humidity was 65%. A wet concrete slab will keep on giving off moisture until this equilibrium is reached, and this can take years.

It works both ways, and this is where the hygrometer reading comes into play. With a hygrometer we seal a small air space immediately above the subfloor. Over a period of 48 hours or so the air in the hygrometer and the moisture in the subfloor reach equilibrium. If the subfloor is dry the air shouldn’t go above 75% relative humidity. But if the subfloor has a high moisture content, the air will absorb more moisture and give a higher reading.

Although it’s a reading of the moisture in the air, it actually gives us a very accurate indication of whether or not the floor is dry and, more importantly, whether or not the subfloor can and will give off moisture that could affect our flooring materials.

There’s a slightly quicker method of obtaining a relative humidity reading, and that’s by using a sleeve inserted into a hole drilled into the subfloor. The hole is drilled to about 40% of the thickness of the slab and readings can usually be obtained in a few hours, rather than a couple of days. But if the hole is drilled through any damp-proof treatments, whether buried in the subfloor or applied on the surface, the moisture barrier will be comprised and this could be a serious problem.

Next time I’ll look at other methods of moisture measurement and how these compare to relative humidity readings
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Richard Renouf is an independent flooring consultant