5% RH or 65% RH? Why do some manufacturers have a lower requirement for the moisture content of a screed? RICHARD RENOUF investigates
THERE are two likely reasons some manufacturers have a lower requirement for the moisture content of a screed – 75% RH or 65% RH. But before we look at these, what do the figures mean, anyway?
RH stands for relative humidity. This is the moisture content of air, not concrete, so it may seem a strange measurement to use, but there’s a reason why it’s quoted like this in the British Standards for flooring.
Moisture in a screed may not be a problem if it stays there, so the important thing to measure is whether or not the screed will give off moisture that could affect flooring or adhesive laid on top of it.
The standard method of measuring is to seal a hygrometer box on the surface and to measure the RH of the airspace when it has reached equilibrium (balance) with the screed. This method even works when there is a dpm close to the surface as this retards moisture and the hygrometer only measures the ratio of moisture to air that is coming out of the floor.
RH tells us how saturated the air is. At any given temperature the air can only hold a limited amount of moisture. The warmer the air, the more moisture it can hold. The relative humidity is amount of moisture in the air as a percentage of the maximum it could hold at that temperature, and this is the factor that determines how the moisture interacts between the air and the screed (or even wood flooring).
A normal screed, whether it’s cement or anhydrite (calcium sulphate) based, will give an RH reading of 75%, and once this is reached it will remain stable at or below this level with very little fluctuation day to day. This level of moisture is suitable for many types of flooring.
In an older property, built before about 1970, there’s unlikely to be a structural damp-proof membrane between the slab and the ground below, so the moisture content will naturally be higher. This will show when taking moisture readings when estimating and so the necessary measures can be included in the specification (and the price) to ensure this moisture remains in the floor and doesn’t affect the flooring.
If testing isn’t done, the moisture will build up and cause the flooring to fail.
When underfloor heating is built into a screed the moisture content will drop below the 75% due to the extra drying effects of the heat.
Screeds like this will stabilise at or below 65% RH, but even when a screed seems dry, the process of commissioning an underfloor heating system will drive moisture upwards out of the slab and this is why all flooring products require underfloor heating to be commissioned and for the environment to be stable before flooring is laid.
That explains the first occasion when moisture readings need to be at or below 65% RH – the presence of underfloor heating makes this necessary.
The other common reason for requiring the lower 65% RH figure is when a moisture-sensitive flooring is to be laid, such as solid or engineered wood. It isn’t always a requirement, so check the manufacturer’s instructions. Changes in the moisture content will always have an effect on wood, so issues such as cupping or other forms of movement can be avoided by complying with the moisture level requirement and ensuring the flooring is fully acclimatised in the location in which it is to be used for the period specific by the manufacturer.
I’ve been to see a number of cases recently where an engineered wood floor had developed what looked like machine (chatter) marks in the surface when in fact it was the warping of the internal cross battens due to changes in moisture levels after the flooring was laid.
It may seem odd that a machined wood floor can warp or cup, but the explanation is quite simple: if you imagine the cross-section of a tree trunk which is usually roughly circular, and if you imagine it shrinking as the moisture dries out the shrinkage will be less along the radius of the tree and more around the circumference.
In fact it will shrink roughly three times more around the growth rings as the circumference is ϖd – 3.1416 times the diameter of the tree. Any piece of wood will shrink more along the growth rings than at right angles to them, so any piece not cut at right angles to the radius of the trunk (known as quarter sawn) is likely to distort as the moisture level changes.
Manufacturers are keen to ensure their products are installed in suitable conditions, so follow their guidance carefully.
In the real world it isn’t always possible to take the time to use a hygrometer as this may take three-to-four days or more, so handheld meters that give instant readings can be used to give a good indication of whether the floor is adequately dry or not.
Different meters use different units of scale, so check with the meter manufacturer’s information so you know what the equivalent of 75% and 65% RH are on their meter.
If the reading is borderline, checking with a hygrometer is well worth the extra time and effort.
Finally, if you’re intending to use the hygrometer sleeve method, which involves drilling the screed and inserting a sleeve into which a probe can be inserted, the CFA have issued new guidance on this method which can be downloaded by CFA members from their website. This gives essential guidance on when and how this method can be used, and on the values to look for.
Richard Renouf is an independent flooring consultant.
