Moisture Matters #1

As promised earlier, we are delving into the changing world of concrete subfloor moisture testing in as much detail as we can (without driving you totally insane).

Resilient Flooring Installation Standard AS/NZS 1884 has been reviewed and the revised version was published in June 2012. Obviously, if you wish to properly limit your liability, or avoid major implications in the event of a future floor failure, standards should be followed. In the absence of a specific timber flooring installation AS/NZS Standard, 1884 is the most applicable.

Essentially, the revisions have done away with the time-honoured “moisture content <5.5%” measurement. For reasons will we go into in a later post, it was widely regarded as irrelevant or inaccurate. Relative Humidity (RH) is now the required indices, and the wording of the standard strongly implies that measuring RH within the slab is the preferred route. It does allow for reading RH at the surface under some circumstances (where drilling the slab is restricted), but this method is regarded as less accurate, and even the standards themselves acknowledge this.

But first, lets revisit some concrete subfloor basics.


Concrete starts wet, and has to dry. The water/cement ratio generally should be between 40-50% water, but even small increases in water can lead to disproportionately higher drying times. A 5% increase in water can DOUBLE the drying time of the concrete. 10% increase- TRIPLE the drying time. The same thing occurs when the concrete is thicker- drying times increase disproportionately. (CSIRO – Ron Denning Feb 2012). A dry, old slab which has been re-wet for some reason (poor drainage, flooding, no vapour retarder beneath the slab, plumbing failure etc) will take longer to dry than a new wet slab.


For Timber Flooring, in simple terms, moisture migrating to the surface of the concrete breaks the bond between the adhesive and the concrete.

Some argue that the move away from solvent-based adhesives towards lower- VOC water based types has increased failures in recent years. It’s also interesting to note that the higher water content in adhesives can itself contribute to failures- that is, it’s not the water itself but the alkaline solutions in the concrete surface activated by the water, which attack the adhesive and cause it to delaminate.

Such moisture from adhesives has also been known to cause the timber to swell, the problem being incorrectly (in some cases) blamed on the slab. That said, drying slabs are the most common culprit but again, it’s not the water itself, it’s the fact that the water carries alkaline solutions from the concrete, which are the main detriment to adhesion.(CCAA paper April 2007).

Other flooring types, such as vinyl, are also susceptible to their adhesives being re-emulsified by the alkaline solutions. It can get very ugly, in the case of “Sick Building Syndrome”- moisture migration causes the alkaline solutions to break down the adhesive, and the resulting toxic emissions (including nasty substances like butanol and ethylhexanol) can exceed safe levels. It is no small matter.

In the case of Timber Flooring, timber being the most hygroscopic flooring material around (it takes up and releases water at a higher rate than anything- see our other post on that subject), as well as adhesive failure, you have the inevitable deforming of the timber itself to look forward to.


Firstly, if you’re on a 1000 square meter job, an effective MB will cost at least $8 per sqm. This extra cost can skew quotes rather badly and is no small matter.

Secondly, and we are not trying to be alarmist here, but MBs aren’t necessarily a moisture cure-all. In layman’s terms, a MB doesn’t “block” the transmission of vapour, it simply slows it down to a point which the flooring can handle (which will be specified in the flooring or adhesive manufacturer’s spec sheets- that’s another story!). They all have permeability ratings, usually measured in grams per square meter, per 24 hour period.

Which means, that if the RH in a slab is high enough, it might create enough vapour to compromise a MB. What if your coverage rates were not sufficient to create the required thickness? The main thing here is following the manufacturer’s guidelines so that you are covered, or at least supported, in the event of a failure.  

Which brings us back to the need for properly undertaking a moisture test at the outset. A manufacturer might have a tolerance level of <85% RH for their flooring product (adhesive, etc). You can’t expect them to honour a warranty if you don’t know what the RH level in the slab was- even if you apply a MB. 

We’ll get into the methods of testing, and why some are recommended over others, at a later post.

5 responses to “Moisture Matters #1

  1. I’ve heard the RH method is more complicated.

  2. great points about testing for moisture transmissions in concrete floors. I have seen many contractors ‘gamble’ and put down coatings or other coverings and not test. with many specifiers and concrete flooring consultant trying to cut cost…they are leaving vapor barriers out.

  3. RH testing is actually very easy using the correct equipment and a little patience. Drill holes, cap them and allow the holes 3 days to reach equilibrium. Return to site, insert probes and then allow them to settle in the holes for the desired amount fo time, an hour is about right. Then measure the readings. Make sure the probes you are using are calibrated and also make sure that they are not damp when used. Surface tests can also be set up using hygrometer boxes in the same manner, fit to slab, allow it to reach equilibrium (72hrs) and then test. If you need to retest, move the box to a new position at every test otherwise the test can get compromised.

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