So you're going to mitigate....

That’s a great idea. You’ve quoted high enough, or have PC allowances, so you’ve decided that you’re going to slap down a moisture sealer/suppressant/barrier, just to be sure that slab will behave itself when a floor goes on top. Surely an accurate and relevant moisture test is not required, right?

Well, not really. Bear with me as I explain two major considerations.


No, this is not about the quality of a hairstyle popular in the 70’s. As oils ain’t oils, so moisture barriers ain’t moisture barriers. Traditional two-pack epoxy Moisture barriers all have permeability ratings. That is, how “gappy” is the matrix structure of the cured sealing material, in that it can allow material to pass through it at a microscopic or minute level. A suitable metaphor would be to compare firing a pellet gun at a wire fence or a metal sheet. The slug probably won’t pass through the metal, but it probably will fly through the wire fence. Some might use the term “porosity”.

Perm ratings measure in the amount of mass (usually in grams) which can pass through a given area of the cured material (usually in square metres) PER a given period of time (usually 24 hours). Perm ratings are measured against their own Standard, but we won’t go into that right now.

However, it’s difficult to directly compare perm ratings for each product, because the Standards dictate how to determine the rating, but not how to express it. Some will give the perm rating for single hour periods, but strictly under certain temperatures and RH levels. Some state a rate per 24 hour periods.

So there are more “solid” products with greater moisture suppressing properties, and lesser ones.The differences are generally reflected in the costs of these systems which can translate to between $8 per square metre to $20 per sqm and above.

So here’s the trick: A moisture barrier is not a cure-all, because some are only rated to perform UP TO A CERTAIN MOISTURE LEVEL EXISTING IN THE SLAB- and most of them will stipulate an RH% figure. Some will perform at over 95%, some no higher, some up to 98%.

Remember, with any permeability, a barrier simply slows down the release of moisture vapour pressure to the point at which the flooring/ adhesive can handle it. If the humidity deep in the slab is high enough, the pressure will be higher. FLOOR TEST Australia have tested many a slab which should be moderate, yet reads 99%.


There are other emerging moisture treatments besides two-part epoxy film builds. Actually, some of them have been around for several decades and have been re-invented, or have crossed over from other applications (such as concrete surface densifiers) and regarded by some to be useful for keeping moisture down. They often boast of being single packs, easy to apply, dry in an hour, flooring able to he adhered within a few hours, and often with extremely low VOCs.

Their technical names include “alkali silicates”, or reactive /colloidal silicates. They react with existing moisture and lime (calcium hydroxide) in the cement mix, to create a gel-like or crystalline substance in the pores of the concrete. This is the effect which made them useful surface densifiers. Some of the product claims range from pragmatic to extremely bold, implying that they will solve all moisture problems.

It is very difficult to wade through the mire of political/ commercial interests which dominate the debate raging over these types of products and remain objective. While Floor Test Australia is INDEPENDENT of those things, we acknowledge the time and money put into researching moisture-related flooring failures by many commercial entities whose only interest is preventing flooring failures.

Efflorescent salts on concrete surface treated with silicates (magnified). Pic courtesy of Ardex Australia

Indeed these are useful products in their own sphere. As I mentioned earlier, some are adapted from surface densifiers which are used to produce spectacular concrete grinding effects. However, like the epoxies and their different perm ratings, these products have their catches as well.

Firstly, since the sole function of the product is to react with the calcium hydroxide (CSH) in the cement mix to be effective, what if there is not sufficient CSH in the mix to react with? There are other elements within concrete mixes which can react with CSH and therefore reduce its availability for the silicate treatments to work.

We also know that colloidal silicates can cause efflorescence. That is, calcium carbonate and other alkaline deposits from the cement mix in the slab can form on the surface over time due to the silicates reacting with Carbon Dioxide in the air. This can definitely compromise the performance of an adhesive. Can the manufacturer provide a (warrantied) procedure for removing these deposits?

Being a different method of moisture suppression, many of the adhesive manufacturers who create a whole moisture barrier/ adhesive system, will not warranty their products over these silicate-based treatments. The silicate product may have its own warranty, but that doesn’t mean they can warranty the adhesive and the flooring itself. Nonetheless, some of the silicate products have made good associations with other manufacturers and specify which adhesives work best over their product.

Like epoxies, these products also require a degree of surface porosity in order to be effective. The rule of thumb is- if the surface is not porous enough to take any moisture treatment, it’s not going to accept a flooring adhesive. Those power-trowelled (“helicopter”) finished surfaces can be a nightmare for any moisture mitigation or floor adhesive product.

Again, there are many different camps with many differing opinions on epoxies vs silicates. But anyone who says that you don’t need a verifiable moisture test before any major flooring installation, regardless of a) whether moisture mitigation is planned and b) what mitigation product will be used…. is kidding themselves.

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