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How to Measure the Moisture of Your Sub-Floor

Posted on Feb 8, 2016, by Michael Eckert

If you’re installing new flooring, it’s vital that you know the moisture levels present in your sub-floor so you can choose appropriate flooring and adhesive products, and protect your investment from moisture damage.

Sub-floor moisture can wreak havoc with your flooring, causing delays and complications in the installation process, floor compatibility problems, and even dreaded mold and mildew.

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Some flooring products have better moisture management properties than others, straight out of the box. So to help you make the best flooring choices, I’m looking at two tests you can use to measure the moisture levels of your sub-floor, and add an extra level of insurance to your next flooring installation.

1) Anhydrous Calcium Chloride (ACC) Test

The ACC test measures the rate of water vapor emission from the surface of your sub-floor. The test is carried out by applying highly absorbent calcium chloride to the floor’s surface for a specified time period, and measuring its weight before and after application.

The calcium chloride absorbs water vapor from the surface of the sub-floor, so its weight change enables you to calculate the amount water vapor emitted over time.

This technique can be difficult to use, and unfortunately, this method doesn’t measure the levels of moisture within your sub-floor slab – only surface emissions. As a result, the flooring industry has moved away from the ACC test, preferring the more accurate (and easier to use) relative humidity test.

2) Relative Humidity (RH) Test

Often referred to as the RH probe test, this involves drilling into the sub-floor slab to a depth specified by your flooring manufacturer. You then insert an electronic probe which provides a percentage relative humidity reading.

As concrete dries, moisture moves from the bottom of the slab to the surface where it can evaporate away. There will most likely be higher levels of moisture within the slab compared with on the surface of the slab. As such, the RH probe test provides a more accurate measurement of the sub-floor’s moisture levels than the ACC test.

Calculating relative humidity levels allows you to identify the most appropriate flooring products for your site conditions. For example, most standard flooring adhesives are rated to withstand up to 80% relative humidity, but if you’re installing flooring products in a more humid environment, you’ll require ‘tougher’ products that are designed to withstand up to 85, 90, 95% or higher. 

Measuring Your Sub-Floor Moisture Levels

While measuring your sub-floor moisture levels may seem like a complicated and time-consuming addition to your flooring installation process, don’t be tempted to skip it to speed up the process.

Sub-floor moisture testing is required to comply with ASTM regulations, and it’s often necessary to follow specific testing procedures to comply with your flooring warranty. If you don’t do the test, and install flooring or use adhesives that aren’t suitable for your relative humidity levels, then your warranty will be invalid and you’ll be left with a costly repair or replacement job.

You can do the RH test yourself if you purchase a RH testing kit. If you decide to perform the test yourself, it’s vital that you follow the testing instructions exactly; not doing so may mean you perform the test incorrectly, which can invalidate your flooring warranty just like if you didn’t perform sub-floor moisture testing.

For an extra level of security, you can contact your flooring manufacturer or supplier who will be able to recommend a flooring or building professional who can carry out the test for you.

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Topics: Installation

Michael Eckert

Written by Michael Eckert