3 Ways to Test Moisture Content

High-moisture substrates can cause the best coatings to fail. Don’t sweat it. Here’s what to do to provide a truly professional finish that will last. 

Most of the earth is a wet place, and while this is a good thing in many ways, it’s also a challenge when it comes to painting. High moisture substrates can cause the best coatings to fail, and that’s why moisture testing is something you need to know about. Saving yourself from hassles and money-losing nightmares is what it’s all about, and there are three situations where moisture testing makes sense for you as a painter.

Breathable Paint

It’s a lot easier for liquid moisture to sneak past conventional paint than it is for water vapour to get out again. That’s why you should never use ordinary paint on exterior masonry. It’s too impervious. No matter how diligently you coat the surface, liquid moisture will eventually soak into the masonry. It might only be in a few small spots, but this is enough to cause trouble. The issue is that moisture won’t be able to evaporate out of the small entry locations, and that’s a problem. In cold climates the high-moisture masonry will freeze and flake. In warm climates the moisture will build up enough to cause paint delamination. If your client insists on painting exterior masonry, always use a paint that has a vapour permeability of at least 75 US perms.

Situation 1: Concrete Slabs

Concrete is mixed with more water than needed for hardening, and this excess moisture needs to get out before you can expect any coating to stick long-term. And since concrete is porous, even an old floor can sometimes still hold way too much moisture for proper finish life.

One long-standing moisture test for concrete involves the ASTM 1869 standard. It was developed in the 1950s and uses a container of dry calcium chloride underneath a plastic cover sealed over the bare concrete floor. Prep the concrete so it’s clean and bare, open the pre-weighed container of moisture-hungry calcium chloride, write the date and time on the container, then use tape to seal the plastic cover to the concrete  with the calcium chloride inside. The more moisture this white powder absorbs the more moisture’s in the slab. At least that’s the theory.

Weigh the calcium chloride after 60 to 72 hours, and then plug the weight figure into the formula that came with the kit to determine the moisture content of the concrete in pounds of moisture emitted per 1,000 square feet per 24-hour period. Most coating manufacturers allow no more than a three-pound moisture rating on concrete for warranties to apply. Some coating manufacturers even require moisture testing results be recorded for warranty coverage to kick in. This is fine except for one problem. The calcium chloride test only measures moisture levels in the very top part of the slab. You can get a superficial reading that’s dry enough for coating a concrete floor, then still have the coating fail later because moisture migrated up from deeper in the slab over time, causing finish failure. Calcium chloride testing is widely regarded as reliable, but in reality it leaves something to be desired because it does not register deep moisture.

pH Testing of Concrete Slabs

Too much moisture isn’t the only thing that can ruin your floor coating. So can excess acidity or alkalinity. A pH of 7.0 is neutral. Any number above this is alkaline, and any number below is acidic. Floor coatings require a certain range of pH values of the surface they’re applied to, and new concrete floors can easily be too alkaline for proper coating life. pH testing pencils and test strips are available and using them makes just as much sense as moisture testing a slab before coating.

If you want to be absolutely sure a slab is dry enough for risk-free coating, the relative humidity (RH) test is an excellent diagnostic tool. It’s especially useful when you’re working with fresh slabs that probably still hold a lot of embedded moisture because the relative humidity test measures deep in the concrete. It tells you what is ultimately coming up to meet your coating in time. RH testing begins by boring a hole about 40 per cent of the way into a concrete slab’s thickness. Clean the dust from the hole, insert a humidity sensor into the hole, and then plug a reader into the sensor. It’ll give you two numbers – one is the temperature of the slab and the other number is the humidity of the air in the hole relative to that temperature. A slab is considered dry enough to coat when the relative humidity near the centre of the slab is 70 per cent or less.

Situation #2: Exterior Wood

Like concrete, wood is an absorbent material, though the moisture dynamics in wood are quite different than concrete. The moisture content of new lumber can range from 12 per cent to over 25 per cent by weight depending on the wood, the region, the season and the time elapsed since construction. This is why it’s especially wise to test new decks and exterior wood siding before coating. New lumber can feel perfectly dry on the surface while still containing damaging amounts of internal moisture that’ll cause trouble later. The best tool for measuring the moisture content of wood is an electric device that comes from the world of woodworking. These days hand-held moisture meters for wood are cheap and accurate. Press the prongs into the wood, hit the button and read the moisture content. Sixteen per cent is the upper limit of moisture content for most exterior coatings, but check with the manufacturer of the coating you’re using. Drier is always better, but don’t be fooled. Wet lumber can dry superficially and give a false reading. If scrap lumber is left over from a new deck job, cut a board in half and measure a fresh end to get a true reading. If that’s not possible, use a moisture meter with prongs that can extend into the wood at least 3/16”.

Beware Wood Siding on Old Houses

While it’s easy to understand that exterior walls get wet on the outside from rain and dew, there’s sometimes more to the moisture issue than this. Paint-killing moisture can also come from inside buildings in cold climates, and that’s one reason wooden exterior walls are trickier than decks when it comes to moisture assessment. Old wood-frame homes are the most common place you’ll see this trouble. Homes built before the use of vapour barriers on warm sides of exterior walls can be susceptible to the formation of frost inside wall cavities during winter. As warm, moist indoor air makes its way into unsealed wall cavities, this air cools to the dew point and moisture condenses out of the air, forming water droplets and frost within walls. As the weather warms when spring arrives, the frost melts and the moisture migrates outwards through wood siding. If you ever encounter an older wooden home where the owner tells you how previous paint jobs never last more than a few years, beware of the internal moisture dynamic. If premature paint peeling happens repeatedly in localized areas that don’t get extra moisture from outside sources you can be sure you’re dealing with an internal moisture problem. It’s a situation no painter can beat without the underlying cause being dealt with. A moisture meter is the fastest way to verify moisture issues on wood siding. 

Situation #3: Post Flood Restoration

As a painter, you face an unfair hazard when it comes to post-flood restoration work. Wall frames, floor frames and concrete slabs will be very wet after a flood, but you can’t necessarily count on a restoration contractor to allow enough drying time before drywall goes on and you get called in. The thing is, the moisture of a too-wet wall frame or floor will eventually come through drywall to your painting, and paint failure will be seen as your fault. If possible, take moisture readings of the wall framing before fresh drywall goes up. If you can’t do that, wait as long as possible before moisture testing drywall. If excess moisture does exist below the surface, you want to know it’s coming out before you apply paint. A woodworking moisture meter works on drywall, too. It might be a few percentage points out because it’s not reading wood, but results are more than close enough to offer useful guidance. High quality painting is about diligence, and sometimes this means more than just care with a brush and roller. Moisture testing is one of those things that is easy to ignore when you’re eager to get going, but it’s also one of the little things that can save big trouble.

Moisture Content % vs. Relative Humidity %

If you’re measuring the moisture content of wood, you’ll probably use an electric meter that determines the moisture content percentage by measuring the electrical resistance of the wood. The higher the moisture content, the lower the electrical resistance, and thus the higher the percentage of moisture you’ve got. The moisture content of wood determined this way is expressed as a percentage of total weight. A 1,000-gram block of wood with a moisture content of 14 per cent contains 140 grams of water. By contrast, “relative humidity” is often used to measure the moisture content of concrete slabs, and though this is also expressed as a percentage it’s entirely different than “% moisture content.” A slab with a relative humidity of, say 70 per cent, is holding 70 percent of the maximum amount of moisture that the air in the hole can hold at that particular temperature. It does not mean that 70 per cent of the weight of the concrete is water.

Pool Painting and Moisture

Painting swimming pools is a specialty that requires more attention to moisture issues than most situations. “There are two times a pool must dry for painting,” says Rebecca Spencer, marketing manager with RAMUC Pool Paint. “First, before applying the first coat of paint, and second before filling the pool with water. The average dry time from the final coat of paint to filling an outdoor pool is five to seven days. If using an acrylic product, the average is three days. If the pool is indoors, the time lapse is about ten days, providing there is good ventilation and air movement.” It is also essential to match any new coating to what’s already on the pool walls. Lab testing of a paint chip from the pool is the best way to avoid blistering or peeling of the new paint. “A high build epoxy can last eight years or more,” says Spencer, “and a traditional epoxy can provide three to five years of durability. Chlorinated or synthetic rubber based paint averages between two to five years, and acrylic based coatings will last up to two years.”

Beware of Efflorescence

If you find yourself asked to paint a masonry surface with white fuzzy stuff on it, you’re very likely to make one of two serious mistakes. Either you’ll deal with the white stuff as mould (which it probably isn’t) or you’ll simply brush it off and forge ahead with fresh paint. Both approaches will end in a failed paint job and grief for you because moisture is the source of the problem. Fine, white, fluffy stuff on masonry is efflorescence, and it’s a warning that tiny amounts of liquid moisture are migrating through the masonry. Brick, block and stone are solid and strong, but they’re also porous. Moisture can easily pass through most masonry, and this is what the white stuff indicates. As tiny amounts of moisture migrate itcarries minerals with it. When the moisture reaches the surface of the masonry, it dries before it appears as visible liquid, leaving the minerals behind. As this process of leaking and drying goes on for months or years, more and more minerals are left on the surface, creating the white fuzz that should be a warning to you. So what should you do when a client asks you to paint a fuzzy, white masonry surface? Explain the dynamics and insist that you need to seal the surface then give it a chance to prove that efflorescence has stopped before applying paint. Anything less and you’ll probably wish you never took on the job.

Sharing is caring!