Hanging with Hafen: How moisture can ruin your floors

Avoid a construction disaster by taking the proper precautions when laying flooring on top of your concrete slab.
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Nov 22, 2011

You may have heard of Paul Harvey and his radio show, “The Rest of the Story.” This month, I’d like to take a page out of Harvey’s book and share the rest of the story about flooring. Since the mid- to late-80s, the construction industry has struggled with using the wrong glue for the right problem when it comes to flooring.

As far back as 1985, the Environmental Protection Agency began moving toward limiting Volatile Organic Compounds (VOCs) used in the construction industry to protect indoor air quality and workers’ health. The EPA required builders to move away from solvent-based glue and paint to latex or acrylic-based products. And while this certainly made for better indoor air quality, the latex and acrylic glues just aren’t as good as the more toxic, solvent-based glues.

The problem with latex-based glues is that they’re susceptible to failure due to the excess moisture often found in flooring subsurfaces like concrete. If the concrete contains too much moisture, the glue doesn’t set and remains liquid—and fails to hold down your flooring.

In fact, moisture issues with flooring are so common that the majority of vinyl and troweled-on epoxy flooring manufacturers have a blanket policy. They don’t warranty their products in “below grade” situations—like basements—because the likelihood of moisture in basement floors is high.

So if you’re thinking about installing a sheet vinyl or troweled-on epoxy floor on top of a concrete floor, how much of an issue are you facing?

Most flooring manufacturers set a maximum moisture emission in the subsurface or concrete slab and will not warranty the installation for their products in conditions above that maximum. For many manufacturers that number is 5 pounds per 1,000 square feet for 24 hours. In some cases, it’s even lower at 3 pounds. These are very tough numbers to hit.

Think of it this way: A concrete slab starts life as a slurry of cement, sand, gravel, and most importantly, water. When it’s first installed, it’s wet with a relative humidity of 100 percent. Over time, it hardens and dries, but this is a slow process. Concrete is still called and looks “green” for up to 28 days after installation because, although it has “set” for the most part, it’s still very wet. The longer you can wait for concrete to set up and dry, the better. In a perfect world, you might want to wait 60 or 90 days from when the building is enclosed with walls and a roof before proceeding further with flooring. Unfortunately, this is often not possible. After all, time usually equals money in the construction industry.

And here’s another factor in the moisture department: A concrete slab sits on dirt or some kind of subgrade, and often this dirt contains water. To further complicate matters, as concrete dries, it can actually act like a sponge and draw water up from the dirt beneath it.

As you can see, expecting to have a truly dry concrete slab may be wishful thinking.

So how can you tell if you have a water problem? There’s a relatively simple test that you can and should perform after every concrete flooring installation. This test is called “ASTM F-1869—Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.” Basically, the flooring contractor sprinkles some “magic dust” (anhydrous calcium chloride crystals) on the floor, tapes a piece of plastic over it, then removes the plastic 24 hours later, picks up the crystals, and weighs them to determine how much moisture they’ve absorbed. That figure is measured in pounds or moisture emission per 1,000 square feet per 24 hours. As mentioned earlier, the typical maximum is often 5 pounds per thousand. Above this level, the flooring contractor can put the product down, but the company won’t offer a warranty that guarantees it will stay down.

What can you do to prevent the problem of having too much water in your slab in the first place? There are a number of strategies you can use:

> Install a vapor retarder beneath the slab. The industry standard is a 15-millimeter polyolefin sheet with the seams taped.
> Use a stiffer, stronger concrete. Most experts recommend a maximum of a 4-inch slump and a 3,500 psi compressive strength concrete. Your architect will understand these values and help you choose the right material.
> Wet cure your slab. While it may seem counterintuitive to cure your slab by putting water on it during the wet cure process, it’s better than spraying a chemical curing compound on the slab, which actually delays the curing and drying process.

What can you do if you already have a slab that contains too much moisture? You have a few options:

1. Wait for it to dry (if it ever does).
2. Take your chances and install the floor anyway.
3. “Paint” the floor with a waterproofing liquid.

The third option has the most potential of the three but also carries the most uncertainty. These waterproofing membranes can vary in price from a couple to a few dollars per square foot, and each product has a different level of effectiveness. This makes it tough for someone like myself to recommend any particular product as a sure thing. Additionally, the warranty these membrane manufacturers offer also varies, but it’s typically quite limited.

The simplest solution when you have a slab that is too wet is to change the flooring to one that’s more permeable to off-gassing moisture. For example, a vinyl floor with a felt backing can be more permeable than one without. Most vinyl flooring manufacturers offer a range of products that are backed or not backed. A tile floor can also be more permeable, as can a simple stained and sealed concrete floor.

Many years ago, my firm worked on a project in the Midwest with a very short construction schedule. Unfortunately, when it came time to test the floor, it failed. At that point, the practice owner said, “We can’t wait—put down the vinyl floor now.” So the contractor did. A couple of months passed, and we received a phone call. The vinyl floor was now squishing underfoot because the glue that had not set oozed out of the seams. It was a mess. There was much screaming and yelling and lots of finger pointing. When all was said and done, the whole vinyl floor had to be pulled up and replaced with a more permeable tile floor. Not only did this delay the opening of the hospital, it cost a lot of money.

Don’t let this happen to you. When making a flooring choice, work with your architect to make the right choice. That way you won’t become part of the rest of the story.