Stop moisture rising through a damp concrete slab

Before choosing the right solution, you first need to determine whether your concrete floor actually has a moisture problem.

A first easy-to-spot sign is the presence of a white, powdery substance on the surface of the concrete. This is probably efflorescence, a phenomenon often associated with moisture in concrete.

There are several ways to check the moisture level:

1. You can use a hygrometer to measure the relative humidity in the concrete. However, this device does not make it possible to determine whether the moisture is coming from the concrete itself.
2. To check directly for the presence of moisture in the concrete, you can do the polyethylene test. Tape a square of polyethylene about 12 in x 12 in to the floor, making sure to seal all four sides well.

Then wait 12 to 24 hours. If condensation appears under the polyethylene or if the concrete becomes visibly darker, this may indicate moisture migrating through the slab.

In that case, it’s best to address the moisture problem before installing your finish flooring.

If your concrete is facing a major moisture problem, you will absolutely need to apply an anti-moisture sealer before anything else.

Otherwise, what happens? The finish product you apply over your concrete slab will trap the moisture. As it tries to escape, the moisture will cause the finish to peel or flake.

The Rock Solid anti-moisture sealer is the first coat to apply on a very damp concrete slab or one that is weeping. It will create a moisture barrier that allows the finish to adhere properly.

Rock Solid also works its way into microcracks and chemically bonds with the concrete. This helps strengthen weak concrete and reduce surface crumbling and dusting.

A) Clean the concrete slab

First, it’s important that your concrete slab is properly prepared. Start by cleaning thoroughly with a concrete prep cleaner such as Krud Kutter.

• Dilute the cleaner using a mixing ratio of 6 parts water to 1 part Krud Kutter.
• Apply the solution to your concrete slab with a sprayer, a roller or a brush.
• Scrub with a floor brush to loosen dirt, then let the cleaner work for 3 to 5 min.
• Then rinse the floor to remove any product residue. If you’re working on a basement slab and there’s no drain, mop carefully instead to avoid water buildup.

When you’re done cleaning, let the concrete dry for at least 24 hours. Then you can move on to grinding the floor.

B) Grind the surface

Grinding is important to remove as much friable concrete as possible from the surface. If your concrete is crumbling, it will cause the finish above to lift.

It’s beneficial even for concrete that’s too smooth: grinding makes it more porous, which helps the product adhere properly.

• Be sure to isolate the room with plastic sheeting to prevent dust from spreading throughout the house.
• Preferably use a diamond-head grinder. Otherwise, use a sander with 40- or 60-grit sandpaper.
• Make sure you grind the entire concrete slab thoroughly.
• Then remove sanding dust using a shop vacuum (shop-vac). Go over it twice rather than once to make sure nothing is left.

You can then apply the sealer to your floor.

C) Apply the sealer

Applying the Rocksolid sealer is fairly easy. Make sure to apply it in a well-ventilated room. Even though the product is water-based, its odour can be unpleasant.

• Start by mixing the sealer for 30 to 45 seconds.
• Apply the sealer with a sprayer or by pouring it directly onto the floor. It’s important to fully saturate the concrete slab, with coverage of about 175 sq ft/gallon.
• Then spread the sealer using a stiff-bristle brush or a squeege.
• Once your concrete slab is completely covered, wet the surface with a garden hose. You must make sure it stays wet for 35 to 45 min after applying the sealer.
• After 45 min, remove excess water and moisture from the floor surface with a squeege.
• Let it dry for 2 hours, then rinse again with a hose. Then remove any water the concrete does not absorb with the squeege.

If your goal is only to reduce the moisture level in your basement or garage, you can use Rocksolid on its own. If you want to enhance the concrete slab or make it more resistant to abrasion and impacts, you can cover the sealer with epoxy, a vapour-barrier epoxy or a floor paint.

If your concrete floor has a moisture problem, Passeport Élite’s PE700228 vapour-barrier epoxy could be the solution.

This 100% solids epoxy was designed to be applied over damp concrete. It acts as a primer coat and helps limit reactions that can occur between moisture in the concrete and the finishing epoxy.

The vapour-barrier epoxy can tolerate a water vapour emission rate of up to 25 lb per 1,000 sq ft over a 24-hour period. Beyond this limit, the pressure exerted by moisture can cause the coating to delaminate. However, it’s important to know that there is no simple method to precisely measure the hydrostatic pressure responsible for a finish coming loose.

In many situations, using a vapour-barrier epoxy is a much less expensive and less demanding solution in terms of work than redoing a concrete slab or installing a waterproofing membrane.

Redoing a concrete floor notably involves waiting several months before you can cover the new concrete. Installing a waterproofing membrane, on the other hand, generally requires major excavation work.

The vapour-barrier epoxy can therefore be an interesting option when you want to treat a moisture problem without undertaking major work.

Where can vapour-barrier epoxy be applied?

PE700228 is a durable epoxy that can be used in residential, commercial and industrial environments. Its acid resistance notably allows it to be applied to concrete with a high pH.

It can be found in demanding environments such as airports, aircraft hangars, hospitals and community centres.

But you don’t need an industrial building to benefit from its properties. You can also use it at home to treat a basement floor or an old garage floor that has a moisture problem.

Once the vapour-barrier epoxy has been applied, it can be top-coated with a 100% solids epoxy or with the polyurethane of your choice, depending on the result you want.

How to apply vapour-barrier epoxy?

For PE700228 to fully do its job as a vapour barrier, you need to apply one or two coats, to achieve a total dry film thickness of 15 to 16 mils.

If you want to avoid air bubbles, it’s safer to apply two 8-mil coats rather than one 16-mil coat.

Concrete preparation remains essential before application. The surface must be clean enough and free of contaminants that could affect adhesion. The steps to follow are the same as those mentioned earlier in the article.

A) Mix the epoxy

Once the vapour-barrier epoxy has been applied and fully cured, you can apply a coat of 100% solids epoxy or polyaspartic over it.

The goal is to create a complete system: the vapour barrier blocks moisture coming from the concrete, while the topcoat provides the look and performance you want for your floor.

Once the moisture problem has been treated, you can choose the finish that best matches your needs and the look you want to give your floor.

Start your project now!

You will find all the products you need directly on our website.

1. Start by measuring the dimensions of your damp concrete slab to calculate the quantities you need.
2. Determine whether you need to apply the Rocksolid sealer, the vapour-barrier epoxy, or both!
   • If your concrete has a moisture level above 4%, you must apply the Rocksolid sealer.
   • If your concrete has a moisture level of 4% or less, you can apply the vapour-barrier epoxy.
3. Place your order online and choose between in-store pickup and home delivery.

Want to learn more about different epoxy projects? Check out the section of our blog dedicated to this topic!