Don’t Epoxy Until You’ve Done This Simple Moisture Test First

moisture on garage floor causes efflorescence
Efflorescence caused from moisture in the concrete


How much moisture does your concrete transmit?  Before you decide to coat your garage floor with epoxy or paint, this is the question that needs to be answered before you make your final decision.  If you don’t know the answer to this question, then you need to perform a simple moisture vapor transmission (MVT) test of the surface of your floor before you attempt to apply any coating to it.  If you don’t, all your hard work may be destined for failure.

As hard and dense as concrete seems to be, it is actually very porous.  Water vapor and moisture beneath the slab can permeate to the surface of the concrete.  Because coatings such as epoxy and paint are impermeable, the water vapor can’t pass through the coating and can cause it to delaminate.

In some cases if the epoxy has a good mechanical bond, the hydrostatic pressure of the moisture can be strong enough to actually cause the epoxy to pull up part of the surface of the concrete with it, creating even more problems for you in the future.

moisture concrete garage floor

You may already have signs that you have a moisture problem and you don’t even realize it.  If there are any areas of your garage floor that always seem to have a powdery residue, then you have efflorescence.  It is created when moisture travels through the concrete, condenses and evaporates, and then leaves a residue of calcium hydroxide.

You may also have an area of your garage floor that always seems damp.  If so, then you most likely have a moisture issue.

So, where does this moisture come from?  In simplified terms, the porosity of concrete causes it to act like a sponge.  If the ground that your slab was poured on is naturally damp due to climate or other factors such as water drainage or irrigation issues, then your slab will naturally draw in moisture from the ground.

If the surrounding air on the surface of your slab is drier than what is underneath, then the moisture that your slab has collected will be transmitted to the surface of your floor as it tries to equalize.

This reaction can be enhanced if your slab was poured below grade, which is lower than the surrounding earth.  If you live in a naturally humid area, your concrete can also absorb the moisture in the air and then release it as the humidity drops. That is why it is good to coat your garage floor in the late afternoon or early evening when temperatures are cooling.

moisture test concrete garage floorThe easiest way to determine if you have a moisture problem is to test for it.  A simple way is to perform the ASTM-D-4263  Plastic Sheet Test. You can do this simply by cutting a 16”x16” piece of plastic sheeting and taping down the perimeter with duct tape to the surface of your garage floor.

After letting it sit for 24 hours, peel it up and look for any condensation on the underside of the plastic or look for a dark spot on the surface of the concrete.  Water on the plastic or a dark spot on the concrete is created from moisture.  If none exists, you should be fine. If you do have some moisture then you will want to do a calcium chloride test to see how much moisture you have.

Great information on how to perform a calcium chloride test as well as check your concrete’s PH level.

A calcium chloride test is used to determine exactly how much moisture you have emitting from your concrete.  It does this by placing a pre-weighed petri dish of calcium chloride under a sealed plastic container on your slab.  After a predetermined amount of time, you remove the petri dish, seal it, and mail it off to be tested.

As long as your results are less than 4 lbs of moisture flow per 1000 square feet per 24 hours you can continue.  Don’t worry; they do the math for you.  These kits are self-contained and can be purchased for around $8 a piece.  If you like, you can purchase them from here.

If your results are higher than that, then you need to consider applying a moisture blocking sealer or moisture blocking epoxy primer first before coating your garage floor. Of course there is always the alternative of deciding on a different garage flooring option.

Just remember, of all the things that need to be considered before you decide on an epoxy or paint coating for your garage, determining if you have a moisture issue is the first step.  It’s easy and fairly quick to do and can save you from the disappointment and anguish of a failed floor coating.


  1. says

    If you are adding water to the concrete when cleaning and efflorescence forms afterwards, is this a possibility of moisture vapor problems? Or is it just because I added water to the surface. Thanks

    • Shea says

      Hello Michael. If you did not have efflorescence before cleaning, then the water caused a reaction when it dissolved excess lime on the surface of the concrete and created calcium hydroxide. When it dried, the calcium hydroxide reacted with the carbon dioxide in the air and created your efflorescence. It wouldn’t be a moisture vapor problem. A solution of vinegar and water scrubbed on the concrete and then rinsed well with fresh water under high pressure will usually remove it.

  2. Barry says

    Hi Shea,
    I have a garage floor that has been getting efflorescence during the rainy season, along with some collection of water in areas where the floor is cracked. Is there any way I can seal this up so that it stops happening.

    • Shea says

      Hi Barry. The first thing you want to do is make sure water drains away from the foundation of your garage and not towards it. After that, you need to repair your cracks to prevent water from coming up through them. Larger cracks should use an epoxy patch repair with or without a sand slurry depending on the size, while the smaller ones can be repaired with a polyurethane or polyurea crack repair product. Once that is done, lightly acid etch the concrete to clean up the efflorescence and open up the pores of any dense areas of the concrete. The last step would be to apply a moisture blocking penetrating sealer to prevent any moisture from coming up through the remainder of your slab.

        • Shea says

          Sorry Barry, we are not familiar with your area. We suggest contacting a couple reputable concrete contractors in your area and ask them who they can recommend that can tackle the job for you.

  3. karen says

    I live in Pennsylvania. My house is 40 years old & garage floor pitted & spalled & I think never seen a treatment. If a humid day and Garage door is open, the floor is damp. I looked up the ardex products and only found CP which tells me to clean floor but not with acid etching products. I want to seal the floor with the silliconate multi-surface sealer which says to clean floor with acid etching products. What else can I use to fill the pits. I was looking at the UGL drylok Fast Plug. I need to do something and am very confused. Can I get the silliconate in color? I also need to clean the old oil marks and have seen a lot of your reccomendations. Thank you

    • Shea says

      Hi Karen. Yes, fixing a garage floor such as yours can be confusing. The dampness is most likely caused from humid air contacting the cooler slab and condensing at the surface. The pitting and spalling is generally caused from freeze thaw damage and road salts judging from your area of the country.

      What we recommend is cleaning the floor as Ardex recommends and then apply their product. Once it cures, prep the entire surface of your garage floor by grinding with a Diamabrush surface prep tool. You can usually find these are your local Home Depot or equipment rental. We talk more about grinding here. This will smooth out your patch work to make it look nice as well as prep it and the surrounding concrete for the sealer. Doing it that way will avoid having to use an acid etch.

  4. Lee says

    Hi! I have a very damp garage floor that also has some significant pits and spalls, like Karen’s does. It is set under the house, and is partially underground on one side. The floor is damp enough that there is even sometimes water gathered in the deeper pits. For kicks I tried the plastic test, too, and it revealed a darkened square.

    I really just want to do something to fix up the floor, which looks terrible currently. Is it possible to seal it well enough to allow me to epoxy it? If so, with what? Would it be better to seal it and then put down something like VCT tiles that stick on, but maybe don’t require quite as perfect of a chemical bond as epoxy does?

    If there is not a way to seal it well enough for epoxy or stick-on tiles, what are the best options? Is there a top coat cement product that will at least make the floor look uniform that can be applied to a damp slab? Or are the snap-together plastic tiles the best option? Is there something moderately affordable and DIY-able?

    And what do you recommend to fill in the pits and spalls given that it will likely be constantly damp?

    I know this is a ton of questions … thank you for any help or advice you can offer!!

    • Shea says

      Hi Lee. Your biggest issue right now is stopping or at least slowing down the water intrusion. Start by making sure water drains away from the outside foundation and doesn’t sit up against it. Water that can pool outside will work its way down under the foundation and intensify the moisture problem. A lack of rain gutters can worsen a moisture problem for example. Once you do that, you will need to clean the concrete well and run fans to help the surface completely dry. Once it’s dry, multiple coats of a deep penetrating sealer will need to be applied to slow down and possibly stop the water intrusion. Epoxy or any other garage flooring option that adheres to the concrete should not be installed unless it passes the moisture test. We’re not saying it can’t be done, but when you have water that actually pools in the pitted areas, it can be very expensive to completely block it enough to apply epoxy.

      Your safest DIY option after slowing down the moisture problem are interlocking garage tiles. They will not mildew and they allow for evaporation of moisture if some still exists after applying the sealer. Unless the pitting of the floor is extremely bad, the interlocking tiles will install right over it without need of a repair.

  5. Collin says

    I just started doing prep work for laying epoxy on a garage floor before moving in. I found a section to one side of the garage that had a lot of white powder on it. When I washed it away with the pressure washer, I noticed one small oval area where the powder was had the smooth top surface of the concrete coming off. I also noticed that there were little spots of bubbling foam after I washed off that area.

    I had thought the powder was a spill of something from the previous owner but after reading this article, I’m thinking it was a long term build-up of calcium like in your picture from this article.

    Any thoughts on how I should address this? My next step was going to be muriatic acid before I found this problem. Should I go with an epoxy primer to block the moisture? I noticed your article on epoxy primers didn’t have any specific product recommendations. (Something available from Home Depot would be great since I need to move forward in a hurry; this is blocking my move-in) Would Seal-Krete Model Lock-Down Epoxy Bonding Floor Primer work for this purpose?

    • Shea says

      Hi Collin. When you first suspect moisture you need to do a test first to determine if it really is a moisture problem or not. If it is, then you need to determine how much you have in order to figure out what you need to do. The spot you refer to may be a chemical spill that the water reactivated or it could be a moisture issue. Once that area fully dries, do the plastic sheet test in that spot to see what you get.

      Also, Seal-Krete Lock-Down is not an epoxy primer. It a latex acrylic concrete primer for acrylic paints and 1-Part epoxy paints. It is not epoxy. If you aren’t sure about the difference, then we suggest you read this article. Epoxy is a 2-Part resinous coating that needs to be mixed together first before application. Standard epoxy primers are not moisture barriers either. You would need a special moisture barrier epoxy for that. Neither of these types of products are going to be found in home improvement centers. This is an example here of a moisture blocking epoxy primer.

      • Collin says

        I had already purchased H&C ShieldCrete from Sherwin Williams but have not opened or started anything so I could return. However, the expenditure of that ($80/box x 3) was close to the top end of what I am willing to put into this project, so all these complications are throwing me for a loop, not to mention delaying moving into the house!

        I have no idea how long the powder was there. This is in California so with the drought I wonder if a moisture test would be valid right now. I’ll put a sheet on today anyhow.

        With the highly porous surface of that slab, I was thinking of using a densifier on it. Would that help with moisture?

        Also, which order of densifier, epoxy fill of pits/hollows, and etching would you go with?

        • Shea says

          Many times a moisture problem is not caused by rain, but by a drainage problem near or under the slab. Ground that is irrigated regularly next to the slab is an example. So is a leaky drain drain or water pipe that runs under it. If your moisture test result is negative then you shouldn’t have an issue. Prep the concrete, do any required repairs, and then apply your product.

          Densifiers will definitely slow down moisture but they are not a sealer. Deep penetrating sealers of a silane/siloxane mix work better. Either way, if you still want to apply a densifier or penetrating sealer; prep the concrete, apply the sealer or densifier, make any necessary repairs, then apply the epoxy.

          • Collin Ong says

            I looked at the staining patterns on the floor and concluded that water was entering the garage below the side door and then pooling in the area with the powder. I did put down two plastic sheets and have not seen any moisture under them. (however, it is 100+ degrees and no rain right now). So I’ve concluded there may not be moisture coming up from under so will just patch and the epoxy.

            (I’ll post my patching question under that separate article.)

          • Collin Ong says

            Actually, I removed my sheets and found that there was a damp spot accumulating under one of them and under the tape as well. So it looks like I will have to use AquaDyke from Legacy as a primer coat. (Doubles the cost of this project! Argh!)

          • Shea says

            Sorry to hear that Collin. Ask to talk to Scotty at Legacy Industrial about the AquaDyke primer. He will give you any tips you may need when applying it.

  6. Collin says

    Is this article linked in any of the categories? without a site-search function (that I can find) I have a hard time finding it again.

    • Shea says

      Yes Collin, it’s under “Application” on the “Epoxy” page. We had deleted the search function on our site because no one was using it. You can also find a list of all our articles and pages under “Site Map” at the bottom of the page.

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