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A sweating garage floor can be a real nuisance than can create a slippery surface and even make your garage damp as a result. If you wonder why your garage floor gets wet, particularly in the spring, then your garage may be suffering from this condition. So let’s discuss what makes a garage floor sweat and what you can do to help stop it.
Why does my garage floor get wet?
Before you make a plan to stop it, you first need to understand what causes it. There are a couple of reasons why your garage floor may be getting wet. The most common is condensation — AKA sweating garage floor.
When relatively warm, moist air comes in contact with a very cool concrete slab, the cold temperature of the concrete will cause the air that comes in contact with it to cool. If the air cools at or below the dew point, it will begin to condense on the surface of the concrete, thus causing the wet and sometimes misty looking garage floor.
As a refresher, the dew point is the temperature at which the air becomes completely saturated. If your concrete surface is cooler than the dew point, it can easily cause the boundary layer of air that comes in contact with it to cool at or below the dew point if allowed to rest long enough undisturbed.
Otherwise known in the concrete industry as sweating slab syndrome, it’s the same condition that causes the outside of a cold beverage container to get wet on a warm, humid day or the surface of your car to get wet when it’s left outside on a cool, damp night.
A wet garage floor generally takes on a darker look due to the moisture and can even suffer from efflorescence as a result. This happens when the moisture that has been absorbed by the concrete travels back up to the surface and evaporates. As it evaporates, it leaves behind white mineral deposits on the surface that the moisture carried with it as it traveled through the concrete.
Sweating garage floors seem to be most prevalent around spring time in areas of the country that experience cold winters. Ground temperatures are still very cool from the winter and tend to keep the concrete cold relative to the air around it.
However, it can also be an issue in areas of the southeast where warm moist air brought in from the ocean comes in contact with relatively cooler concrete slabs during the winter and spring time.
Another condition that can make your garage floor wet is moisture that travels up from under the slab. This is different from a sweating garage floor and is often misunderstood as the same thing. It happens most often with concrete that does not have a moisture barrier beneath it.
Though concrete is hard and dense, it actually has small capillaries within that act like a sponge. If the ground beneath the concrete is wet or has high moisture content, it can absorb that moisture and distribute it through the concrete. Basements commonly suffer this same condition.
If the air above the concrete is warmer, the moisture within it will rise to the surface and evaporate. This can create a wet surface if there is enough moisture. It can also cause efflorescence and just like a basement, it can make for a damp garage as well.
How to test for a sweating garage floor
If you are uncertain whether you have a sweating garage floor or a moisture issue from below your slab, the easiest way to tell is to test for it. Tape a 16” square piece of heavy plastic down to the concrete when it is dry. Make sure to seal all the edges. Leave it in place until your concrete shows signs of being wet.
Remove the plastic and then look at the concrete. If the concrete under the plastic is dry and the surrounding surface is wet, then you have a sweating garage floor. However, if the underside of the plastic is wet and the surrounding surface of the concrete is mostly dry, then you have a moisture problem from below the slab.
Another sure indicator of a sweating slab is if you remove something from the wet garage floor that has a flat surface and you find the concrete dry underneath. An example would be a floor mat or a cardboard box.
On the contrary, if your garage floor looks dry and you remove an item to find wet concrete underneath, then you have a moisture problem from under the slab.
How to stop a garage floor from sweating
Stopping a garage floor from sweating can be more challenging to accomplish than you may be aware. It’s actually a problem involving the physics of condensation that you are trying to prevent.
For example, you cannot stop your garage floor from sweating simply by applying a sealer or coating. The surface of the garage floor will still get wet (sweat) because it is cooler than the dew point. If the air that comes in contact with the concrete has a chance to settle long enough, it will condense regardless if there is a sealer or coating.
In order to stop a garage floor from sweating, what you need to do is eliminate the conditions that make a garage floor sweat to begin with.
Think of it this way. If you park your car outside on a cool night with damp air, the surface will be wet with dew from condensation the next morning. If you wax your car to seal the paint it will not prevent it from getting wet, it will just cause the water to bead up nicely. It’s the same principle with your garage floor.
In order to stop a garage floor from sweating, what you need to do is eliminate the conditions that make a garage floor sweat to begin with. In other words, you need to do at least one if not more of these 5 things.
- Decrease the dampness/humidity of the air
- Increase air circulation across the floor
- Increase air temperature of the garage
- Change the garage floor surface
- Keep the garage door, man door, and windows closed and sealed
So let’s discuss each of these items in more detail, including what may be required.
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Decreasing the dampness of the air
Decreasing the moisture in the air is one of the more effective ways to prevent concrete sweating. If the humidity level in the garage is kept at acceptable levels during the times of year that sweating is most prevalent, then you will greatly reduce the chances of it happening at all.
The best way to reduce the moisture content in the air of your garage is with a dehumidifier. If set to run during the times of year when you encounter sweating, it can greatly help to reduce if not prevent it.
The key is to use the right dehumidifier for the job. For a typical 400-500 square foot garage, we recommend using a unit rated for 2500 ft² spaces or larger.
The reason for using a higher rated dehumidifier is that they will work much more quickly at removing the moisture from the air. Plus, garages are far more drafty than the inside of your home and will continually let in the relatively warm, moist air at a greater pace. Dehumidifiers rated closer to the actual size of your garage will be too inefficient to get the job done.
Also, it’s important to use a dehumidifier that is rated for working in colder temperatures. Many of the in-home units are not rated for temperatures low enough to work in a non-heated space and will freeze up if the temperature dips into the low 60’s.
One of our favorites is the 6 gallon Dehumidifier by hOmelabs. It’s rated for 2500 ft² and works well for a standard 2-car garage. For a 3-car garage or larger, the 9 gallon unit would be better suited.
The hOmelabs units are UL and Energy Star rated as well as rated by the manufacturer to work in temperatures as low as 35°F.
Price the hOmelabs Dehumidifier here
Though larger units such as these emit dry heat when in operation, it is not enough heat to affect the temperature of a cool garage.
Of course, running a dehumidifier requires that you only open the garage door and man doors when necessary. Leaving doors or windows open defeats the purpose of using one.
Increase air circulation within the garage
Another method that can be effective at reducing garage floor sweating is to run a fan to increase air circulation across the floor. If the air near the concrete surface cannot rest long enough to cool, then it will not condense. If it is allowed to rest long enough and form a cooler boundary layer of air at the surface, then it will condense.
A larger sized oscillating fan, like this Huricane Fan pictured above, can move a lot of air.
Let’s go back to the example of a car sitting outside on a cool, damp night. You may have noticed that dew will not collect on the car if you are driving it, but it will begin to collect sometime soon after you park it. The reason is because the air does not have a chance to settle on the surface and condense while you are driving. It’s the same concept with your garage floor.
A lower floor height is good for moving more air under parked cars if necessary, while the wall mount or upright stand will help to circulate more air throughout the entire garage. Just set it up in an area where it will cover most of the garage floor.
If you have a larger garage or more space, a larger wall mount fan is a better choice. They will move much more air and can create a circulation effect when directed towards the floor.
One of our favorite fans that we have used in our garages is the brand Hurricane. They come in a variety of sizes and configurations depending upon your needs.
Price Hurricane fans here
Many times the combination of moving air with a fan and using a good dehumidifier will resist some of the worst conditions for sweating garage floors.
Heat the garage
One option that is not always practical but is worth mentioning is to heat the garage. What this will do is bring the surface of the concrete closer to room temperature. When the conditions for sweating exist (relatively, warm moist air), the concrete will not be cool enough to condense the air that comes in contact with it.
The problem with heating a garage, however, is that it can take up to a week for the concrete at the surface to warm. Plus, running a heater for long periods can become expensive since most garages are not insulated. Amazon has a compilation of popular garage floor heaters if you like the idea of heating your garage, even temporarily.
If you are in the planning stages for building a home or separate garage in an area of the country where concrete sweating is common, you may want to investigate radiant floor heat for concrete instead.
Changing the flooring of your garage is arguably the most effective way to stop garage floor sweating. We don’t mean sealing the garage floor or applying a coating. What we are referring to is installing a different flooring surface.
Great examples are interlocking garage floor tiles, vinyl mats, and even automotive approved carpet for a garage. When such a garage flooring system is installed, the surface of the flooring will no longer adopt the temperature of the concrete. Instead, it will adopt the temperature of the air in the room.
As a result, when warm moist air is introduced to the garage, it cannot condense on the floor since the floor will closely adopt the temperature of the air. The other benefit to this type of flooring is that it will help to insulate your feet from the cold temperatures of the concrete underneath.
Seal the garage from drafts
Lastly, you can lessen the effect of garage floor sweating if you take steps to slow down the intrusion of relatively warm, moist air into the garage to begin with. This is particularly helpful when running a dehumidifier and/or fan. You can achieve this by ensuring that your garage door and man door are properly sealed and that windows are kept closed.
For the garage door, a garage floor door seal can be very effective at achieving this goal. It works well at preventing the moist air from traveling across the concrete floor and settling to cool.
Side garage door seals are something to be considered as well. Not only will they help to seal the garage door better, they also work well at keeping blowing rain and other critters from entering your garage.
For your man doors, make sure the door seal gaskets are in good shape. Also, don’t forget an under the door seal as well. Like the garage door, this helps to prevent outside air from flowing across the surface of the garage floor.
Make sure to check for any other areas of your garage that may be drafty or letting in air from the outside. Do not block off or seal any venting that may be required by local or state building codes.
We do realize that the minute you open your garage door to park or exit with your car, you will be introducing large volumes of outside air that may be conducive to sweating. However, many times this air can move in overnight or the early morning when you keep your doors closed. Making sure your garage is sealed properly can go a long ways towards reducing this air from getting in.
Final thoughts
As a reminder, sealing your garage floor will not stop it from sweating, but it will prevent some of the adverse effects such as efflorescence. You will also benefit from easier cleanups and stain resistance as well. Changing the flooring of your garage is the most effective method for stopping it completely.
You may have been surprised by the methods required to help prevent your garage floor from sweating. However, once you introduce one or more of the procedures we stated, you may significantly reduce or stop the problem completely.
This is a great article, thank you very much for publishing it. I have a question that is more of a puzzler than anything.
I live in the SF Bay Area. 50% humidity all the time and never too cold.
I have an 80+ year old garage that was settling badly in one corner, making a huge crack down 1/3 of the slab. My contractor cut that slab out, hand dug out the foundation and repoured it, then poured new 1/3 slab. Everything’s great.
The new garage door has been on back order, and now that we’re in the rainy season, the floor damp after a rain, but no water is really pouring into the garage. After reading this great article, I’m 99% sure it’s just condesation, and when we get the doors on, it should go away. (I’m planning on putting down carpet squares too, which I had in there before. Only the sunken corner ever got wet underneath the carpet squares.)
Here’s the puzzler: The old 2/3 of the concrete slab is sweating, but the new 1/3 slab does NOT seem to be sweating. At least, it’s not looking damp, dark or misty. How come?? Any thoughts on why?
I believe both slabs are the same thickness. Old 2/3 of slab almost surely would not have a moisture barrier under it, but I don’t recall seeing if the contractor put a barrier under the new 1/3 of the slab before repouring. Would that change the temperature of the slabs relative to one another, affecting the sweating? Or new concrete different density than old concrete?
Quite the puzzler, and mostly idle curiosity, since I’m not longer worried about it, after reading this article.
Thanks
Hi LH. Most likely there is a temperature variation between the new pour and the old slab. The old slab has been subjected to the cooler temps for a much longer time. It takes a while for concrete to change temperature and is why sweating is most common in the spring. If you want, you can test it to make sure. When the old slab is dry, tape a 16″ square sheet of plastic to the surface. Seal it all around the edges with good tape. When the concrete shows signs of being wet, pull up the plastic. If the concrete underneath is dry, then the slab is experiencing condensation. If the plastic is wet underneath, then it’s a possible moisture issue.
Shea,
I’m sure it’s condensation. I move a box today, and the floor under it was dry.
I also found out today that the new 1/3 slab DID have a moisture barrier put underneath, before pouring the slab. That might change the temp of it relative to the old slab.
Once again, great information! Thanks so much.
This article covers sweating concrete but not moisture from below. How can I solve that problem?
Hello Sue. Moisture from below the surface is a much different problem entirely and one that can be difficult to stop. Coatings and penetrating sealers do a good job at stopping moisture that comes in contact with concrete at the surface. Moisture from below, however, does not interact in the same manner. Coatings and sealers are poor performers in most cases when trying to stop moisture from the other side.
Concrete is essentially a hard sponge. It is porous and will absorb moisture in the soil if it’s there. For above grade slabs, atmospheric conditions above the concrete (typically relatively warmer and dryer) will draw the moisture up to the surface that the slab has absorbed . This moisture will either evaporate and leave behind the minerals that traveled with it (efflorescence) or it will be so much moisture that evaporation cannot keep up with it and result in damp or even visibly wet concrete. Sometimes application of densifier will help slow down this moisture and sometimes even stop it all together. This article here explains how a densifier works.
If the concrete slab is below grade, then moisture may be pushed up through the concrete due to hydrostatic pressure. Hydrostatic pressure is very hard to control and sometimes may require grinding of the concrete and special moisture vapor blocking coating applied (expensive). Even that may not be enough and is why it’s not unusual to sump pumps in basements.
Your first course of action is to determine why you have moisture to begin with and see if it’s something you can fix to help mitigate the moisture problem. Is it due to poor drainage around the slab and water is pooling up against it when it rains? Are there planter beds up against one side? A leaky water or drain pipe? Or, is something you can’t control like one or more sides of the slab below grade and up against or near a dirt bank?
What if I want to enclose the sweating carpet/garage into a living space? Same principals apply?
Hi Eon. Yes, the same principles apply. However, enclosed spaces are generally heated or air conditioned and not subjected to the immediate outside weather changes that can cause sweating of a slab.
Shea,
My garage floor has been plagued for years with what I think is a combination of sweating and moisture from beneath. I live in north Florida where it is almost always very humid with frequent rain, yet can be cool/cold in the winter months. Your article about sweating was very informative and I will be implementing some of the sweat reduction things you outline as well as possibly sealing the floor. In the past I’ve intentionally left my main garage door up about a foot, day or night, rain or shine, so my cats can have access. That will have to change!
But the reason I write is, I have a curious thing going on at a few spots on my floor for which that I cannot come up with a logical explanation. The long story is that the garage was completely cleaned out for a paint job. The clean-out was done after several days of no rain and low humidity so all the usual wet zones were dry. Two or three days after the clean-out high humidity rolled in (heavy fog at night and morning but no rain) and as expected the usual wet zones became wet again… except for two or three once hidden spots. From the shape and location of the dry spots I can see that they are the outline of where a plastic storage box sat virtually unmoved for months/years. They are completely surrounded by wetness but for some reason water isn’t condensing right where the boxes sat. No special treatment was ever done to those spots or the surrounding wet zones. I’m baffled as to why these spots are exempt from condensation and wish I could extend their magic to the surrounding areas!
Let me add that the entire garage floor usually doesn’t get wet. The wetness happens most frequently (but not exclusively) around the outer walls where I am pretty certain water also comes up from below. Once a wet zone dries there is typically a white residue “ring” left behind. The floor is at ground level and during moderate to heavy rain the ground outside often saturates.
Any theory about the mystery dry spots?
Hello R. Great mystery! Our educated guess has to do with the plastic storage boxes and the fog. Many plastics will slowly release monomers (small particles of polymers) and other substances during their life. Most likely, these monomers have leached over time into the upper surface of the concrete, thus making it much less porous. When the overnight fog rolled in, the more porous concrete was able to absorb the moisture in the air that came in contact with it. That is why it looked darker and damp. The temperature of the concrete was most likely not low enough to cause condensation. This technically isn’t sweating, just a result of the wet foggy air coming in contact with a porous surface ready to absorb moisture. Assuming the concrete where the plastic boxes sat is much less porous, the concrete would not absorb the moisture out of the air and appear dry.
Thanks Shea. You might be onto something. Thinking of all the tendencies that I can remember (hopefully correctly) over time, I would tend to agree that my floor’s not really “sweating”, though it definitely correlates to high moisture in the air. During the recent fog event the floor wasn’t really cold. So the simple absorption of the saturated air (fog) in some zones could be what’s going on. In fact, as I type, the floor is drying with a light continuous rain going on, i.e., humid air but less humid than that foggy night. And I think the floor is cooler now than during the fog — cold front moving through. Seeing this makes me less fearful that the problem is water rising up from below. Especially, since you state above that efflorescence can even occur from condensation and absorption. Seeing efflorescence made me just assume that ground water was rising up.
So if you’re are correct and I’m getting absorption into the porous surface I suppose a penetrating sealer should fix the problem. But before I do anything I will experiment to nail down once and for all if there is water rising up.
Thanks for your insights.
Hello,
This has been a helpful forum – thank you. We have a sweating floor near a lake in Michigan and I fear moisture is coming from below (it’s a poorly built slab-on-grade cottage( as well as humid air above. Wondering if anyone has heard of installing a ventilation system below the floor using fans similar to those used in radon mitigation to keep air flowing through the floor with concrete or tile above? Thank you.
Hi Margo. That’s creative thinking, but moisture vapor from wet soil is not going to be alleviated with such a system for a variety of reasons. The first thing we recommend doing is a moisture test to get a much better idea of the problem you need to address. I’m assuming the floor is bare concrete?
Hello! I have a recurrent water stain on my garage floor that keeps showing up in the same area. However, it never feels wet to the touch. The intensity of the stain darkens and lightens. Sometimes this happens when it rains and sometimes not. I live in Florida so due to the year-round warm weather, this happens all the time. The house is less than 3 years old. Recently, I had left an object on part of the stain for a while. When I lifted it, I used a moisture meter (don’t know if those things are actually useful for concrete) and it read 28% for the area under the object. The surrounding areas which were exposed to air and had the chance to dry off were 12%. If pictures will help, I can send them. Thank you for your help!
Hi Pesi. This is not uncommon in Florida due to the high water table. The dark area being in one spot is an indication that you have moisture traveling up through the concrete from below. When it reaches the surface it evaporates. It would only be wet if the moisture content traveling through was at a rate faster than what could evaporate at the surface. If the area under the object was just as dark or darker, then than confirms it’s moisture from below. Sometimes moisture in one spot may indicate a leaky pipe that is running under the slab. It may be worth checking if you are not sure if there is plumbing or a drain pipe of some sort below the slab.
Thanks for the info. You said the answer to a sweating floor is to add flooring to the garage (but not a sealer) I’m not understanding what the difference is. I added a cheap plastic underlayment and some vinyl flooring and now there is moisture between the underlayment and my vinyl floor seeping up through the cracks of my vinyl. I am not sure what is going on or how to solve it. Thanks
Hi Sarah. Have you first verified that you have a sweating floor and not a floor with moisture issues? We discuss how to determine that in the article.
Finally, somebody actually figured this out! Everyone else claims it is moisture from below and applying a coating of some type is the answer.
A way to prevent this in new construction is to put your slab up high on fill well above grade so it is high and dry and can quickly heat up. Better yet, pour your slab over insulating foam.