Moisture in Concrete Raising Relative Humidity in Shop

[John Karam]

Hello everyone!

So I've got a moisture problem. My shop is uninsulated, and unheated currently. Things have been fine, occasionally I find some surface rusting on my tools. However, recently I have been finding condensation build up under the safety guard and blades of my jointer. In efforts to combat the rust, I've used boeshield and pastewax, it has done its job for the most part but the condensation it too much for it.

My area isn't very humid, temps have been about 40s but the relative humidity in my shop has been over 80%. The air itself isnt "humid." I can see that the moisture is coming from the concrete. Whenever it rains for a few days, the concrete absorbs the water from the ground and it begins to appear dark. Now it has gotten to the point where it is moist to the touch and has stayed this way for a long time. The wood I have stored in the shop has raised in moisture content levels as well (I've made a few little things took them into the house to test, they shrunk).

With that said, Im working on solutions.
1. I will be insulating as soon as I can, however I fear that could make it worse as it may "trap" the humidity inside.

2. My concrete is unsealed, and have been considering sealing it. Would this prevent the moisture in the concrete from getting into the shop? Also, would the excessive moisture in the concrete become a problem to the concrete itself because of sealing it?

3. Dehumidifier is not something I really want to do, but will if I come up with no other solution.

Any suggestions? Things I have not consider? Comments on effectiveness of the above ideas would be greatly appreciated. Ultimately, I want the shop relative humidity to mimic my house.

Thanks ahead of time

[Steven Wilson]

Every spring I have condensation problems as the ground thaws and the shop goes through the magic 50F mark. Besides Boeshield I throw covers on my tools and it keeps the condensation from collecting. No problems since I started doing that.

[Josiah Bartlett]

Ventilation is a good thing in this case, even with insulation.

Another trick is to put little local vapor barriers under your tools. This keeps the moisture laden air from your concrete from percolating up the chimney of your tool cabinets and collecting under things like the guards. While the shop air may be 80% RH, I bet the air inside the tools is closer to 95%.

This is a common trick when storing vehicles: Drive the vehicle onto a tarp and cover the top with breathable fabric. You can skip the cover if you like but the bottom vapor barrier really works.

Running a fan in the shop may also help, it would at least equalize the temperature.

[David Miller]

Hi John,

Does your concrete floor have a vapor barrier below it?

Insulating your shop and heating it will help. The goal is to keep the temperature above the dewpoint so the moisture in the air does not condense on your tools.

Running an exhaust fan will help if it is less humid outside the shop. This will help lower the dewpoint in your shop (a little).

You won't be able to properly seal the concrete if there is excess moisture in or below it. The moisture will work it's way through. This is why I am curious about a moisture barrier.

-Dave

[David Giles]

John, here at 6ft above sealevel, we have the same problem. Concrete absorbs moisture like a sponge. A plastic sheet barrier between the concrete and ground works wonders at reducing this problem, but it's a difficult retrofit to existing structures. I use a couple of strategies that seem to work in combination.

First, every cast iron surface has been cleaned and coated with CorrosionX, Boeshield and paste wax in that order. Second, the rust problem is worst when the temperature has been cold for a long time, then starts to rise. Especially if the warmer weather is humid. I use halogen lamps to warm up the cast iron surfaces. A radiant heater would work even better. I don't open the doors and windows until the machines are warm.

The outside shop perimeter is lined with plastic garden edging is keep water away from the concrete edge. The ground is sloped away from the building and gutters direct water away. Finally, inside the shop, each tool is separated from the concrete floor using a 6 mil plastic sheet and 1/4" waxed hardboard piece cut to fit the base. It makes it easy to move machines around as well. The wall area is also lined with plastic and hardboard so that I can lean jigs and boards against the wall without moisture diffusing into the end grain.

I looked at a floor epoxy, but was warned that it would flake off due to the moisture transfer. Might be worth a try, though. Floor sealants that use silane/siloxane will prevent liquid water from absorbing into the concrete, but will still let water vapor pass through. So that doesn't help much.

[Anthony Whitesell]

A while ago I was doing some minor renovations in the basement and ran into a few issues.
1. A crack in the concrete foundation that was leaking behind an insulated sheetrock wall.
2. An area were the concrete was lacking cement and was leaking due to this.
3. Moldy insulation in the basement walls.

Before I attempted to remedy my problems I did some research. I researched topics regarding concrete foundations; including sealing, painting, fixing, and insulating and on the materials to do all of the above. I talked directly with Quikrete, Owens-Corning, and UGL (manufactures DryLok and the like) representatives about the issues and I was able to put a few things together that will be very relevant to your problem.

Things to know about concrete: It's solid but porous. Water will seep through it, period. Efflouresence (water powdry stuff on concrete walls) is evidence of this. Efflouresence is lime drawn out of the concrete by water passing through and left behind when the water evaporates. Sealing the concrete will help some but not a 100% solution. It works best if sealed from the outside, but can a little if done from the inside. Keep in mind that basements are generally cooler and the lower temperature will also result in a higher humidity.

The purpose of insulation is to create some dead air pockets. Moving air is leak heat. Dead air is insulating the house.

Take those two items and add them together. Insulation (dead air pockets) + concrete (damp surface) = mold and moldly insulation.

In a second call to Owens-Corning I was able to speak with a product engineer. He said I was correct and pink fiberglass insulation should not be placed directly against concrete. The foam board type on insulation would be better, but the real issue is the lack of airflow. Airflow between the insulation and the concrete would help prevent mold, but will partly defeat the insulation. If the pink insulation is used, the Owens-Corning representative also suggested that attic rafter vents (styrofoam W-like things that keep the insulation from the bottom of the roof sheathing) could be used.

[Bernhard Lampert]

Looks like you have a drainage problem. Is the surrounding area properly graded and sloped away from the house?
The way you describe the problem, it seems that surface water after rain is collecting near or under your structure.
I would look into a French drain design to divert the water. It is best to attack the problem at the root cause: divert the water before it gets near or under structure.

Good luck!
Bernhard

[Dennis Putnam]

I looked at a floor epoxy, but was warned that it would flake off due to the moisture transfer. Might be worth a try, though. Floor sealants that use silane/siloxane will prevent liquid water from absorbing into the concrete, but will still let water vapor pass through. So that doesn't help much.

I haven't any direct experience with epoxy but I've found polyurethane works well (the stuff specifically for floors). I use the stuff for auto repair shops as it is less vulnerable to petroleum based solvents. However, you have to etch the concrete first so it does not flake off. The other drawback, unlike epoxy, it does wear so you will have to reseal every few years. The biggest benefit to sealing concrete is not preventing moisture but rather lime dust as it ages.

[andy brown]

Hi,
Interesting post! We have just bought an old house in France, which has concrete floors...so similar problems with the damp as yourself. Curiously enough the house only has this damp because of the concrete! and I'm thinking of getting rid of it and instead of all this fighting damp with barriers etc change the floor to 6" of LECA (Clay balls to those what know!) and Limecrete instead of concrete. Lime has the ability to breathe and pass any moisture through it without the dreaded damp. You may need to think of redoing your outer walls with a lime render up to a metre or so to let the moisture pass. Just a thought though and there's loads of info on lime and LECA on the web of course!!

Andy

[Lee Koepke]

Bernhard is on the right track

Simply coating the floor may stop water NOW, but if it truly is an issue, it will only get worse.

If your house is recent (10 yrs or so) you should have had a vapor barrier installed. thats intended to help vapor transmission.

There are several moisture tests that you can do to see if that is really your problem. You need to search around a bit for the correct steps, but you can duct tape some visqueen to the floor tightly and let it set for a few days. If you have condensation on the underneath of the plastic, you have vapor issues from the concrete. You should then contact a waterproofing subcontractor in your area for recommendations.

[John Karam]

First, thanks for the thoughts everyone.

The shop is a separate building from the house, recently built. A vaper barrier was placed before concreting and yes, it is on a hill. Sorry I neglected mentioning those two details previously. Two drain ditches have been dug along two sides of the building. One is intercepting the down slope and drains into the other that goes down the hill. Both are right next to the building, which may be a problem if its allowing a lot of water to rush quickly to the concrete, therefore allowing more absorption.

I would love to attack the problem from the outside, and will be attempting to do so as soon as I can. By adding another ditch further out, perhaps digging the closer ones deeper. Also, might add a plastic barrier along side of the building.

My main concern is if the humidity is too high and raises the moisture content of the wood that I'm building with, when I move projects into the house they will shrink. I'm hoping by fixing the concrete, it will stabilize to be much like my house.

[Lee Koepke]

well john, that would lead me to believe, you either have some water issues on the wall sides or foundation areas. sometimes a dehumidificator would be the trick. air itself is laden with tons of moisture, and proper ventilition can keep the moisture from accumulating.

if you waterproof the exterior walls down to the foundation, make sure you do it right. gravel / french drains, etc. it helps move ground water away from the building. you still may need to dehum the area.

get a hygrometer (sp??) and track your relative humidity in the air for awhile to see if thats your problem. try the visqueen on slab to see if thats where your moisture is coming from. when you know where its coming from, its easier to rectify the problem.

[michael osadchuk]

"My area isn't very humid, temps have been about 40s but the relative humidity in my shop has been over 80%. The air itself isnt "humid." I can see that the moisture is coming from the concrete. Whenever it rains for a few days, the concrete absorbs the water from the ground and it begins to appear dark. Now it has gotten to the point where it is moist to the touch and has stayed this way for a long time. The wood I have stored in the shop has raised in moisture content levels as well (I've made a few little things took them into the house to test, they shrunk)."

John, the above sounds unusual/extreme - relative humidity levels of over 80% in winter and the concrete floor appearing dark/being moist to the touch after rains.....

.... based only on my own homeowner experience, I wonder if there is proper drainage under the concrete slab, something like 6" or a bit more of gravel under the slab with some drainage tiles to prevent rain water from getting into contact with the concrete slab and to get the water drained away so it doesn't accumulate........ do you know what is under the concrete slab?
..... without removing all of the concrete slab, I wonder if excavating a 3' x 3' hole in a 'lowest' corner of floor area, and installing a sump pump to drain away rainwater, might be an idea......

when I built an extension to my house, the basement workshop concrete slab was poured to rest on: 6mil vapour barrier, then 2" white foam slabs, then 6"+ pea gravel, then undisturbed earth.
The walls of the workshop are also well insulated and the concrete floor remains warm and relative humidity in the shop moves between 25% in winter to 65% in late summer along with wood moisture content cycling between 7 and 10%, no rust issues and all areas of shop being comfortable to work in.

good luck

michael (a bit north of Toronto, Ontario)

[David Miller]

John,

Is the floor damp only along the uphill wall or is it damp all over? If it is along the uphill wall then I would lean towards sealing the outside walls and ensuring proper drainage. If it is damp all over then it's not getting in from outside, especially with a moisture barrier present below the concrete.

IMHO, if the entire floor is damp then the only thing that will help is to 1) reduce the moisture in the air, or 2) increase the temperature of the air. Insulating your shop will help keep the shop warm, and heating the shop will help even more

Since you have a moisture barrier underneath then you should be able to seal the floor. You'll have to wait until it's dry enough to apply the sealer or paint. This will prevent the concrete from absorbing moisture from the air.

It's all about that pesky dewpoint.

-Dave

[David Giles]

You could humidify your house so that the projects wouldn't shrink.

If your shop has brick walls, they can absorb a tremendous amount of moisture which can diffuse into the work area. A silane\siloxane coating works wonders at keeping liquid water out of the brick while letting it breathe.