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.

[michael osadchuk]

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.

hmmmm, more information (above)
...john, IMHO the solutions to the moisture problem are outside the separate workshop structure: better drainage around the outside of the structure would appear to be primary; if digging up the outside immediately adjacent to the footings to provide this better drainage, I would consider adding 'water barriers' (coal tar, sheet plastic, whatever 'wonder' barriers have been developed) on the footings although this would likely be secondary to drainage...

perhaps consulting with someone on site to see positioning of the workshop, the grading and quiz you on exact construction of the existing drainage, would be very useful; you could still do the work yourself, but such an onsite consult could save you embarking on a 'partial' solution....

good luck

michael

[Peter Quinn]

John, sorry to hear about your problem. As a small remodling contractor (several basement remodels) and basement woodshop owner I have dealt with several moisture problems. I am not a professional waterproofing contractor, but I will share my observations and research if it helps.

Concrete is very porous. A typical foundation can absord 400/gallons of water per hour during heavy rain until it reaches saturation. Ground water in the soil to the exterior of the foundation creates incredible hydrolic pressure whose force increases logarythmically with depth. Any attempt to Waterproof a foundation should be conducted from the exterior by professionals using bituminous products applied continously from footing to grade level in conjuction with proper footing drains and additional french perimeter drains as neccessary. Attempting to truly waterproof from the interior is typically catostrophic to the concrete in the long term; it will crumble. Simply putting plastic near theexterior foundation is typically inefective.

Sealing concrete from the interior with a vapor permiable product discourages minor running water problems and mitigates vapor transfer to a level a dehumidifier can eliminate. Vapor permiability is essential to relieve the hydrolic pressure created by soil moisture. Sealers cannot solve major water table issues and are not a substitute for handling ground water runoff effectively. Most below grade concrete spaces, even those with exterior waterproofing, will require fresh air exchange and dehumidification to maintain desired humidity levels.

There is little point to insulate unconditioned space. Insulation will help maintain the temperature of conditioned air (heated or cooled) relative to the buildings external environment. Insulation should be used in conjuction with any heating/air conditioning system to maintain energy efficiency. Interior insulation below grade should not contain a vapor barior.

For concrete below grade EXP (extruded polyurathane, the blue stuff) is prefered as it is vapor permiable and inhospitable to organic growth (mold). Fiberglass batt insulation is inefective when moist and readibly supports mold growth if not well vented, thus a problem for below grade applications. Water vapor must be allowed to pass through the insulation to discourage mold growth and allow the concrete to dry to the interior.

EXP is a serious fire accelerant and must be covered with fire rated material, normally min 1/2" sheet rock. Normal instalation is 2-3" thick EXP T&G, all joints sealed with tape/foam, fire stops at upper termination point, 2X4 studwall to the interior of that, sheetrock to cover. Never leave EXP uncovered in occupied space! If you don't believe me, take a piece outside and ignite it.

Warm air can hold MORE moisture than cold air, not less. In an unheated space moisture vapor enters, cannot be absorbed by the cold air and condenses readily on cold surfaces. Heating the air will allow it to contain more water vapor. Warming the air should reduce the condensation problems on your iron tools but it wont solve the wood movement issue. Keeping your wood storage area temperature above the daily dew point will avoid wild swings in moisture content but again wont solve the wood movement issue created by the difference in relative humidity between your shop and home. You'll still need a dehumidifier and or air exchange.

If the humidity outside your shop is lower than the humidity inside, fresh air exchange is a cheap and effective way to lower the humidity. Fans, open windows, open doors etc. help considerably. A good comercial grade dehumidifier run 220V, with a squirrel cage fan (like an HVAC system uses) while more expensive to purchase initially is cheaper to operate and far more effective than consumer models.

I have seen shops where a small 'dry' area was created using vapor bariers, stud walls and insulation, then the space was conditioned and humidity controlled. Wood and parts of existing projects were stored in the conditioned space. This is more typical of instrument makers and may not suit your needs depending on the wood volume you process.

[David G Baker]

I didn't read all of the posts on this thread but this topic is hotly debated on another forum that I belong to. The most acceptable theory is that the concrete does not store moisture or allow a lot of moisture to pass through it but it acts as a thermal mass that is considerably colder than the air above it thus causing all of the condensation on things above it especially in areas where the ground freezes.
My garage is not heated and the slab is setting on at least 5 feet or more of sand yet I have serious puddles of water that show up when the weather warms up. The sand under the concrete is dry so no moisture comes through that way.
My pole barn on the other hand is kept at around 50 degrees during the colder seasons. There is 2 inches of foam board under the concrete so the slab does not get that cold as the ground freezes. There is no condensation on my tools and there are no moisture puddles on the floor, so until it is proved to me other wise, I have to agree with the moisture showing up due to the temperature difference between the slab and the ambient air.

[John Karam]

Sounds like fixing the problem from the outside is my best solution. I'm looking at ways of improving the exterior drain ditches that are currently there. The concrete appears to be the most moist at the upper corner of the building, where the two ditches come together. This is where my first effort will be held.

Thanks for the advice everyone!

[Ron Coleman]

My shop is in a freestanding 4 car garage and I heat about 3/4 of it. Everything in the heated part stays nice and dry, but in the unheated part the floor gets wet and turns dark from condensation sometimes in the winter months. My concrete floor is well above grade and rests on about 18 inches of gravel so there isn't any dampness moving up from the ground.

One good test for dampness is to tape a piece of clear plastic sheet to the floor. Do this when the floor is dry and seal the edges with duct tape. Keep an eye on the plastic and see if it fogs on the bottom or top side. Another test is lay a rubber floor mat on the floor and see if it stays dry underneath. A really scientific method it so measure the temperature of the concrete and see if it's below the dew point.

Condensation can be a big problem where you have a mass of cold concrete and the weather warms up and it's rainy for a few days. I've even seen condensation form on our large 90 gallon plastic trash can stored in the unheated side of the shop. The floor and foundation of my shop weigh about 50 tons (all one pour) and it takes a long time to adjust to sudden changes in the weather.

Heat is about the only real solution to combat condensation when you are dealing with cold concrete. Ventilation will help if the outside air is dry but only bring in more moisture when it's raining.