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Old 10-27-2011, 02:24 PM   #1
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Need Help Fixing Damp Slab Problem in Shop


Hi all,

Hoping I can tap into the collective wisdom on this one: I have a detached pole barn shop that is about 800 square feet located in the temperate Pacific Northwest where it rains a lot and is often humid but not really ever really fridgid. The building is sheet metal sided/roofed on large timbers/trusses with a 2" vinyl backed/fiberglass insulation sandwiched between. There are two large track-sliding doors, one man-door, and four windows. 12 foot ceilings at wall; 16 foot at the peak.

There is a 4 inch poured concrete floor that I'm having problems with in terms of dampness. There never is standing water and the dampness only rears it's ugly head on high humidity days but does so enough to start rusting things in the shop. The rest of the shop stays dry -- that is, there is no condensation on the walls, windows, doors, etc., only on the floor. It can get damp enough to wet a finger when drawn across the floor. There is a 220v forced air shop heater that is used only when cold out (below 50 for me).

The slab was poured over rip-rap fill, topped-off with regular 1.5 inch clean gravel. The slab is about 8 inches above the old dirt grade at one end, and about 7 ft above grade at the other. So there is about 70 tons of well-draining rock under the slab. There is no moisture barrier between the slab and the rock on which it's poured -- a fact about which I kick my own ass daily in an ongoing rite of repentence.

So what's up with this and how do I fix it? I suspect this is a condensation problem and not a water contact/wicking issue since the slab is well elevated above the old grade. I have been investigating the following as partial fixes:

1) leaving a low temp (oil-filled radiator) heater on in the shop -- problem is that it isn't always needed and it's a hassle to turn off/on over time predicated on the humidity level). Cost: about $40 plus ongoing elect. costs. Maybe a ceiling fan to go with this. Cost: another $50-100.

2) installing a dehumidifier (problem is that these don't work well below 50 degrees and I'd prefer not to heat a frequently unused space 24/7). Cost: a couple of hundred plus ongoing elect. costs.

3) installing a humidistat-operated vent fan about a foot above the floor on an exterior wall to help vent the air when the humiditiy rises (can't see a downside to this except that the air that's vented has to be replaced by...you got it...outside moist air!). People say this works but darned if I can figure-out how. Cost: about $230 for a humidistat-operated auto exhaust fan and ongoing elect. costs.

Lastly, 4) remove the slab, put-down the proper moisture barrier and re-pour the slab (what if that doesn't solve a condensation problem ?). Cost: a couple of grand.

so I could use help in figuring-out which solution or combination thereof might work best to cure the problem of the damp, weeping floor.

Sorry to be long-winded but I know that as much info as possible is
useful.

Thanks up front -- great forum.

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Old 10-27-2011, 03:54 PM   #2
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Need Help Fixing Damp Slab Problem in Shop


Sounds like you mentioned the solutions already. Most require money to solve. You just need to decide which one you want to do.
I don't see repouring the slab as a solution. Even with the vapor barrier, the concrete will be cooler and attract condensation. With the base you have, it doesn't sound as though the moisture is coming from the ground up. More like from the air, down.
Radiant floor heating will help that, but that will also be at a cost. Might not be feasible based on the building use.
And the slab wasn't insulated, so the heat will migrate down as well as up.
But warming the floor slightly might do the job.

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Old 10-27-2011, 07:03 PM   #3
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Need Help Fixing Damp Slab Problem in Shop


Check out E Z Breathe. Not sure of the cost of install or cost of operation but seems like it might work for you.
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Old 10-27-2011, 09:57 PM   #4
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Need Help Fixing Damp Slab Problem in Shop


Quote:
Originally Posted by Ron6519 View Post
Sounds like you mentioned the solutions already. Most require money to solve. You just need to decide which one you want to do.
I don't see repouring the slab as a solution. Even with the vapor barrier, the concrete will be cooler and attract condensation. With the base you have, it doesn't sound as though the moisture is coming from the ground up. More like from the air, down.
Radiant floor heating will help that, but that will also be at a cost. Might not be feasible based on the building use.
And the slab wasn't insulated, so the heat will migrate down as well as up.
But warming the floor slightly might do the job.
Honestly Ron, I think the vapor barrier would help alot. I can't explain why it works so well, but we almost exclusively put them down in every garage/shop floor because they simply don't sweat, ever.

In my own shop, I have about 60% of the floor that I tore out & re-poured with 6 mil. vapor barrier. The remaining 40% I kept in so I have an area that carries no guilt on my part when I abuse the floor. During extremely humid weather, the new side is completely dry, where the other side of the cold joint is the old floor, which sweats to the point of actually running water to the drain.

As for the OP's question, I certainly don't buy into "snake oil" products for a remedy like this, but I'd at least look into a sealer that's marketed to serving as a VB as well. Generally in the past, a sweating floor wouldn't do well at holding most sealers, but with current technology, it would certainly be worth a try from a fiscal standpoint IMO.

Your next best feasible option would be trying option number 3 IMO.

Good luck.
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Old 10-27-2011, 10:20 PM   #5
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Need Help Fixing Damp Slab Problem in Shop


Wow, thanks for a bunch of ideas to ponder.

I did check out ez breathe and another product called Wizzvent -- the ez breathe is designed for inside dwellings and definitely works by replacing the dampest air in the house with heated air from your furnace. In other words, if you don't have heated indoor air, you won't improve upon the moisture situation. I fear that this may be the same verdict with the wizzvent (and the similar but slightly cheaper humidistat ventilator I found at Grainger) though perhaps the air at the floor is moister than the replacement air from outside.

I don't know if just raising the temp in the building a little bit (like with an oil filled radiator heater) will make a difference.

Similarly, I don't know if the moisture barrier would prevent condensation like Ron and Jomama were discussing. It's a little bit of an investment in time and grunt work to get into it without more certainty.

One thing to do when uncertain is to experiment. The cheapest things I can try are: small radiant heater, ceiling fan, and ventilator at the floor. Moving up from that in terms of cost would be a dehumidifier then replacing the floor.

I like that there is some real world evidence that the vapor barrier might work. Jomama, what sort of sample size are you talking about with regard to seeing the vapor barrier prevent condensation?

thanks all
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Old 10-27-2011, 10:35 PM   #6
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Need Help Fixing Damp Slab Problem in Shop


Oly,

My shop is about 2500 sq. feet. I'd say about 1500 is new with VB, while the remaining 1000 sq. feet is old concrete with no VB. The old isn't even sitting directly against soil to be completely honest. After tearing out the existing floor, I realized the interior was backfilled with sand and had settled out a few inches. Most of the floor is literally suspended and hollow underneath. The airspace underneath the floor should be less prone to sweating b my beliefs, yet it still sweats.

As a contractor, we pour alot of floors, and if something isn't right, I end up getting a phone call, even if it means the call is 5 years later. I've never seen a floor that we've poured with vapor barrier sweat, although I've gotten calls about those w/o.

I'd still suggest spending a little time researching a sealer that is intended to double as a vapor barrier before spending any money.
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Old 10-27-2011, 10:50 PM   #7
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Need Help Fixing Damp Slab Problem in Shop


Roger that, Jomama. I will indeed look into sealers and see if there are any designed to mimic a vapor barier.

Worst comes to worst, it really wouldn't be that big of a deal to tear-out the concrete and re-pour. Your experience is reassuring.

thanks!
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Old 10-28-2011, 11:16 AM   #8
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Need Help Fixing Damp Slab Problem in Shop


Quote:
Originally Posted by jomama45 View Post
Honestly Ron, I think the vapor barrier would help alot. I can't explain why it works so well, but we almost exclusively put them down in every garage/shop floor because they simply don't sweat, ever.
The reason as far as I know Joe is that if the slab gets damp from the ground it becomes a bit colder than a dry slab. Therefore it's more likely to suffer from condensation. It's the same with solid brick walls getting penetrating damp and being colder.
It's been a building reg here since 1965 to put a damp proof membrane (DPM) under concrete slabs in houses.
Another problem with rising damp is that it can bring hygroscopic salts from the ground to the surface. These attract condensation at a relatively low R/H.
If sulphate salts are brought up it can wreck the slab.
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Old 10-28-2011, 06:29 PM   #9
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Need Help Fixing Damp Slab Problem in Shop


Good point -- hadn't even thought about the chemistry. Water follows salt such that increased (or in this case concentrated) salts would of course, become wetter than if the surface was less salty.

I have noticed this ring of efflorescence form on the perimeter of puddles left in the garage from the dripping snow melt on the car...
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Old 10-29-2011, 06:27 AM   #10
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Need Help Fixing Damp Slab Problem in Shop


If you rip up the slab it might be worth putting some insulation boards under as well as a DPM.
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Old 10-30-2011, 12:35 AM   #11
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Need Help Fixing Damp Slab Problem in Shop


I developed a partial solution that mimics the wizzvent. I found a 250 CFM fan mounted inside of a 6 in galvanized pipe that is designed to be a booster fan in heating duct systems presumably to even-out those runs of heater ducting that don't put enough heat in a room. So this is a 0.3 amp motor that moves some air ($29 at HD). Also picked-up a humidistat for another $29 that can be used as the switch to automate a humidy removal system.

So I mounted the fan-in-the-pipe thing about 3 inches above the floor with the pipe in a vertical position and coupled it so a 4" duct elbow and out through the wall of the shop. I now have a 250 cfm floor-level venting solution that is regulated by humidity levels I select. Cost about $80 total for all of the duct, the fan, humidistat, and louver. Some 1/4 inch hardare cloth prevents rodents from moving-in through the louvers.

Also bought one of those oil-filled radiant heaters (killer deal at HD on these right now for $36) and intend to rig it to another humidistat such that it comes on when the humidity rises to a certain point. This will limit the heater needing to be on constantly or be regulated by temperature which isn't as useful with regard to my problem as humidity is.

Lastly, I'll be putting-up a ceiling fan to gently blow warmer air from the upper reaches of the building to the floor and to move the heat from the heater to the floor where it will do some good. It will be interesting to see if all of this works. I decided that if none if this makes any difference, I'll tear-up the floor and do it "right" before I settle on a humidifier due to the long-term costs of running one.

Thanks for all of the pointer, opinions, and insight.

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