Ok, think I might know the answer to this, but thought I’d tap the breaktime pool of knowledge….
Built a barn/ shop 2 years ago. 4′ frost wall, poured concrete slab. Building is weather tight.
I have a problem with dampness on the concrete floor. Whenever we get a lot of rain, or even when it’s humid for a couple days. It is bizarre to me in that I did this building the same way I’ve done a zillion paying jobs, and have never seen this before. I did not have my concrete guy put a vapor barrier down before pouring, but we never do. Not done much or at all in my neck of the woods…. I also don’t think it’s an issue of water or vapor coming up through the slab because if there’s something on the floor when the dampness sets in, and you move it, it’s dry underneath. Then in a few hours, that spot will be damp, too.
My lot is pretty wet, but the slab on both the walk out cellar in the house, and the shop floor, are about 2′ above highest water lavel. Never had water standing in the yard….
My guess is that the high water table keeps the slab so cool that when the air is high humidity and temp, when it hits the conc floor, it condenses and makes the floor wet. This sound likely? Now, I had already planned on insulating and heating this space before next winter, which will dry it out from October through April or so, but what’s my best bet for warm months? I’m thinking De-humidifier in there, but wondering how bad the monthly juice bill will be.
Any input will be appreciated.
Bing
Replies
around here no one puts a vapor barrier under cement either.
i have a pole barn,galv roof no insulation,none on the walls either. i found that when the sun came up in the am it would warm the roof up so much and so fast,that the inside would condense and leak water on the floor, also the floor would look wet,i would guess it was condensing also.this would last most of the morning and then be fine. if it rained and the sun came out,the floor was really wet.
i don't understand dew points etc. but i had a hail ,put on a new white roof and if fixed all this. kind of a unexpected benifit.
YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
DUCT TAPE.
Your observation about a covered spot staying dry, then eventually becoming damp after exposure to the air would seem to nail it. The temperature of the floor is below the dew point of the air, so water condenses, just as it would on the cold surface of a can of your favorite brew.
What's under the slab, anyway? If it's tightly packed and conductive, like wet clay or bedrock close under, that would conduct heat down easily to the mass of ground under everything. That's probably around 50 F well below the surface.
>>just as it would on the cold surface of a can of your favorite brew
It would do that on a can of cold horse piss, too.
You hang onto cans of horse piss?
You can get horse piss in any store that sells beer, it's the stuff in cans that's $2.99/12-pack. At least according to my mom. Sometimes at her house all that's in the frig is horse piss, other times there's Heineken or something in a bottle.
I've never even heard about, let alone seen, a can of horse's piss. Didn't even think it might exist. I guess there's a market for just about anything. Where do you hangout?Who fills the can?Frankie
Flay your Suffolk bought-this-morning sole with organic hand-cracked pepper and blasted salt.
Thrill each side for four minutes at torchmark haut. Interrogate a lemon.
Embarrass any tough roots from the samphire. Then bamboozle till it's al dente with that certain je ne sais quoi.
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Your favorite brew is cold horse piss?
no vapor barrier the problem,
why not, vapor barrier is cheap insurance, no vapor barrier your problem now, how to fix, jackhammer, maybe french drain but more likely jack hammer
It's all about the RH. No matter if you have a vb or not. The humid air hitting the cold concrete slab? What happens in the after a warm stretch?
The shop where I do a lot of work has the same problem, and it has a well sealed vb under the slab. Any time it is humid the slab sweats - and this being the PNW that is most of the year. I wonder what sealing it would do?
I've seen this happen if there's been salt on the floor. It attracts water from the air like a magnet.
Mike Hennessy
Pittsburgh, PA
Everything fits, until you put glue on it.
Thanks for all the replies.I guess the consensus is somewhat as I was thinking. BB, I hear you on the VB, but it's not something I've ever put much importance on. The guy I use for my flatwork has a sterling rep; both for the appearance of his work and the longevity. When I've mentioned VB's to him, the reaction has been that he'd do it, but it tends to take forever to set up. I was working on a frame where he was doing the flatwork, homeowner running the job, used thick poly for VB, and even pouring the conc what I would consider very dry, it was still 6 hours before he could hit it with the FIRST pass, so I can see why it isn't his favorite thing. Maybe it's the conc we get, or maybe it's the humidity factor in setting up. I dunno. I actually figured I'd get crucified on the vb issue, glad that didn't happen. he thing is.....I never put em, and this is the first time I've seen this. I also think the fact that it's dry under stuff seems to indicate that it's not H2O coming up through the floor.I've thought about sealing or painting the floor, but I'm thinking that if it is temp/ humidity related, it won't really change anything, ( although I may do it anyhow, to make cleaning easier and to control dust)Someone asked what happens after the weather dries up; When the relative humidity goes down, it's dry as a bone. In fact, I have a big tarp on a frame that is on a pulley..hinged at the 10' point to act as a garage door til I get mine made...If that's closed, it will stay PRETTY dry til I open it, then within a couple hours, the floor is damp.The material underneath is silty hard pan that came out of the hole when dug...compacts well so I never gave it a whole lot of worry. I put about 6" of stone dust on top, compacted, and it's worked fine in regards to cracks, but you may have a point as to conduction of heat....Larger stone might have made a difference there.I figure that if it's the humidity thing, there are only two solutions, one of which it's too late for. If I'd known ahead of time and had put radiant in the floor, I guess if I ran water through at the same temp as the air, there wouldn't be any condensation problem....Too late for that. The other is to get the place sealed up, insulated, run a de-humidifier, and hope that after it's dried out, it doesn't have to run 24/7. Am I missing any other solutions? I'd hate to throw good $ on a quality de-humidifier if there's something dumb that I'm not thinking of because of tunnel vision.As for the horse piss, well, the beer tastes good out there whether it's damp or not. Sam Adams usually tastes better, tho...;)ThanksBing
Edited 6/25/2009 8:26 am ET by Bing187
but it tends to take forever to set up.I run into this everyday, it take so long to set up because he pouring wet, your concrete finisher is a hack, I see them everyday. It what keeps me in business. By not using a VB he has a scape goat, the soil will leets out the water and it will crack because of this. That why they say all slabs will crack, because they are hacks. They want them to crack so they quality be lower, dont have to work as hard.Fine homebuilding a term for craftmans. Fine concrete building, thats me. I hope to think I can be consider a craftman when it comes to concrete because its an art. the typiclal high school drop out drunk cannot do it.if you had a crew that was craftman in concrete they could name their price and have people waiting on them. The c rew that the HBA sees and try to train, I see everyday in concrete.
once concrete set up and cures, its going to maintain an moisture of around 4 % unless it has moisture from another source. Humity is not going to do it. dew point is not going to do it. standing water, moisture is an outside source.
Well.....I guess you're entitled to your opinion.I have a lot of confidence in my concrete guy, and that's why he's been doing my work for years. I have no problems with cracks in his concrete, and his work has made me look good a hundred times. You seem to be alone with your opinion about vapor barriers. Curious as to what your reason is to why the floor is dry under anything on the floor. I value your opinion as to the placement of concrete, I lurk on this site far more than I post, and generally agree with what you've said in the past, and have learned a thing or two from you. I would have a real problem with you nastily calling my guy a hack, except that I've seen his work after 20 years, I know his rep, and all you did by throwing that phrase at him is show your own ignorance. Maybe you're having a bad day, if so, so be it. As much as I'd like to take the highest road possible, I have to say that with your spelling and grammar, I laughed out loud at the "High School dropout drunk" comment.I can tell you which guy matches that description, and it's not MY concrete guy.Bing
Spelling?
Cause he pours more concrete a day than your guy probably pours in a year. The 500-750 yard pours brownbagg does have to be done right.
Personal insults are no longer allowed here, take them somewhere else.
If your guy is so good, why don't you ask him where he screwed up? He doesn't have an answer right?
Edited 6/27/2009 9:07 am ET by catfish
That is one where you are speaking from your experience in the south where the slabs just don't get cold enough to cause the condensation issue enough to be noticeable.you average AIR temp is higher than out average soil temperature
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Thanks for the input.I'm not trying to get in a whizzin contest here; I know BB is THE concrete guy. What I also know is that this guy takes great pride in his work, and goes out of his way to do nice work. I've never had a complaint about his work. I know precisely what you're talking about as to the chemistry of concrete, I know that it's a chemical reaction, and I know that the stiffer it's poured, the stronger and more crack resistant it is when done. I learned most of these things from him. Maybe it's been overlooked that I mentioned that I've had virtually no cracks in this guys work. He doesn't pour souped up concrete. As I said previously, he is THE guy to get around here. Done a ton of stamped work, exposed aggregate, etc. He's also done 700 yard pours on commercial work too, so he's no stranger to slump tests or the other standards in place. The Vapor Barrier is, imo, a locality thing. Ask Mike Smith if he uses one on all his cellars; maybe he does. It's not the norm tho. I came here to ask a simple question about what I thought was a condensation problem, got a dozen valuable responses, and one guy who goes off about my guy being a hack. Whatever. In addition, the evidence CLEARLY shows that the VB has NOTHING to do with the dampness. Funny he didn't respond with a reason for why, if it's moisture coming from below, there's no moisture under anything sitting on the floor. Must be something else my concrete guy did wrong........ To Catfish.....I find it ironic that you're breaking my nads about being insulting; all I did was question his grammar. He went off about how a guy with 30 years in the business and a sterling rep is a hack. I tend to defend guys I've worked closely with for a long time. To all who offered opinions, sincerely, thank you. I appreciate the input.Bing
I am not in with calling himn a hack, but I was just trying to explain some of the particulars. I don't think your particular problem is caused by lack of a VB.But your guy was not used to using a plastic, so when he did pour over one, he was caught waiting for evaporation because he had too much water in the mix. There is simple evidence of this in that he had to wait six hours to get on it. There is no way in any kind of weather he would need to wait that long with a proper mix.
Last one I poured was a 24x24 garage slab in cold weather just before Thanksgiving, over a VB. We were troweling it in two hours.
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I have to say that with your spelling and grammar, I laughed out loud at the "High School dropout drunk" comment.I can tell you which guy matches that description, and it's not MY concrete guy.
That was your statement to brownbagg. If you read the link, you know he's right about VB.
Right. I think maybe one guy in twenty doing concrete work has some basic understanding of the chemistry involved. The rest just know how to do it fast and easy and get out of there. Not interested in a good job.
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"In fact, I have a big tarp on a frame that is on a pulley..hinged at the 10' point to act as a garage door til I get mine made...If that's closed, it will stay PRETTY dry til I open it, then within a couple hours, the floor is damp."
thers the problem i think,before doing anything get a door up.i'll bet yourproblem goes away.warm air hitting cooler shaded concrete. when i was doing our house,it was standing open and one of the basement walls had so much moisture on it that it started to mold. i was freaked out. once it got enclosed and the temputure stabilized a little,never had a problem,YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'TMOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THEDUCT TAPE.
a hack : a person that in it only for the money, a person that doesnt care about quality or any type of quality control, a person that has done it wrong for so long, wont accept new idea or methods, a person that set in his ways, hardheaded, stubborn, old school. Doesnt not try or want to improve the craftmanship of their product.this is my opinion and only my opinion. I love hacks, without them it put me out of business.
http://www.architects.org/emplibrary/UnderSlabVaporBarrierPPP.pdf
"When I've mentioned VB's to him, the reaction has been that he'd do it, but it tends to take forever to set up. I was working on a frame where he was doing the flatwork, homeowner running the job, used thick poly for VB, and even pouring the conc what I would consider very dry, it was still 6 hours before he could hit it with the FIRST pass..."Sounds to me like he pours too wet. When the mix is right, it doesn't take any longer to kick because there isn't excess water to wait for bleed-off or evaporation. He gets away with that normally, but the VB exposes his sins to the naked light of day.too much water makes the pour go easier physically, but it causes far more shrinkage cracks in the finished product, and a lower cured strength.
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It's the humidity hitting cool crete and condensing.
You can test to prove it by taping a 2 foot square plastic down over a dry spot of crete for a few days until you get the moisture show up on the rest of the slab. Moisture on top of plastic - it is condensation. Moisture under the plastic and it is weeping up thru.
Fans can help cheaper than de-humidifiers.
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Taunton University of Knowledge FHB Campus at Breaktime.
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>It's the humidity hitting cool crete and condensing.<
used to work in a hangar that had 2 separate 100x200 slabs under one roof. Metal partition wall dividing the building by the slabs but with doors between the 2 hangars. One slab was slick as snot every spring as the temps changed and the other dry as a bone. The dividing line between wet & dry was along the expansion joint between the slabs, no dry spots on the wet one, no wet spots on the dry one. I thought condensation was the issue too but why did one slab condense and the other not? Had to be in the prep, either too little gravel or too litle VB.
Source of heat for one and not the other?
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both the same Gas blowers hanging from the ceiling but heat wasn't on at the problem times. Hangar doors open and closed several times/day meaning all new air but mostly open all day. Hangar halves are/were identical. Slabs acted completely different.
Edited 6/28/2009 9:59 am ET by john7g
Mystery to me.If Bing will do the test I mentioned with a square of plastic, he can confirm what his is.
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for me, ther only logical difference is lack of vapour barrier. maybe I'll get back there for something that will call for drilling through the slabs and then I'll know.
I will do the plastic test Paul...makes sense to me. Tho, if there's a cardboard box, a pile of scrap, even a car parked inside, any part of the floor covered by something is dry underneath. I'll do the plastic test, but i think I know the results already.As to the fans, I had thought about doing that, one blowing in, one blowing out at the other end type deal. My thinking was that it would help it evaporate, but I'm wondering how that'll happen if the air blowing across it is humid as outside......Would be cheaper to do that way to start out, tho. Have been planning a cupola with a copper roof and vented louvers in the big attic room upstairs; might mount a whole house fan in there, that would draw fresh air in through the door and expel through roof upstairs. Thinking that fans directly on the floor would keep it dryer tho, not sure why I think that...instinct I guess.I'm going to send a message to Mike Smith. I'm curious as to whether he uses a vb or not.ThxBing
Any input on this thread, Mike? You're right around the corner from me. Although being from RI, I know it's a far enough ride for a small suitcase and a hotel room...;)Bing
"any part of the floor covered by something is dry underneath"still sounds like condensation.On the fan idea - just keeping air moving is what you want to help things, nit one in and one out.I can't explain all the physics, but I know it helps, just like with windows in winter. Heck, for all I know somebody could make an arguement based on science that the fan can't help, but I know it does.
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bing.... the ground is infinitely wet.....
and where we build we have high watertables
we've been using 6 mil poly vapor barriers since the '70's...
the typical deal for us is to over dig the hole 12", spread 12" of 3/4 " crushed stone,
install a 4" drain system,install our vapor barrier, then pour the slab...
after the slab is finished, we cut our control joints
if it's an insulated slab, we place our insulation on top of the stone and then the VB
long ago , we'd have cement finishers poke holes in the vapor barrier... but most of them nowadays are familiar with pouring over vapor barriers
i can't control the humidity in my basements if i don't use vapor barriers
Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
According to Lstiburek, those holes they poked aren't a problem. The VB is 100% effective in the areas they didn't poke, so you still have 100% effectiveness in 99.9% of your area. Not bad... better than the condoms they pass out on college campuses.
IMO... water reducer... man's best friend.
I hear ya on the humidity, and frankly, between Brownbagg and other comments I've read....I'm man enough to say that I think in the future I'll start asking for one, and keeping an eye on the wetness of the pour, though I still think my guy is A-1. It seems that technology says it makes sense.....I'll try to use this experience to grow as a builder...:)Do you go VB under garage floors, too? Insulation? (Aside from instances of radiant heat) I'm thinking that's something else that might have helped in my shop; insulation would've kept the floor more ambient temp than ground temp.Thanks for your input.Bing
I'm thinking the fan works primarily by helping to bring the slab temp up closer to ambient temperature, which hopefully is higher than the dewpoint. Depends on how fast you can warm up the slab by moving ambient air over it vs. how quickly the ground can suck it away.
Interesting related example: This afternoon I was over at my father's place. He's got a 36x48 pole barn workshop we built almost 30 years ago. Half was closed in with overhead doors and a slab shortly after it was built - no VB or insulation. The other half just got closed in last year. I convinced him to put some insulation underneath - he went with 1" EPS, don't think he used a VB. Ground water level here is pretty close to the surface.
Both sides have been open for the last couple of rainy, humid days. The old side with no insulation is damp and slippery. The other side with a little slab insulation is dry.
Other related scenario. Same place, the old farmhouse is 2 stories with attic storage, full fieldstone cellar/2-car garage.
On hot days, a big attic fan will pull air through the garage, then the cellar, up through the house & exhaust out the attic. Cools the house nicely, but the garage & cellar floors & walls are dripping wet.
Don