Thoughts on slab moisture issue

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I have a new house with walk out basement. 1/2 the basement is unfinished (the part that is fully underground) and this is my mechanical room it is roughly 15' by 60' with a sump pump near the center of the long side, and against the framed bearing wall (so 15' off the exterior wall). There is double drain tile (corrugated) around the perimeter inside and out, and long the center footer. The drain tile is also connected to daylight drain as a backup. 2/3 of this area has been painted with Dryloc latex concrete paint from Lowes - just as a clean storage area surface. We moved in October 1 of last year.

The sump pit never gets water from the drain tile, but will get ground water through weep holes in the bottom after a very have rain.

The problem observed was after days of rain, and the sump pit will fill high enough to kick on the pump - it would run once an hour or so, until the level dropped below the float, and then water stayed in the pit for a few days.

Anyway, the issue was moisture spots on the floor. I am mostly sure it is not condensation and most spots leave behind white residue. The spots are only appearing in the painted area of the floor, but not 100% of it - but it does seem to coincide with the areas outside the house that are landscaped - the areas with no water spots, get closer to the garage on the outside wall.

I am 99% convinced it is a moisture issue from below, but there is a vapor barrier under the concrete, we have gutters and downspouts with black pipe carrying the water away (50' or more). All grading against the house flows away, but the house is lower than the street, and the front yard does slope towad the house.

I verified the vapor barrier by drilling a 3" hole, and I filled up the drain tile via the sump pit and observed water flowing out of the daylight drain. There is no leaking in the basement - cold joints between wall and floor are dry. All cut control joints are dry, hole that I drilled is dry.

So, could the slab still be sweating from moisture below under these conditions? If so, is there anything I can do about it? All I can think of is some kind of pit near the foundation, lower then the water level observed in the sump pit and then sloped trench from there - all filled with gravel and then covered over to relieve any sub surface water?

Just looking for ideas at this point - TIA

 

[Shea]

Hmm... there may be confusion of terminology. A sweating slab is essentially caused by condensation at the surface - not moisture from below. When relatively warmer air that contains moisture comes in contact with a surface (concrete floor), and that surface temperature is below the dew point, condensation occurs. That's what looks like is occurring in the image.

I can see boxes on the floor in the upper right of the image. If you remove those boxes, is the floor wet or dry underneath? If it's dry, then you have condensation, AKA sweating slab. If they are wet underneath, then you have moisture from below.

 

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thanks for the reply - yes we had moisture under boxes and plastic tubs so that and the efflorescence (white spots after water evaporates) make me think this liquid is coming from below. Also, using a laser thermometer showed the slab at the same temp as the copper water pipe and lower walls - yet no moisture on any place other than slab. Also, it appears to be in only one end of this unfinished space, which also makes me think it is not condensation.

I have taped plastic down but my test has been inconclusive - most of the moisture pictured has no recurred. But we also have not had that level of water in the sump for such a long duration as we did when this event happened.

I guess I am surprised that with a thick poly vapor barrier, I saw that much vapor move through the slab, but I have confirmed the water level is coming up from the bottom - the sump is filling from below, and the water is not high enough to ever get into the drain tile around the footer

 

[ConCretin]

To add to what Shea said, moisture from below a slab doesn't typically appear as liquid moisture on the surface. It will condense on a plastic sheet covering the floor but usually dissipates too quickly for the surface to appear wet.

If it is water vapor from below, your vapor barrier has failed. Unfortunately, it's pretty common for poly vapor barriers to be damaged during construction or installed without taping the seams. This might explain what you are experiencing.

I'll leave it to those more knowledgable than I am to advise you but they do make surface applied chemical vapor barriers that can block moisture from below.

 

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@LLWillysfan I agree with all observations - I can't seem to reconcile this as below slab because of how much I saw (not so much puddles as the picture implies, but many damp spots). Even with a vapor barrier failure I was surprised to see actual surface moisture. I thought i ruled out condensation due to it only on the slab, below boxes, and not on any walls.

Still puzzling to me on what next steps should be. I am going to continue to solicit opinions and monitor the situation. Hopefully i can figure it out when it happens again.

 

[rdenney]

Get a moisture content reader, and read the moisture content in the slab. But it seems to me you have two vapor barriers--one below the slab and the paint on the top of the slab, which would indicate to me that it is a condensation issue despite your other observations. Also, an open sump pit keeps the air in a basement moist. Have you tried to provide a good seal around the sump pit lid? Those that I've seen are just pieces of plywood (if even that) thrown on the top of the hole, which a slot cut in from the side for the sump drain pipe.

Also, make sure the sump drain is really draining away from the house. A new house I moved into plumbed the drain to the uphill side of the house and dumped it on the ground, where it drained right back into the sump. I routed it to the downhill side of the house and my sump pump ran only a quarter of the time it had been running.

But I will say this: I never had a wet floor in a basement that had a radon ventilation system. That pulls a vacuum on the gravel bed below the moisture barrier, and in my experience reduces moisture content in the slab no matter what the state of the vapor barrier.

Concrete is porous unless it's treated to be waterproof, and if it stays at a high moisture content from the humid air in the basement, it won't take much for it to sweat.

Rick "wet basements are no fun" Denney

 

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I did put a lid on the sump pit soon after i moved in due to a damp smell - it is a radon mitigation solution and seals well. Also, I have confirmed the sump pump dumps to a drain run from the house and is working. The sump pump rarely runs and when it does, water is coming up from below, not from the drain tile - so water level below the house has to rise 6" into the bottom of the sump (24" below the slab) - that is a lot of ground water.

Yes, I still have not ruled out condensation as we had the door open in the spring and the floor was cold, but I still am confused on 1. why we didn't condensation in other areas of the basement and on walls. 2. what the white residue is left after evaporation.

I did purchase a calcium chloride test kit - trying that on top of the slab (and on top of the paint) - if the moisture is migrating from below, this should give me some indication.

I also have a dehumidifier running in the unfinished space - ever since this incident happened. I still find occasional small damp spots on the concrete (dime or pea size) under things, or in the open - so more reason I think it is not humidity. I have many different measures of humidity so I guess the average is roughly 50-60%. The slab is 64-66 degrees F. Our AC doesn't run that often as we keep the house at 78 and did Foam insulation.

 

[mcbane]

I agree with rdenney that the slab may not yet have dried out. If it went in last October and you have had high humidity and relatively cool temps in the garage since then, the slab would still be holding some moisture. This is why new concrete still forms new shrinkage cracks even a year after it is placed. When the air is dry enough, moisture coming out of a slab evaporates as quickly as it appears so the slab appears to be dry. But after a few days of rain and potentially higher humidity, moisture wicking to the surface of the slab wouldnt evaporate as well and you would be looking at wet spots.

 

[Magnify]

We moved in last October, but the slab was poured in January before that. I just did a calcium chloride test, on top of the painted concrete and I got back 3.5 lbs per 1000 sf. So not really high. If both under slab vapor barrier and paint were failing I would expect higher.

I think I have to assume condensation at this point. We had the house open, and the furnace fan runs 15 minutes per hour. I am thinking that we pulled in ultra humid air - we have a screened in porch with French doors - and had the doors open a lot, heat off no ac either. The basement has always been colder than the main floor by a lot.

I can’t explain why I find condensation under items, and not on walls, but taping down plastic has not indicated any moisture underneath.

Still not 100% sure on anything though.

 

[CombatNinja]

Just get a good capacity dehumidifier and set that thing to run continuous during a nice rain and see if you have the same issue. I'm willing to bet once you dry the air out, your problem will 'evaporate'. I have a 50-pint Hisense that my wife uses in her musty classroom during the school year and I run in the garage when she is off for the summer. It can rip my 500 sq. foot garage down from 70% relative humidity to 40% in about 30 minutes.

 

[rancherbill]

I have a new house with walk out basement. ..... We moved in October 1 of last year.

That should be covered under your New Home Warranty.

It always help when you list where you live, although sometimes it's a National Secret.

 

[Magnify]

Just get a good capacity dehumidifier and set that thing to run continuous during a nice rain and see if you have the same issue. I'm willing to bet once you dry the air out, your problem will 'evaporate'. I have a 50-pint Hisense that my wife uses in her musty classroom during the school year and I run in the garage when she is off for the summer. It can rip my 500 sq. foot garage down from 70% relative humidity to 40% in about 30 minutes.

I am in Dayton Ohio area - summer humidity is actually very low (compared to east coast), and basement slab has warmed up. I have a dehumidifier running now - entire house is down in the 40-50% range and daytime humidity outside tends to run the same or lower.

Even with basement down to 40%, I have found some moist spots under things (I think the condensation got trapped there) - but I am fairly certain that opening our house in the Spring when the air was really wet for a few days, and the basement slab was very cold was the cause. We have a screened in porch with french doors on 2 sides that open to the house - and I have the furnace fan set to run 15 minutes every hour - so lots of moist air pulled into the house when this problem got noticed.

Unfortunately we haven't had a week's worth of rain like we did in the Spring so I am in wait and see mode.

And my builder would be happy to warranty the problem if we can identify it. Now i Just need to figure out a way to make sure that I don't contribute to this again in the Spring - assuming I am correct and it is humidity.

 

[CombatNinja]

Anything propane/natural gas in that basement (water heater, dryer)?