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Old 09-29-2010, 09:49 AM   #1
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Should I seal the perimeter gap of my floating slab?


I have a two-story colonial with a full-basement in upstate NY. The basement has cinderblock walls and the basement floor is a poured concrete slab. The basement walls also have some sort of stucco/cement parging applied.

I have a 1/2" to 1" gap between the basement walls and the basement floor.

Based on the question asked in this article:
http://www.stopmold.com/don_t_finish_your_basement.php

I'm fairly certain I have a floating slab vs. an interior French drain. I have a sump pit--no sump pump, but the sewer clean-outs and incoming water line are in the pit too. The previous owner had run the AC condensate into the pit too, but I've hooked the AC up to a condensate pump, and then to the outside.

In the three years that we've lived in the house, we've had a persistent mold odor in the basement. I've tried dehumidifiers to knock the moisture level down, and I've got it down the 20% range, but the odor remains.

A few months ago, I tried an interesting experiment. I went to Home Depot and got a garden sprayer and two gallons of silver-based anti-fungicide. I sprayed the antifungal into the perimeter and the results were amazing. For a few hours, the air was completely odor-free.

I've never had any water in the sump pit or flooding in the basement.

I'm considering sealing the perimeter gap to stop the mold from entering the basement.

Does anyone have any thoughts or experience with this?

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Old 09-30-2010, 01:33 AM   #2
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Should I seal the perimeter gap of my floating slab?


I see no advantage to a large crack, and I would sure seal it. I'd pour a healthy dose of 10% bleach down the crack, let it sit several days to dry, then seal it. You may have no vapor barrier under your slab, which would then be wicking moisture. THAT is a big problem. Seal the crack first, and see what goes.

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Old 09-30-2010, 10:48 AM   #3
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Should I seal the perimeter gap of my floating slab?


Concrete is porous and allows moisture through it. Mabye some paint will slow this down.
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Old 09-30-2010, 11:06 AM   #4
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Should I seal the perimeter gap of my floating slab?


You undoubtedly have a floating slab.

The gap dimensions you mentioned are far to great to be due to the concrete shrinkage. Does the gap go the full depth of the slab thickness? Does is appear to have clean edges and is it formed?

At one time in much of the east, the gap was formed and even used as a sales gimmick to collect water that had already leaked into the basement envelope and collect it to keep the floor dry. Often it was directed to a sump or a floor drain. It was really ineffective and a poor substitute for building a dry basement, even though the the extra cost during construction to build a dry basement was extremely low. Some codes now require the fllor slab to have the 3 5/8" of the slab cast against the wall.

Filling the joint with a cement based material will result is a small crack later on that may not be a problem. If you feel you must fill the formed void, get a foam backer (rod foam in various diameters) and force it into the void so that the top of the rod is 1/2 the crack width below the slab surface. Then caulk with a quality flexible caulk designed for use in concrete control joints.

Dick
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Old 10-01-2010, 07:22 AM   #5
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Should I seal the perimeter gap of my floating slab?


dick's right not that he needs any validation from me it was called a ' slot drain ' & promoted by many bldrs ( correctly or not ) as dick posted,,, in theory, it worked fine til theory ran into practice it filled up w/dirt, etc, & became a breeding place for insects, mold, mildew, etc.

i'd just stuff closed cell backer rod into it but forget the caulk unless appearance is a concern,,, apron stores won't necessarily have the diameter rod you need ( 25% larger than the space ) so find a const supply house.

filling that space w/cementitious mtl will result in a smaller shrinkage crk & many other crks as the filler cures - not my choice, either.
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Old 01-19-2011, 03:02 PM   #6
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Should I seal the perimeter gap of my floating slab?


I just came across this post; I have the samething (1" wide and 4" deep/ht of slab - a Floor/Wall joint - they call it cove joint) in my basement that builder put it. It's allover the whole perimeter.

I was going to finish my basement but by-chance I found that there was some dampness (spread across 12-14") at the base of joint - rest of the joints all around the perimeter of my basement is dry. I am not sure if it should be concern as it's only little (no standing water) bit of dampness at the bottom of the joint/gap - not touching any wood or framing but just concrete around it;
Question - Is it just humidity in that corner (~1 foot) or water seeping through under the slab (possible high water table there). Can I just go ahead put drywall on the frame (already there - 4" away from basement wall) without worrying about any condensation in the particular small-area or should I pour some liquid hydraulic concrete of small layer (0.5 cm) to seal any small holes/porus or air gaps to joint to clay under the slab. I could put foam-backer and seal with caulking but will it stay in place and wouldn't water vapours/condensation will stop wicking any water from underneath the slab. I was reading about Xypex high’n dry concrete waterproofing crystallization product?? Any comments will be appreciated. Thanks.

Last edited by Dera_Mik; 01-19-2011 at 03:13 PM.
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Old 01-19-2011, 03:13 PM   #7
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Should I seal the perimeter gap of my floating slab?


Any advise to above questions are much appreciated in advance. Thanks

Last edited by Dera_Mik; 01-19-2011 at 03:16 PM.
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Old 08-30-2011, 07:18 PM   #8
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Should I seal the perimeter gap of my floating slab?


Quote:
Originally Posted by toddmanqa View Post
I have a two-story colonial with a full-basement in upstate NY. The basement has cinderblock walls and the basement floor is a poured concrete slab. The basement walls also have some sort of stucco/cement parging applied.

I have a 1/2" to 1" gap between the basement walls and the basement floor.

Based on the question asked in this article:
http://www.stopmold.com/don_t_finish_your_basement.php

I'm fairly certain I have a floating slab vs. an interior French drain. I have a sump pit--no sump pump, but the sewer clean-outs and incoming water line are in the pit too. The previous owner had run the AC condensate into the pit too, but I've hooked the AC up to a condensate pump, and then to the outside.

In the three years that we've lived in the house, we've had a persistent mold odor in the basement. I've tried dehumidifiers to knock the moisture level down, and I've got it down the 20% range, but the odor remains.

A few months ago, I tried an interesting experiment. I went to Home Depot and got a garden sprayer and two gallons of silver-based anti-fungicide. I sprayed the antifungal into the perimeter and the results were amazing. For a few hours, the air was completely odor-free.

I've never had any water in the sump pit or flooding in the basement.

I'm considering sealing the perimeter gap to stop the mold from entering the basement.

Does anyone have any thoughts or experience with this?
Hello, I know that you posted this awhile ago, but I am having a similar situation and need some help. Our basement floor is a floating slab. We also have a sump with a pump. Never have had any problems with visable moisture or flooding, but we just finished our basement. Installed new carpet and put up new baseboards. It went through the winter and spring last year and now we have a 1 to 2 inch gap between the bottom of the baseboard and the carpet. Also have that musty smell. Ahh.... Any suggestions on what we should do it now to fix it?
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Old 08-30-2011, 11:57 PM   #9
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Should I seal the perimeter gap of my floating slab?


who is ' you ' ? zypex may well work

IF its a builder-installed sump system, chances are good its not filtered w/soil cloth,,, if i'm reading the ' gap ' thing right, could shrinkage be from green wood ? the only ' fix ' i know is rip it out & do it right but you probably already knew that
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Old 09-01-2011, 02:39 PM   #10
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Should I seal the perimeter gap of my floating slab?


It is common practice to leave a space between a floating slab and a block wall, Here (ky) it is called an expansion joint and concrete materials sells a mesh type expansion joint product especially for that purpose. It is spray glued to the block wall before the floor is poured and then the floor is poured to it. The purpose of the expansion joint is to allow a space between the concrete floor and block wall so that when the concrete floor expands and contracts, as concrete does, it does not expand and push your block off your footer. Over time the mesh will rot away and just leave the 1/2" or so gap and people are confused about why it is there.
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Old 09-01-2011, 04:08 PM   #11
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Should I seal the perimeter gap of my floating slab?


guest -

A concrete slab on grade does not expand, but has a long term shrinkage that essentially stops after a couple of months. In some situations, there MIGHT be a very small short term thermal expansion IF the slab and soil got heated to over 150F or so.

Some codes even require the slab to be poured with a 3-1/2" contact height with the floor to give additional support to the wall to resist the lateral soil loads in a basement. It is a method to use the slab as a diaphragm. Normally when the slab is poured against the basement wall there gets to be a 1/16" to 1/8" gap due to the initial curing shrinkage, but still can provide lateral resistance if there is any movement. A lazy contractor using a wet mix will be on the high end of that range and good, low slump concrete will have less shrinkage.

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Old 09-01-2011, 04:19 PM   #12
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Should I seal the perimeter gap of my floating slab?


guest, this is a great answer & definition as many of us define 'expansion joint' differently,,, in 39yrs, i've never seen any bsmt slab 'expand and push your block off your footer.' conc does expand/contract in direct relation to temp changes( heat expands conc & cold contracts it [remember high school physics ?] however, most pro's agree expansion jnts aren't used in bsmts where temps remain fairly constant - its more likely a bsmt floor would contain contraction jnts while expansion jnts are to separate slabs from structures,,, then again, i've never placed conc in ky,,, the ONLY time we've seen cont expand to do any damage is on conc hgwys w/100' slabs - a method never used anymore

Last edited by stadry; 09-01-2011 at 04:43 PM.
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Old 09-01-2011, 07:16 PM   #13
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Should I seal the perimeter gap of my floating slab?


They have been doing it that way in ky for decades.
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Old 09-01-2011, 07:56 PM   #14
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Should I seal the perimeter gap of my floating slab?


KY is just one place and not necessarily applicable in other situations or places.

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Old 09-01-2011, 08:23 PM   #15
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Should I seal the perimeter gap of my floating slab?


True, we get all kinds of weather here and never know what we will get. I just know they do this here and I had to clean mine out with a putty knife and shop vac to get rid of the rotting expansion material which was feeding the mold. The expansion material had served its purpose to create the space between the floor and wall, after that it starts to rot over time and feeds the mold, I guess dirt and debri in the gap will also feed mold given the dampness of basements.

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