# condensation under plastic sheet in finished basement



## Mike_TV (Nov 25, 2015)

Hi all, first time home buyer here in need of some advice. My wife and I just bought a townhouse, originally built in the early 1900s, with a finished basement. Every time we visited the basement has been dry without any hint of a musty odor. It's divided into two parts: a water-sealed storage area in the back and a drywalled portion in the front that I was hoping to turn into a game room.The former owners disclosed that there was a bit of leakage in the back, but that was fixed by adjusting the drainage spout and they fixed our gutter concerns before closing.

We've been painting some rooms before moving in and last night we noticed that there was condensation under the plastic sheeting in the basement. The sheet has been there since Saturday and carpet was wet enough that there was some water on my hand after pressing down on the carpet. It's a berber carpet and feels like it's laid directly on the slab. We weren't sure what to do, so we opened the door between the front and back of the basement where the humidifier is and turn on the fan in the house's HVAC in hopes that it'd dry out over night.

So here's my question: where do people think the water is coming from? The drywall of the basement is not completely flush with floor, could vapor be coming out from there and getting trapped under the sheet? Could there be a leak in the slab? Would resealing the slab do us an any good at this? Do the former owners have any liability here? TBH I'm freaking out a little, one of the main reasons we chose this place is because the basement seemed really dry.


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## oldognewtrick (Nov 25, 2015)

to House Repair Talk!

Have to had any rain lately?


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## Mike_TV (Nov 25, 2015)

Ah, I should mention the moisture was mainly along the sides of the walls that faced the neighboring houses, and not the front of the back that faces the street and backyard, respectively.


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## Mike_TV (Nov 25, 2015)

It rained Monday, I believe, but not terribly hard.


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## bud16415 (Nov 25, 2015)

I&#8217;m not a fan of any permeant carpet in basements. I know people do it all the time and many don&#8217;t have any issues but I personally think there is always some moisture going on with concrete basements. It can seem very dry and that&#8217;s because normal air flow is taking away anything that&#8217;s working out of the walls and floors/ when you put plastic down things can&#8217;t breathe and moisture builds up. I would rather see tile or a stained floor and area carpets where needed. 

That&#8217;s just my two cents and I am by no means a flooring expert. There are some experts that post regularly and they should be along to comment.  

Welcome to the forum.


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## nealtw (Nov 25, 2015)

If the carpet does not dry quickly you want to roll it back and get it to dry or you wil have mold problems.
You do have a moisture problem, the question will be, is it bad enough that you will have to fix it. or will normal air flow keep it at bay.


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## Mike_TV (Nov 25, 2015)

Just got back from the new place (it's only a couple of blocks away), and the basement carpet is bone dry now. Some friends suggested it was just the cement sweating and that we can try having the sump pumps go a little lower, or just to plan around it and use elevated shelving. Thoughts?


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## nealtw (Nov 25, 2015)

If the dainage on the outside is working, there would be no water to wick up thru the floor.
Plastic on the slab is a test for moisture, so your test failed.
There are things we don't know.
Is the foundation properly waterproofed.
Was there ever a perimeter drain and is it working
How were the walls constructed, is there wood in contact with concrete.
Are the walls insulated to prevent heat from getting to the concrete wall, that could cause wicking
What conditionthe walls were in before painting, any signs of mold?
How will you be heating this space?


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## Sparky617 (Nov 25, 2015)

Given the age of the house (115 years old give or take) you can be fairly certain there isn't plastic under the slab and the walls could very well be rubble stone construction.

That the carpet dries out when not covered with plastic indicates that the HVAC and/or dehumidifer can keep the moisture in check.  I wouldn't be a fan of wall to wall in this space either, nor would I go with any type of laminate or wood flooring.  Is there a sump pump in this basement?  Having one with a perimeter drain would keep the ground water lower than the slab most of the time, but you can still have water vapor working its way up through the floor, since it likely doesn't have a vapor barrier under it.

Have you checked with your new neighbors about what is on their side of the basement walls?  Are their basements finished?  It is kind of strange that the wet spots are near the common walls and not the exterior walls, because you would assume their basements are fairly dry as well and any water issues would start on the exterior walls and work in.   At some point, I'd be interested in knowing what is going on behind the drywall.  Is it damp, moldy or dry?  I'd want to look mainly at the exterior walls but might be tempted to look at the common walls as well given the wet carpet.


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## Mike_TV (Nov 25, 2015)

Unfortunately I don't have the answer to many of those. The house was fully renovated back in 2012 and I can't tell you  about the foundation, perimeter drain. I think there might be wood in contact with the concrete but it's mostly brick. This is a row home in major city with a small back yard with a facade that's directly on the sidewalk. Also, I don't know if the walls are insulated to prevent heat from getting to concrete. I can tell you, however, there were no signs of mold or musty smell in any part of the house, and I'm pretty sensitive to that sort of thing. Heating will be done via forced air.


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## bud16415 (Nov 25, 2015)

I would say it got wet once it will get wet again. It could well be sweating but you mentioned it was mainly on one side facing the neighbor house. I would think sweating would be more even. You opened up the area and turned on the de-humidifier I think you meant to say. So overnight things dried out. Is it still wet under the plastic? Mold likes areas that are wet then dry then wet. Get to the bottom of the dampness problem before you go too far with the remodel project would be my advice.


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## Mike_TV (Nov 25, 2015)

So this isn't a remodel project; the house has already been remodeled and we're just painting and moving in. In general, the house is in really good shape and received an excellent appraisal from our inspector (he's independent and trusted). This is the first time I've seen any moisture in the basement, and we have two sump pumps in the back dealing with any water that may come in. I don't know the state of the basements next door, but I suppose that's something I can ask about when we meet the neighbors. 

A friend suggested that the water could be more along the walls because that's where the french drain is and is transporting water to the back of the house from the front to get to the sump pumps. Does that sound right? Either way, I think I'm going to do an experiment where I seal up a space near the wall and seal up a space in the center of the carpet and see what happens moisture-wise.


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## bud16415 (Nov 25, 2015)

I think I totally misread your post I&#8217;m sorry. You had carpet down and then put plastic over the carpet to protect it from the painting you were doing and overnight water collected on the plastic on top of the carpet. The next day you removed the plastic and the carpet dried on its own with the help of air movement and a de-humidifier. 

Do you know what&#8217;s under the carpet? Is there a pad? Is the carpet glued down or just resting on the basement floor? Any plastic under the carpet?

When you look in the sumps is there water in them? How far is it from the floor level? Do you hear then run? Is water coming into them?


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## Mike_TV (Nov 25, 2015)

So here's what it looks like when I pull the carpet. It's not glued to the floor, just placed on top and tucked under the molding. As you can see it's dry and clear of mold, but there is one part where if you place your hand it's cool. Not exactly damp, but cool. Also, I checked both sump pumps and they seem like they're set at decent levels (about 6-8" below floor level) and working.


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## Mike_TV (Nov 25, 2015)

Another question: should I consider running a line of caulk between the baseboard and the floor to seal everything up?


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## Sparky617 (Nov 25, 2015)

Mike_TV said:


> Another question: should I consider running a line of caulk between the baseboard and the floor to seal everything up?



I wouldn't.  Caulking in this situation would be for air infiltration not water. You'd want to know if you have water coming in sooner rather than later.

As long as you can pull the carpet back you could try doing a dampness test along the walls.  Tape a piece of plastic sheeting sealing all four edges.  If water collects between the plastic and the concrete you have water vapor coming in through the slab.  IF it is just water vapor a dehumidifier should keep it in check. Though I'd probably still opt for something other than wall to wall carpeting, laminate or wood flooring.  You'd want something to allow the floor to breathe into the room and let the dehumidifier pull it out of the air.  Area rugs without a rubber backing would be best.


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## nealtw (Nov 25, 2015)

Is the common wall foundation or just a stud wall, perhaps they had a flood problem next door. Go meet the people next door.


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## vinny186 (May 1, 2016)

I have a similar problem in my basement. I intended to lay down carpet but first taped down a plastic, dimpled subfloor to create an air-break between the cold concrete and your feet. I came back to the project several months later to find lots of mold and water under the dimpled floor. However, for some reason this was only happening in half of the basement area. Next thing I did was jackhammer the wet and dry areas of the basement to see what was going on; turns out the useless prikks who laid the floor only used a vapor barrier under half the floor.

My workaround for this was to install a radon fan to get airflow under the slab. What I've noticed so far is I still get moisture accumulating under the dimpled floor but a temporary piece of backer free carpeting I laid down next to the dimpled floor is dry underneath.

Perhaps you could use outdoor rated carpet squares instead of a traditional carpet.


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## Rusty (May 1, 2016)

I have seen carpet last for 20 years in a basement. Sounds like the floor is sweating due to temperature change or temperature difference between the top and bottom of the slab. I personally would not worry about it. The water vapor will go through the carpet and be taken out by the dehumidifier.


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