Condensation between original and addition concrete slabs

Old 02-16-14, 09:54 AM
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Exclamation Condensation between original and addition concrete slabs


I would like to know if there is a way to block or stop condensation between two concrete slabs in my home.

The previous owner added a room to the house. What I have now is the original concrete slab, what appears to be a 2X4 piece of wood, and the new concrete slab for the addition.

The original floor has radiant hot water heat in it. The previous owner told me that he extended the radiant heat in to the new room when he built it on, but the floor of the addition is only warm within ~2 feet of the original room. The addition's floor, and the room in general, is much colder otherwise.

The original floor has padding and carpet on top of the slab. The addition room has a green rubbery adhesive and tongue/groove hardwood flooring on top of the slab. There is a wooden threshold that matches the hardwood floor spanning the 2 x 4 between the rooms, that sits atop the hardwood and has the carpet butted up against it. There was nothing between the 2x4 and the threshold as far as barrier, the threshold was simply glued down with some type of construction adhesive.

I noticed the wood floor darkening near the threshhold, and so pulled up the threshhold and found it wet on the bottom. The 2 x 4 was also wet and beginning to rot.

I first suspected a leak where the previous owner attempted to extend the radiant heat, thinking this would also explain why the addition room remained cold. But, since having the threshold removed, the 2 x 4 has dried even though the heat has been running as normal.

Can the wetness be due to condensation beneath the threshold between the 2 x 4 and the concrete slabs? The slab of the addition is pretty cold, and the slab of the original house is pretty warm.

If condensation is likely, is there a way to prevent it, or to protect the wooden floor and threshold?

I attempted to provide an illustration in the attachment for reference. I appreciate any help or suggestions.


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Old 02-16-14, 12:54 PM
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Do you have any idea about the construction of the concrete floor in the addition?

I might suspect that there is no vapor retarder under the slab and the green coating on top of the slab is a vapor retarder. If that is the case then the heat in the slab of the original house could be pulling moisture from under the addition to the junction of the two slabs at the threshold where it can migrate through that 2x4 and up to the threshold. The threshold may be cold enough to be at a dew point if you have enough cold migrating from the outer edges of the slab in the vicinity of the threshold.

Is the evidence of condensation occurring evenly along the back of the threshold or only at specific spots?
Old 02-16-14, 04:18 PM
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I have no details about the addition's construction, unfortunately. I believe you are correct that the green coating on top of the slab is a combined adhesive and vapor barrier however.

The condensation appears to occur only at the middle of the threshhold and again to one side. The threshold is L-shaped, a little deeper on carpet side than on the wood floor side. The thicker piece on the carpet side rested directly on the 2x4. The thinner side sat atop the wood floor. Also, my drawing is not entirely accurate, as the last two planks of the wood floor also sat on top of the 2x4. So, in other words, there was some of the green adhesive on the 2x4 to hold the last two planks of flooring. The construction adhesive that held the threshold down was applied only to the thin part, on top of the wood floor. There appears to have been nothing between the threshold and 2 x 4.

I edited the drawing to be a little more accurate. I also included a few pictures. The pictures show the threshold flipped upside down. The black / dark areas are where the condensation was when I removed it. I have arrows point to that, and also the areas on the 2x4 that were wet.

Thanks for taking a look.

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