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Discussion Starter · #1 ·
I am bidding on finishing out a new basement with a prefab house above. There is to be a stud wall built around the entire exterior perimeter of the basement with bedrooms and a bath to be framed in also. My building materials sales man is telling me that I don't need to waste my time and money on putting a vapor barrier between the concrete walls and the stud walls with bat fiberglass insulation. His theory is to set the wall a half inch off the concrete wall and the paper backing on the insulation will act like the vapor barrier. But if the paper backing is not 100% sealed not just stapled in place, then the vapor barrier wouldn't be 100%either. To me that sounds like a disaster waiting to happen. I was thinking that there needed to be a vapor barrier in between the concrete wall and the stud wall, thus not allowing condensation to come in contact with the insulation and causing a mold issue. Either a poly film, a concrete sealer applied to the walls, or an asphalt impregnated product that is ran from the top of the concrete wall, down to the floor and under the bottom plate to help protect it. Also to protect the bottom plate one could use sill sealer under it to allow air and vapor flow? To seal the rim joist use spray foam on the rim and sill plate and rim and subfloor joints and then place faced fiberglass insulation into the joist opening? Any help would be appreciated, thanks
 

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Love me some Concrete
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I have done a bunch of research on this, it really is 50/50 on which way is best. They have found mold behind the vapor barrier on walls with it installed, I have seen recommendations on no vapor barrier at all.

On the other hand, ALOT of builders put a vapor barrier in and some codes call for it. The problem is concrete absorbs water in some way, if there is no way for it to escape, it can be a problem.

It has also be found that the insulation with no vapor barrier has been found to wick up the moisture and causing mold in the walls. Houses from back in the day didn't have many mold issues, but on a wind day you could feel the breeze inside, lol.

So when you find the correct answer, I would like to know.:thumbsup:
 

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seal the block with a water proofing paint and call it good that is what i have done for years. when i have went back to do more work on the homes i have not found mold :thumbup:
 

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Sean
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North West Kansas right? aka Climate Zone 5

Poly is a bad idea as it will trap any moisture coming in, same with fiberglass in contact with it.

Yes you should keep the studs off the concrete not only for moisture reasons but how many basement walls are perfectly flat? You should be using a pressure treated bottom plate & you could even wrap the bottom if you so choose to create a capillary break

As for condensation forming against the wall - the primary cause is from outside & if you have that issue you need to fix it first. If you are worried about diffusion (the reason people went with plastic) I would stop worrying. Just control the moisture levels in the basement & house. The only other reason that maybe an issue is due to air leakage so make sure you take care of that when you are doing your building.

Just as a simple rule, if moisture is a concern, you should only use closed cell foam if it is going to be in contact with the walls
 

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I had the same issue on a basement year ago. Vapor barrier was between studs and concrete which after researching found out is a big no no. At least it has to be on the conditioned side of the stud. Between the drywall and stud. I ended up going a whole different root though. I used 2" styrofoam on the wall to vapor and insulate and then unfaced batts in the framing. Next time I'm gonna give the batts a miss and go with thicker styrofoam.
 

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Discussion Starter · #6 · (Edited)
Thanks for the replies guys. Obviously a poly vapor barrier is a big no no, and after posting was reading about it on buildingscience.com. I will lay some options out and see what the home owner wants to try. Might look into the closed cell foam.
 
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