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   I have a project on a small house built early 30's, two floors and basement and very leaky. Basement is dry and is part of the conditioned space from the beginning, with wood paneling, carpet and a fireplace. Since the new owners would like to use the basement about 10 hours a day instead of the possibly 3 hours a day on a space originally designed as a family room, I was thinking to recommend insulating the concrete floor as well with Poliiso rigid foam that already comes with the plywood attached to the surface of the insulation. Even Though the basement is dry, the humidity level is high and I guess it has not been an issue because the house is leaky.. so my question is can a 6mil vapor barrier be applied before the insulation? but then, what would happen with any possible humidity build up between the insulation and the vapor barrier? Would sealing the concrete floor be an alternative instead of the vapor barrier?   

   Your help is greatly appreciated, 


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The foam is a barrier & besides in all cases - no plastic sheets above concrete. If you are really worried about water you would use a product like dricore.

As for high humidity - you might want to try to find out why - dry basements should not have high humidity levels. Old floor layers trick, tape down a piece of plastic - if condensation forms the area is to wet (aka a quick & easy way to find leaky areas) 

I don't know anything about dricore, but I used to be in the concrete flooring industry, polishing and coatings specifically.  I agree that a plastic sheet is a bad idea that would just trap vapor and possible create pooled moisture.  Plastic barriers are for under concrete.

First thing I would do is to test the concrete with a calcium chloride moisture test.  Sean suggested a "poor man's" moisture test which can work but for the few dollars a real quantifiable test costs I would not risk it in a paying customers house.  Make sure that basement in conditioned as it will be when used to best test the moisture vapor emissions (MVE).  If you find you have an issue, there may be good products to use as a subfloor as Sean suggests, or I would suggest a moisture block epoxy such as an Ardex MC system if you want to put down floor covering.  Ardex (if installed by a certified installer) will warrant the floor from covering to concrete or at least they used to.  I haven't been doing that work for the last 3-4 years.

Hope that helps.

Curious as to why "trapping moisture" against concrete is likely to be problematic.  Can  you please share?  Many thanks.

I wouldn't want the potential for any mold growth which could be possible.  I can't say that I have ever seen that happen, but I can't think of anytime I have seen a plastic sheeting used as an above slab barrier, more "best practices".  

In the case of pouring a topping slab I don't know enough about flat work to comment on, but you would not want to do that with any kind of self-leveler as you run an extremely high risk of the topping cracking since there is nothing to bond to.  I would think that the same thing could happen with a topping slab but it would probably depend on if you were using any reenforcement, the mix design and the thickness, but again flatwork was not my area of expertise.

As a long time resident of an older neighborhood, I've seen plastic vapor barriers put over existing concrete floors before a new floor was poured.  In our area floor drains were normally installed in the basements, and the floors sloped toward the drains.   When people wanted to finish their basements it was necessary to eliminate the floor drains and add a new floor (either a second layer of concrete or a wood floor installed over tapered sleepers) in order to get a flat floor.   When the basement in my house was "finished" (vintage 1960) a wood floor was installed over wood sleepers.   It seemed to hold up well for 40 years or so---until termites got the best of it.

If the owner is wanting use the basement 10 hours a day I would strongly recommend RADON testing (and abatement, if needed).

I second Tom R. to test the indoor radon levels - if it was tested in the past - test it again. What part of the country is the home located? Site drainage issues are often the problem with basements holding moisture and not "drying to the outside" properly. Installing proper moisture barriers and drainage on the exterior walls of the basement may help to abate moisture best. Also, consider adding a ventilation system to the basement room to allow for multiple air changes throughout the day. SmartExhaust systems come to mind. New exhausts are so quiet you hardly know its on. The air quality and humidity could be improved this way as well.


Blue Bear Home Inspection 

Go with this style but treat all the cracks with www.xypex.com

Then look into Echo basement design.. Indoor Air Technology Association.

An appropriate type and thickness of XPS rigid insulation will be a vapor barrier (check the perm rating for your local products). If the seams and perimeter are sealed, and especially if the XPS is glued to the concrete, you will have a vapor barrier. You will also have a barrier between the inside of the house and the other side of that vapor barrier, IF any mold were to grow. Mold needs moisture, warmth and oxygen. Even if mold were to grow in any voids between the XPS and concrete, the mold would be outside the house and the mold spores could not get into the atmosphere in or out.

We just completed such a project with a slab-on-grade. We glued down 1" XPS and laid a floating laminate floor on top.



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