People that I have talked to say that the first layer of poly which is against the cold block walls is not needed and will cause more harm then good. Is there any truth to this or is it just a myth? Can I leave that first layer of poly there or should I remove it and only install the last layer which would be sandwiched between the studs and the drywall?
Also, I wish to lay down a layer of the same poly on the concrete floor prior to just laying down an underpad and final carpet. I am not going to build a subfloor. Is this acceptable in most cases?
A vapor barrier on the outside of the insulation would only add an extra layer of protection against moisture from the outside. In a basement, that should have already been done by the people who laid the foundation; they should have either painted the outside of the wall before filling the dirt in fromt he outside, or should have poured the concrete with a vapor barrier on the outside. If they did not, and if you have water or moisture seeping in from the walls, then an outer vapor barrier would be a good idea to protect the insulation from getting wet and then rotting.
The downside of using an outer barrier is that you might lock any moisture that gets into the insulation between the two vapor barriers. Thus, if you do use two barriers, be sure to leave a few feet bare at the top of the outside layer for moisture to escape from the insulation should it get in between the barriers. (It will "want" to evaporate upwards when it gets warm.)
As for the flooring, using plastic sheeting as an underlayment will help protect your carpet from moisture. The problem it solves is that you might have moisture coming up through the floor that evaporates easily and unnoticably on an unfinished floor. Putting carpet or wood floor directly down on the concrete might trap that moisture in the carpet, allowing the carpet to rot. You might not need it, though, because your floor might already have a proper vapor barrier underneath it, just like the walls. The only downside I can think of is that your carpet might slip on the plastic if you lay the carpet directly on the plastic.
vapor barriers/basement finishing
Bowing of Trex decking
Options for flooring for black tar under vinyl tiles
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