I am in a new home (since Feb 2010) and looking to finish off my basement. My builder has placed insulation (r12) from ceiling to floor (8ft ceiling). The insulation is a roll and covered with poly plastic.
My question is can I build my wall studs on top of current insulation OR do I need to remove the builder insulation and place vapor barrier against cement wall and then build my walls?
If I keep the builder insulation, how do I build my wall studs and will this pose any damp/moisture issues in the future?
It is best to remove all the insulation and Vapor Barrier and build a 2x4 wall one inch away from the concrete wall you need an air space behind the 2x4 wall and the concrete wall. However if you want to save time and double insulate then you can leave the builders insulation but you need to cut the vapor barrier every 6 inches from floor to ceiling and then insulate your 2x4 wall and install Vapor Barrier with 6 mil poly.
Your instincts are right, the way your basement is insulated right now is not ideal, although very common. There is a risk of condensate forming inside the walls as well as a moisture seeping from the soil through the concrete wall and creating a perfect mold environment inside your insulation. You want to leave at least one side of your wall to be vapor permeable so the wall can dry in case any moisture gets in. Since it's impossible for a basement wall to dry to the outside because it's below grade, it should be able to dry from the inside. In your case poly plastic will prevent it from doing so.
Although having poly wrap on the inside is very common, in order to avoid creating a perfect environment for mold growth, you are better of removing it. The best way to proceed would be by placing 2" rigid insulation board directly against the concrete wall, place your 2X4 framing and fill the gap with the mineral fibre insulation (mineral fibre is preferred because it doesn't trap moisture, but you can reuse your builder insulation if you want). After that, you place gypsum board and paint it with the latex paint, NO poly plastic.
There is an excellent article on this topic that describes everything I've written in more details. Here is the link http://www.ecohome.net/guide/basement-renovations-prevent-mold-retrofitting-basements.
Good luck with your renovations!
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