basementinsulation
I'm finishing my basement in southeastern Wisconsin. This is a poured full basement below grade. We are ready to insulate the walls and are going to use closed cell spray foam.
I've had two different contractors come out and recommend different thicknesses for the foam. One said I only need 1" against the walls, and 3" in the sills. The other said 2" against the walls and 3" in the sills.
What kind of performance gain would I be looking at between 1" and 2" sprayfoam?
At this point, it's not a DIY project. You need to consult an engineer or an architect/engineer that is used to these kind of building envelopes. A sealed envelope with an ERV needs to be designed and PROPERLY sealed, and the ERV capacity needs to be determined and designed by an engineer in order to meet building code and pass inspections.
Note: I'm in TX and familiar with building code for high efficiency projects here, and my girlfriend is a PE but not in HVAC.
"Should I use Tyvek or tar paper on basement walls?"
Neither. No building paper or wrap against the foundation. Just use foam it is not really all that difficult to cut.
The best practice really is to use rigid foam against the foundation wall, then a framed (possibly insulated) wall with drywall and no vapour barrier.
Batt insulation in the framed wall is debatable. Green Building Advisor has articles that say it's not a good idea because it can absorb moisture, but others sources have no problems with it. If you do use it (I would if you are in a cold climate and don't have existing moisture problems in your basement), I personally prefer mineral wool over fiberglass.
Don't forget about the cavity above the wall in the joist spaces.
Good Luck!
Best Answer
Closed cell foam typically has an R6 value (that's R6 per inch)
http://www.fomo.com/resources/technical-bulletins/opencellvsclosed.aspx
So 1" of foam would be R6. 2" of foam would be R12
I've lived near you - Wisconsin gets cold and stays cold - and in a basement the longer the ground stays cold the more thermal break you want. so frankly in your climate, I'd be looking for foam to completely fill between the joists with foam - 3.5" if you're using typical 2x4's - giving you closer to an R20.