Basement Insulation Question

[Brad H.]

I am working on finishing my basement. When we built our house we put 2” foam around the entire exterior of the foundation, then this was sprayed with the tar coating before being back filled.

My question is, will I need a vapor barrier on the inside between the foundation and my fiberglass insulation?

What would be the proper way to insulate to avoid moisture issues. I live in a very sandy area, if that makes any difference.

[shawn]

What size framing? Ultimately an air space between the foundation and insulation is desired. This stops the transfer of moisture from the foundation tn the fiberglass. The insulation on the outside can be considered in the equation for r value but the outside waterproofing is just for the outside, to keep water out.
If you have 2x6 framing I believe the recommendation is to use 2x4 insulation along the walls that are against the foundation,  keeping it towards the inside of the stud cavity and 2x6 elsewhere. With the vapor barrier installed on the inside of the wall cavity prior to sheetrock or whatever else you use. that airspace behind the insulation does a couple different things, it offers a space for any incidental moisture a space to run down the foundation and enter the drain tile that should be installed around the perimeter of the building and it adds insulation value itself.  kind of like, It is the air trapped in fiberglass insulation that offers the most r-value. That is why they say fluff don't stuff, when you are installing fiberglass insulation. 

[Brad H.]

It’s 2x4 framing on the inside, and the builder left 1/2” -3/4” of gap between the framing and the foundation. There was no drain tile installed on the inside or outside because we are in such a sandy area.

[shawn]

OK then use 2x4 insulation but make sure to not push it against the cement and then cover with typical vapor barrier.