Can I use 1“ XPS and fiberglass batts instead of 2” XPS insulation in my basement?

Question

I have read multiple sources on the internet suggesting use of 2" XPS foam boards to insulate basement foundation and then building a 2x4 wall in front of it.

2" foam board is very expensive when compared to 1" foam board and I am wondering if I will get the same/better results by following EPA air plus recommendation (link below) of using 1" XPS on the foundation and then filling the 2x4 cavities with fiberglass insulation.

http://www.epa.gov/indoorairplus/technical/moisture/1_12.html

As I missing something or do you think I have the right plan ? I'm in South East Michigan and I have a walkout basement(last year temps dropped below zero for 2 weeks- usually worst night time lows are close to 15 deg F) . The basement gradually goes from 90% below grade to 100% above.

Answer 1

The issue here isn't how well to insulate your basement but it is how to insulate it without trapping water in your walls.

If you double insulated your entire basement you are basically trapping water in your walls Building Science report covers this well. Water can also sit in fiberglass for a long time - it doesn't dry out fast. I'm not even proposing that you have a major leak, I am just talking about the normal bleed through concrete.

For your situation I would do the foam board everywhere. Then I would rock wool to one foot below grade. So your basement will have different amounts on insulation on different walls. On a wall that is ground level I would stop a foot off the ground. Moisture should wick to the bottom and you are giving yourself an air pocket that will help evaporation. If you feel that you don't want to leave that pocket then I suggest that just the ground level wall gets the 2 inch variety (and I would still rock wool over that).