I thought the recommended basement wall R-value is 20 but just found the government's recommendation for interior wall is R10 but not R20 for zone 3. Form the zone map, it looks like MA is in zone 3.

does the R10 recommendation assume there is a good exterior insulation? or R10 itself is enough for interior wall no matter the condition of wall exterior insulation.

I have a full basement with window wells. It only have about 7" above ground. It requires less insulation for underground wall, I think. If R10 is enough, I can save some money which is planned for fibre glass batts.

Bying using this calculator, http://www.ornl.gov/sci/roofs+walls/...on/ins_16.html, I got R11 or R15 in my area with 018XX, 024XX and 017XX zip code. It seems R20 is overkill?

Dana, I know you have done yours with R20. What's your opinion about this?