Fibreglass Insulation for basement walls question

The description is confusing, where did the extra 2" of room come from ? Insulation IMO should be installed in such a way that it is not compressed at all; I don't see the need for the "extra few inches" gap.

I think he means he built his 2X4 wall about 2 1/2 inches away from the basement wall. Thats where he got the 6 inches from. I do not think the extra gap makes a difference. Unless you have severe dampness on your basement walls, if you can get 6 inches of insulation in there without compressing it, then do it. I

Any basement even visably dry may be to wet to close up resulting in mold, get a moisture meter and test your walls after big rains. If dry you don`t want an air gap, I used 2" R 7.2" foamboard for R 14.4. on basement walls, much thinner than fiberglass, On damp areas it is all designed for easy removal and future repairs. Another consideration for foam is pipes brake , basements flood.

Is your "IMO" anything other than a wild-assed guess, though?

(A)What's the R-factor for 6" fiberglass in a 2x6 studwall? (B)What's the R-factor for 6" fiberglass stuffed into a 2x4 studwall? (C)What's the R-factor for 4" fiberglass in a 2x4 studwall?

I suspect that you will find that A>B>C.

In any case, OP would be better off spraying in Urethane foam. and not using fiberglass at all.

Note to OP: If you're really sure that you won't get any groundwater wicking through the foundation wall, then you shouldn't need the airspace, but you should be really anal about the vapor barrier on the inside surface of the wall. If you're NOT sure, then I strongly recommend a closed-cell foam product against the wall.