@westend. Ya I looked at that a while ago and saw what you did. Just checking the options.
Was thinking of using 2- foam type boards with a small air gap between them. Biggest concern is the moisture. Here is what I found at lowes here.
Blue board 3/4=4R 1/2-3R
R-max ( if I remember right it had foil on one side)
1/2------5/8------3/4
3.2r-----4r--------5r
White foam with foil on one side- need 3/4" air to get full R value.
1"-----------3/4----------5/8---------1/2
Foam--------------3.8----------3------------2.5---------2
Foil-----------------.17----------.17----------.17---------.17
3/4" air gap------2.8----------2.8----------2.8---------2.8
Totals-------------6.82---------6------------6.5---------5
I'am thinking either of the 1st 2 at 1/2" then 1/2" air gap then the white foam at 1/2" would give me roughly 8R. I don't know...just thinking out loud.
Did some more demo today. Forgot to take pics. Here is one when I started, after I removed the back
Click For Full-Size Photo.