Insulation, Radiant and Vapor Barriers - air space between wall and insulation

Hi am building sauna in my shed on backyard.
winter temperature goes down to - 20 C


All inside framing done with 2 X 4 , but 2 X4 does not directly attached to walls of the shed .

If I fill space between stads with R12 , I will have 1 - 1 1/ 2 ' of air space left.

If I use R20 I, have to press it to feed between stads

My questions
Can I leave air space ?
if yes
a) leave space between exterior wall and insulation
b) leave space between interior wall and insulation.

if no
what should be used for fill air space?

thank you

Albert

I would use the thicker insulation, unless you have to compress it too much. Compression takes away the insulating capacity. R-20 requires 6.5 inches of space. R-13 requires 3.5 inches of space. If you are going to compress it more than one inch, you might consider filling the space with the foam board, which is thinner. Leaving dead air space provides some insualting effect on its own. One thing I consider is that too little insulation will cause significant energy loss. I would be inclined to find a solution that insulates all the air space.

Hope this helps.