I got mine from EXPOL, they delivery after a certain amount.
I got new sheets as I could not be bothered finding cheaper! I have already made up the $400 spent long ago! Its worth while
I used 40mm for the ceiling, all the ceiling and high areas. I used 25mm for the walls,
I tried to use as thick as possible on areas where space isn’t a problem as in taking up too much space and making the rooms maller.
To be honest, its amazing stuff, the temperature difference outside3 and inside is MASSIVE, my garage has a back area that I blocked off with a makeshift door, the temp there is same as outside….
The difference is massive! I mean massive.
In the summer, my power bill with 2x 1200 litre tanks, 1x 360 and 1x 300 litres, all having at least 100% turnover over a week was about $200 ish. For so many tanks that’s very little I think. Also tanks hold temperature better.
To be honest, its not your garage, iw ould be inclined to line the tank and line it well, make sure that the poly is stuck as much as possible, but lining the garage is better.
Don’t know the size of my garage, but it is a little smalle than yours law, but maybe you can find cheaper poly and have more patience hunting around for it.
I say insulate it, it will pay dividends later.
This hobby is long sighted:D
Tinfoil barbs ill eat plants and destroy them