No worries on fermentation heat - this is strictly for cold conditioning and fermentation will be done (i.e. Secondary).
A vacuum...whew, nice thought but too complicated for my build :)
The insulation I plan to use (from Home Depot) is 2 inch thick 4'x8' "Super Tuff-R" which has an R-value of 13 (it is closed-cell foam). So two of those gets me R-26, plus the foam board rigidity is a plus for what I am doing. The rock wool looks to be about R-15 for 3-1/2.
I might have to...
I am planning to build a no-electric "passive" cold conditioner for beer brewing with the concept being maintaining a keg at 40-50F by periodic replacement (every few days?) of a portion of the water surrounding the keg with ice. I have attached a sketch of two options...one using insulation...
