garage insulation

You pose an interesting question. I sprayed a STS coatings product in my metal shed years ago and have been satisfied...like being in the shade on a hot day...and it gets hot here in TX! Don't think it's going to hold heat in, but I don't know for sure...think it's more an emissivity thing and not an R Value deal. A good website is Heatbloc.net-you could contact them and get good info. STS Coatings was rated one of the best in the country a few years back...if going with a spray on radiant barrier I'd check them out. From Heatbloc.net website FAQ:

DO RADIANT HEAT BARRIERS HAVE AN "R" VALUE?

No. R-values apply only to solid insulation or to insulating systems that control heat flows other than radiation. In independent testing of the product at the University of Kansas, the R-value of the insulation, after applying HeatBloc to the roof deck, increased by 87% to 98%.

I'd certainly look at insulation if I was trying to trap heat in...have the plus of holding cool air in...I was on a budget and was trying to have a shed I could work in that wouldn't heat up and bake me! Lol. Good luck.