https://www.amazon.com/Prodex-Total-...pe=all_reviews
It's r1.2 by itself. This is better than rzero and if it all that fits in the ac unit, it's the best that can be done.
Reflectivity requires a gap to reflect back to so that works except where the foil touches something. It'll sink some heat into the something it touches. Again, it's what fits, it's about all that fits, so use it.
Aerogel is only r15 and it's the best in the world and lab and experimental only.
To reach the rfactor touted it requires three layers with a 4 inch air gap between them, a roof and a ceiling.
I'd use it because it's all that will fit in there, but you're off by a factor of 12 or so on what it the rfactor does.
I use something similar to make a waterproof envelope for the refractory cloth I use for insulation.