MEh, I'm old for canncom but young compared to the REAL growing oldtimers... 30ish...
Mylar is not as easy to work with as panda film and lets light through. Panda is what you want to use for a 'door'.
But keeping heat 'over there' is not your goal... you want your PLANTS to be comfortable, not yourself.
I'm not kidding, I literally sleep downstairs on the sofa in my own home when it gets into the upper 90s outside, so that I can dedicate all my a/c power to making the girls happy. One must suffer for one's art