Clean up !
So... since Mylar is a brand name (by Dupont Tejjin Films afaik ?) I don't really think theres a confusion in between languages regarding "Mylar", so for me and us here Mylar is polyester (PET) sheets with some deposited metal (aluminum I think) layer (mostly only one side is metallised so the other is translucent to that layer). That makes Mylar a "boPET" material.
Yours is one of the cloth ones for good ventilation and all, maybe its the same brand so its reffered to as "Mylar" too ?
Also "Foylon" is out there.
I think we should clear up and decide what Mylar is or just go "Polyester sheet" or "Fabric"
However: PET film mylar is incredibly cheap, strong, lightweight and can be highly reflective. Also the Al layer makes it mostly gas proof.