Well I mean if your pinching penny's and you so happen to have a box of those blankets why not right but for pretty much the same price a nice matte white paint is what I'd do. But if you got the green I hear panda orca and diamond are top notch. Putting up these films miiiight be a prodject... I wouldn't know from experience but I know how to use a paint brush lol
I remember reading a data sheet that said like white paint has something like 85% reflectivity. Mylar 90% those other 95% could be wrong so don't take that as the one true gospel