On mylar....there are fabrics that due use mylar to create a usable fabric. For example is InsulBright, which we use to insulate our FBC Cozies. This fabric is mylar that is then needle punched with poly. So you have a finished fabric that reflects heat but also breathes and is flexible without crinkling. It is also washable.

Does mylar work? Yes and no. You have to use it correctly and plan for condensation. One trick I have used with E-blankets (the cheap ones) is to line my tent floor with one in cold weather, then my sleeping pad goes on top. This barrier stops the cold from coming up. I can sit on the tent floor in bare feet and have no cold. Fantastic for staying warmer out there. The key is I don't sleep directly on it! By having my gear on top, well raised, my evaporation during the night is blown away by breezes coming through the tent and my back is never directly on the mylar.

So don't say no to using it - but do think it out.