To answer the mylar question at walmart
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No we do not sell rolls of mylar, but as someone pointed out we DO sell "emergency" blankets or "space blankets" as others call them, which is just a mylar sheet, in my sporting goods department, the company who makes them that we have, is called Ozark Trail.
i use mylar on my tank and have not had a problem with it. its just a pain to keep straight and wrinkle/scuff free... but if you can manage to keep from moving it around or having prying hands play with it, it is great stuff...
Mylar burning cannabis, sorry but these plants are certainly not cannabis, which i have experience with growing (legally of course, when i was a designated grower for a family member i shall not name for security reasons). And even then, I never had a problem with mylar causing leaf burn on them, if the plants are kept well hydrated and the light source at the right distance, there should be no reason for the mylar itself to cause any form of leaf burn to the plants... more likely the problem would be the heat from the lights, and how mylar (holding in more light) will keep it a wee bit warmer, drying out the plants faster than normal and boom, brown leaves...