what about walls painted white? would it be worth it to go with mylar or something else or is white paint close enough?
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Flat white paint will kill mylar hands down, mylar is only within 3-4% if it is perfect without folds and bends [i.e. not stapled]. It needs to be glued down and smoothed out to be anywhere near effective and why waste all that time.....when white flat or even white flat with a little more titanium dioxide mixed int [have to ask, they will do it though]....
Panda plastic is great for covering white without paint. Anyhoo, pretty basic 1st run stuff.
Mylar has no place in the grow room imho, its really just a lazy solution from newbs.....Even the the thick stuff that is commercially viable is crazy expensive and flat white paint will still provide about the same. You can get "safer" environmentally paint as well with lower VOC's if you look around, for those with that concern.