whats better to use mylar or just simple old white paint.

depends on if you are talking about cost or reflective capability. Flat white paint is 85% reflective hile Mylar is 95% reflective. White paint doesn't affect heat retention while Mylar reflects the heat.

If you do the white paint, use flat white.

Vi

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Flat white paint is the absolute best for reflecting light

I like to use large white plastic party tablecloths. These are thin plastic with a very white matte like finish I think works very well on walls or hung to get closer to plants to maximize reflective light.
They are about a buck a piece at party or dollar stores...

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