I don't know if you have any contacts in the flat roofing industry but have a look at the white, green roof membranes they're using, with a 98% solar reflective index it actually better than the mylar lining the inside of some grow tents
That's a rating for solar heat rejection, not
light reflectivity. It might still be okay (I have no idea), but not because of the above. People use Reflectix duct (et cetera) insulation. Probably for the same reasons - it's not uncommon to walk onto a build site, politely ask if they have any leftovers, and walk away with the last few to several feet of a 4' wide roll for free. Plus, it looks shiny, lol. But, while it's often better than nothing (depending on the color of the walls one hangs it on
), it's not great.
Orca Film is the most reflective - in terms of
light - that I am aware of. It's not real cheap, but is extremely good for the purpose. There's a video on YouTube of someone holding a PAR light meter sensor on the floor of a standard grow tent (that already has mylar/reflective walls) while a helper reaches in and hangs a precut sheet of Orca Film on each of the other three walls. Each time they hang one of the sheets, the reading on the meter goes up significantly. It's durable and can be cleaned over and over without degrading it (don't use a Brillo pad on it
).
Mylar is okay, but it's thin, easy to tear, is often used incorrectly (should be tight, not hung so it's "wavy like a curtain," wrinkled, or creased), and while it does appear to be pretty reflective... if you shine a strong light on it, you can see light escaping through it from the other side.
Properly prepped, primed, and painted walls with the brightest white paint you can find is an
excellent reflective surface. If you use flat paint, you get nice diffusion with no glare or "hot spots." Unfortunately, you can't clean flat paint (try scrubbing your ceiling (no, don't, really, unless you've got some spare ceiling paint)). That might not be an issue if you repaint after every harvest or two, but accidents happen. A finish such as eggshell is almost as good (and still better than most every product that isn't actually made with the intention of reflecting
light) - and you can clean it. Try to avoid satin, and definitely avoid semigloss and gloss. This thing (properly painted walls) isn't a solution for grow tents, obviously, but can often be the best solution for cabinets, closets, and rooms.
You're already paying for 100% of your light's output each and every month on your electric bill. With that being true, it makes sense to do whatever you can in order to actually get to
use as close to 100% of that light as possible.
