Don't use glossy, use a flat pure white (make sure it's not "superwhite" which has a drop of blue in it). If you want the absolute most efficiency, line it all with mylar, businesslights dot com sells it cheapest buy the thicker one.
I use mylar for my 3x5' grow closet. If the room is of any size, paint may be more practical, and perform just about as well. Sativa.

Flat white paint diffuses light better than mylar. Generally
diffusion is better because of better distribution due to
multiple angles.

With a mylar-enclosed room, the temperature is generally
hotter than with white paint. Because mylar contains the
infra-red more.

It seems like we are having yet another white paint vs. silver/aluminum
flame war!!!!!

As an experiment I used an off the shelf KMART Liteway brand shop light.
These shop lights cost about $11 US for a dual bulb 40 Watt T12 fixture.
I used the same two Philips F40T50U (Daylight) bulbs in this experiment.

I measured the light levels at various points around the light
using a Sub-Lux Light meter. I measured several points around the bulbs,
since one measurement directly underneath the bulbs may not tell the
whole story.

One of my comparisons was between the standard white reflector that
comes with the shop light and regular aluminum foil. For the aluminum foil
case, I just lined the inside of the reflector. I did not bother to
get all the wrinkles out.

****************

What I found is the the aluminum foil covered light fixture had a consistent
10% IMPROVEMENT in measured light levels compared to the off the KMART shelf
white reflector at all the various points measured!!!!

****************

I also looked at other variables using this set up. I looked at several
types of T12 bulbs (Cool White, Vitalite, Agro Lux, etc). I also looked at
a half dozen different T8 electronic ballasts and also compare them to
a 2x55 Watt Compact fluorescent lamp setup.

If anyone is interested, I'll try and "publish" these results somehow on
either on the NET or maybe the (if they let me) AGA magazine.

Even aluminum foil does better than white paint. Mylar is the absolute best. I don't really notice that it caused my grow room to heat up any significant amount either.

Ron Wozniak Allentown PA, USA
rjwozniak at lucent_com

Thank you for the info, Sativa. :) I owe you!

Also, I forgot to mention, the sides of your grow closet should be acting like extensions of your reflector. Because flat paint will reflect in all directions, it is not as effective, in my opinion, as mylar, which acts more like a mirror, which in a grow closet, is better. As far as infrared goes, I looked it up, and the titanium dioxide in white paint reflects infrared, just the same as the aluminum coating on mylar reflects infrared, so there should be no difference at all based on infrared reflectivity, in the temperature of your room. But do observed the angles that the light will be travelling from the lamps, to judge whether it is worth the trouble of using mylar, vs. the convenience of the fairly good reflectivity of just Ti dioxide in the flat white paint.

"Fortunately, TiO2 reflects well in the infrared. So one can make a cool pastel shingle by adding a little light color to a modern cool white TiO2 shingle.)" from article below:

Here is a link that might be useful: Actual reliable government publication, that shows Titanium dioxide paint reflecting in the region of 80%, which matches the results of the experiment given earlier in the thread. Are you suprized?

Hi cannibus grower, I checked businesslights dot com and I did a search for mylar. I doesn't list it, am I overlooking something?