Is there a major difference between using the two when building a sealed sub box. If 1/2" is fine then I'll just use that b/c I have plenty of it laying around.
My box is made from 3/4 mdf. I think it might have something to do with what sub and how much power you have to the size of the MDF. With some of the real powerhouse subs i have seen them use 1" for the face to hold the sub. I wouldnt go any smaller the 3/4 for the face.
You may be able to get away with 1/2" if the box is ported; if it's a sealed box, invest in the 3/4" as it will be more rigid and produce tighter bass. IMO
For the front, where you actually mount the sub...don't skimp out on that piece, go 3/4 at least. For the rest of the box, if its a light sub (under 1k watts rms) then 1/2 should be fine. I always just use 3/4 to be on the safe side.
the reason behind using thicker materials is rigidity. if the box flexes (and it will with a good sub and amp) it is wasted energy. i personally think you should use the 3/4 at the very least. for a while at the shop i worked at we were sent 5/8 mdf, and you could tell the differance. what i have done with that stuff is laminate 2 peices together. you could do that' you said you have alot laying around. make sure to put a good layer of wood glue in between the peices.
on my current sealed 15 box it is the double thick 5/8 mdf. my old box could bounce a quarter straight off the top of the box, the new one the quarter barely moves.
1hotz is almost dead on, but forgot to mention that if the box is under 1 cubic foot (roughly), or has short dimensions, then it won't flex and 1/2" would be fine. But, I'd go one step further and use 1/2" Baltic Birch plywood. It is stiffer and more rigid than MDF of 3/4" thickness, and best of all, it is lighter than the 1/2" MDF. SOOOOOO, you can make your box more rigidly damped AND lighter weight. It does cost about double what the MDF does, though. I also find the birch ply easier to work with as it is real wood and not dust/glue, and it can be finished in more versatile ways.
and all this is only if you're uilding a highpower system--
if u don't wanna buy more wood--just wanna use what u got so u can make space---i've lined the inside of a box with fiberglass....if having a "stiffy" is that important....
and man--if you're running like a single JL 12W0 or something--don't waste any money---that 5/8 is plenty rigid...and if in doubt fiberglass....