tinfoil

is it a good idear to use tinfoil on the walls untill i can get mylar

Depends on who you ask. I wouldn't use the stuff(hot spots, poor reflectivity), but some people do. I prefer flat white paint or b&w poly film.

Space blankets or emergency blankets (different areas call them different things) cost very little. I would think they would be a better option.

If youre using CFL's, it wouldnt be the end of the world to use foil. I defintely wouldnt do it with HID's though.

someone say "tinfoil"?

Tin foil is not recomended by experienced growers. Like CR said hot spot and poor reflection. Flat off white paint is much better for reflection!

FDD you should put up that picture everytime someone asks abot tinfoil! LOL