What exactly are you thinking of doing with it. If you just apply tinfoil to an existing surface -- e.g. glue it to the plywood walls -- it does nothing to speak of, because you still get exactly the same ol' heatloss due to conduction that you would've gotten anyhow. It only does some good when installed with some sort of airspace behind it, to break conduction. For instance foil bubblewrap does prevent heat loss on heating ducts or whatever, whereas just *foil* applied to that surface does NOT. Even if there is just minor accidental airspace behind it, like if you taped it to walls without glueing all of it down, that will do very very little to redirect heat loss (exception: the very very little it does can still be useful if you have a *large* source of heat output such as an electric light. But not relevant to just 'chickens in a normal chicken coop')
The problem is that if you apply foil with an airspace behind it, the chickens will be able to damage and eat it when they experimentally peck at it. This is absolutely guaranteed true if you put foil across the stud spaces, and quite honestly I would not give foil bubblewrap great odds in the coop either. (Bubblewrap, plain, is good on windows but only when they're up where chickens can't peck at 'em)
Feedsacks stapled to the walls don't really do much of anything either in terms of insulation, although if it makes a person happy then there is nothing wrong iwth it other than being a dustcatcher, and I suppose if you had a very drafty coop (gaps between boards) they would help with that by diffusing air movement.
IMHO if you actually care about insulating the coop, do it with "real" insulation (covered with something peckproof), either batts or rigid foamboard. Also quite honestly a lot of need-for-insulation can be gotten around be good design and management. (See 'cold coop' page, link in .sig below, for some suggestions)
Good luck, have fun,
Pat