insulating an old 2x4 shed

I have an old shed I want to insulate and use as a workshop. It is about 14x20 ft. and 2x4 construction with 2 ft. centers. My natural instinct is to fill the voids in the wall with fiberglass batts and put sheetrock drywall over that but I hate fiberglass and would like some more sustainable materials. Does anyone have any suggestions? Thank you!

Check out cellulose insulation

sheep wool?

straw light clay? but that has not a very high R-value. but maybe it s good enough for a workshop.

I've had the same dilemna.

The problem with cellulose, i.e. old shredded newspaper treated with nasty fire retardant chemicals, is that it settles. It's great for attics as blow-in insulation, but in walls it will lose it's loft and thus most of its insulating power.

You can buy wool insulation in batts just like fiberglass but it's very pricey. I once used old packing material, mainly bubble wrap. It was a very temporary solution but it definitely did the job. I suppose if being to code is not a concern you could use that and old styrofoam. You need to mak sure the walls are sealed, though, as any air flow (it seems to me) will affect it even more than with fiberglass.

It isn't sustainable, but you can keep a bunch of stuff from the landfill.  Take a scrap piece of board and put a bunch of nails thru it (think bed of nails, but smaller).  Take old Styrofoam from a dumpster somewhere and kind of "grate" it with the board o' nails.  It will go back into those little round beads.  Fill the wall space with them.

Try rock wool. Holds up to being wetted and and can still dry out and regain its R value.
It's fireproof.
Doesn't itch.
Holds it's shape.Cut it a little big and it will hold itself in cavities.
Widely available.