Modern cellulose insulation is made from 75 to 85 percent recycled paper or denim, treated with boric acid, borax, or ammonium sulfate.
To install insulation in a crawl space, cover the vent openings with crawl space vent covers, seal the rim joists with rigid foam board insulation, and use foam sealant to seal gaps between the crawl space wall and sill.
Use R-13 or R-15 kraft-faced fiberglass or mineral wool insulation for 2x4 walls to ensure proper insulation.