What is the best insulation for the money?

The best insulation for the money depends on where you are insulating, but for most existing homes, blown-in cellulose or blown-in fiberglass for attics offers the highest return on investment. For about $0.60 to $1.20 per square foot in materials when you use the free blower rental from Home Depot or Lowe's, you can raise an attic from R19 to R49, which can cut heating and cooling bills by 15 to 30 percent. That payback is often under two years. For walls in new construction, the best value is R15 high-density fiberglass batts for 2x4 walls or R21 for 2x6 walls. They cost only about $0.15 more per square foot than standard R13 but give 15 percent more R-value with identical labor. Johns Manville ComfortTherm and Owens Corning Pink Next Gen are both excellent value because they use formaldehyde-free binders and achieve labeled R-value consistently. For air sealing plus insulation, closed-cell spray foam is expensive at $5.50 to $8.50 per square foot, but it is the best value in rim joists, crawl spaces, and cathedral ceilings where air leaks are costly. Rigid foam board like EPS at about $0.80 per square foot for R4 is the best value for insulating basement walls. Overall, air sealing with caulk and then adding blown-in attic insulation gives you the most energy savings per dollar spent.