How the insulation cost comparison works
The three contenders
- Fiberglass batts — cheapest per R on paper, but performance lives or dies on fit quality. Gaps and compression are silent R-killers. DIY-friendly.
- Blown cellulose — the attic default: fills irregularities, treats penetrations reasonably, adds air-density benefits, made from recycled paper. Slight settling (~2–4%) is normal and priced into its R rating.
- Spray foam — closed cell delivers R6+/inch and air-seals in one pass; costs 3–5× per R and belongs in roofline/conditioned-attic designs more than attic floors.
Cost per R is the honest comparison
Raw price per square foot misleads because materials deliver different R per inch. Cost per R-point (price ÷ R added) levels the field — and the calculator applies it to your actual area and target, DIY or installed, with your utility rebate subtracted.
What rebates change
Utility and state programs routinely rebate attic insulation 10–50% and sometimes require their contractor list — check before DIY-ing, since a 30% rebate can beat DIY economics. The federal credit (up to $1,200/yr for envelope upgrades under current law) stacks in many cases.