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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Insulation savings = your annual energy bill × the expected saving: a typical attic upgrade saves about 15%, so a $210 monthly bill saves about $378 a year, and a $2,400 job with $600 of rebates pays for itself in under five years. Enter your own bill, project cost and rebates for the payback period and 10-year return.
With these inputs:
| Result | Value |
|---|---|
| Annual energy savings | $378 |
| Monthly savings | $32 |
| Net project cost after rebates | $1,800 |
| Payback period | 4.8 years |
| 10-year savings | $3,780 |
| 10-year net gain | $1,980 |
| Return on investment | 110% |
Annual savings = monthly bill × 12 × savings % · Net cost = project cost − rebates and credits · Payback = net cost ÷ annual savings
The DOE puts typical whole-home savings at about 15% of heating and cooling costs from air sealing plus attic insulation. Badly under-insulated houses see far more.
Not federally. The 25C credit (30% of insulation material cost, up to $1,200 a year) applies only to property placed in service through December 31, 2025. Many utilities and some states still offer insulation rebates, so check your utility's current program before you buy.
It depends on how much you are losing now, which is why any single published percentage should be treated with suspicion. The variables that matter are your current R-value, your climate's degree days, your fuel price and how airtight the house is. The calculator asks for those rather than applying an average, and the honest output is a range you sanity-check a quote against.
Usually because of what each assumes about the starting point and about air leakage. Insulation models assume heat moves by conduction through the assembly; in a leaky house much of it leaves with the air instead, and no amount of R-value addresses that. Estimates that ignore air sealing tend to over-promise.
Attic top-ups are usually the fastest because the material is cheap and access is easy; wall retrofits and spray foam take considerably longer because the job costs more. Rebates change the arithmetic significantly and vary by location and year. Work it out from your own fuel cost and quote rather than from a generic figure.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.