Insulation R-Value Heating Savings Calculator

The heat that escapes through your insulation is calculated from the temperature difference, the area, and the inverse of the R-value.
Doubling the R-value cuts heat loss in half through that surface, dollar for dollar.

annual_BTU_loss = (heating_degree_days × 24 × area) / R_value
annual_savings = (BTU_loss_old − BTU_loss_new) × heating_cost_per_BTU

Heating degree days (HDD) is the standard climate measure.
It is the sum, across the year, of degrees that the average daily temperature falls below 65°F.
Approximate HDD totals for US climate zones:

• Miami: ~300 HDD
• Atlanta: 2,800 HDD
• Washington DC: 4,200 HDD
• Chicago: 6,300 HDD
• Minneapolis: 7,800 HDD
• Anchorage: 10,800 HDD

A worked example.
1,500 sq ft attic, currently R-19 (typical 1980s construction), upgrade to R-49 (the cold-climate code minimum in the 2015 and 2018 energy codes; the 2021 edition asks for R-60 in zones 4 to 8).
Located in Chicago at 6,300 HDD.
Heating with natural gas at $1.30 per therm. A therm is 100,000 BTU, so ten therms make a million BTU: enter $13.00 in the cost field, not $1.30.

• Old loss: (6,300 × 24 × 1,500) / 19 = 11.9 million BTU/year
• New loss: (6,300 × 24 × 1,500) / 49 = 4.6 million BTU/year
• Savings: 7.3 million BTU/year = 73 therms = $95/year

Same upgrade in Atlanta saves only $42/year (less HDD).
Same upgrade in Minneapolis saves $118/year.
And same upgrade in Anchorage saves $163/year.
The colder the climate, the bigger the payoff per insulation dollar spent.

Cost of the upgrade.
Blown cellulose to reach R-49 from R-19 in a 1,500 sq ft attic costs roughly $700-1,200 for materials and rental, or $1,500-2,500 if hired out.
At the $95 a year above, that pays back in Chicago in about 7 to 13 years on do-it-yourself work, and 16 to 26 years with a contractor.
That sounds slow, but the insulation has a 30-50 year functional life and the savings continue annually.

Wall insulation is harder.
Adding insulation to existing walls without opening the drywall requires drill-and-fill cellulose ($2-4 per sq ft) or spray foam ($4-8 per sq ft).
Wall area is 5-10× attic area in most homes, so the upgrade cost can be $5,000-15,000.
Energy savings are real but the payback is closer to 15-25 years even in cold climates.

Three practical points to remember.
Air sealing matters as much as R-value: a poorly sealed attic with R-49 leaks more than a well-sealed attic with R-30.
Spend $100-300 on air sealing (caulk, foam around penetrations, weatherstripping) before adding insulation.
The math above assumes ideal performance, with no thermal bridging through joists or rafters.
Real heat loss is usually 10-20% higher than the calculator says.
And there is no US federal credit to count on any more.
The Inflation Reduction Act (IRA) credit paid 30% of the cost, up to $1,200 a year, and it ended for insulation installed after December 31, 2025.
Some states and utilities still pay rebates on insulation and air sealing, so take any rebate off the cost before you enter it.