Attic Insulation Savings Calculator
Enter your attic area, the old and new R-values, your heating degree days and fuel price, and this tool estimates the heating-season energy and dollars that added insulation saves.
Calculator
Raising 1,000 sq ft of attic from R-13 to R-49 over 5,000 HDD cuts conduction loss by about 75.4 therms ≈ $113/season (at 90% furnace efficiency, $1.50/therm — your rate). Conduction only.
Adding attic insulation pays back by slowing the heat that conducts through the ceiling all winter. The savings depend on four things you know or can look up: how much attic you are insulating, how far you are raising the R-value, how cold your winters are (measured as heating degree days), and what your fuel costs. This calculator combines them into a heating-season estimate of energy saved — in BTU and therms — and the dollars that represents at your fuel price and furnace efficiency.
Because it works from the numbers you enter and holds no price list, the estimate stays honest whatever fuel prices do. It models conduction only — the steady heat leak through the insulation — which is the part insulation directly reduces; it does not credit air-sealing or cooling savings, so treat the result as a conservative floor.
Formula
BTU = area × HDD × 24 × (1/Rold − 1/Rnew)
therms = BTU ÷ 100,000 ÷ efficiency · cost = therms × rate
The (1/Rold − 1/Rnew) term is the drop in heat-loss rate per square foot per degree; 24 converts degree-days to degree-hours; dividing by 100,000 BTU/therm and by furnace efficiency turns delivered heat into the fuel you actually burn.
Worked example
Raise a 1,000 sq ft attic from R-13 to R-49 in a 5,000 HDD climate, burning gas at $1.50/therm in a 90% (0.9) furnace:
1,000 × 5,000 × 24 × (1/13 − 1/49) = 6,781,200 BTU
6,781,200 ÷ 100,000 ÷ 0.9 = 75.3 therms → × $1.50 = $113/season
About $113 a year from this one upgrade — and note the biggest gains come from the first jump off a low R; going from R-13 to R-30 captures most of it, with R-30 to R-49 adding less.
What this estimate does and does not include
This is a conduction-only, heating-season estimate. Real bills also depend on air leakage (often a bigger loss than conduction in an old attic), duct losses, thermostat habits and summer cooling — none of which this model claims. Air-sealing the attic floor before you insulate typically boosts the real saving well beyond the conduction figure here.
For an electric-resistance or heat-pump home, convert with the window energy-savings tool's kWh path instead of therms, or set efficiency to 1.0 and price in $/therm-equivalent. Whatever the fuel, the honest takeaway is a planning estimate, not a guaranteed number — get a real energy audit for a firm figure.
Frequently asked questions
How much does attic insulation save on heating?
It depends on your climate, fuel price and how far you raise the R-value. As a worked example, taking a 1,000 sq ft attic from R-13 to R-49 in a 5,000-HDD climate saves roughly 75 therms — about $113 a year at $1.50/therm in a 90% furnace. Colder climates and higher fuel prices save more.
What are heating degree days?
Heating degree days (HDD) measure how cold a location is over a year: each degree the daily average sits below 65 °F counts as one degree-day, summed across the season. A mild climate might be 2,000–3,000 HDD; a cold northern one 7,000+. Look yours up at degreedays.net or NOAA.
Why does the saving shrink at high R-values?
Heat loss is proportional to 1/R, so each added inch cuts a smaller share of what remains. Going from R-13 to R-30 removes far more heat loss than R-30 to R-49. That is why the code targets a "good enough" R rather than the maximum possible.
Does this include cooling savings?
No. The estimate covers winter conduction only. Attic insulation also reduces summer cooling load, and air-sealing adds further savings, so your real total is usually higher than the heating-only figure shown here.