Insulation Savings Calculator
What an R-value upgrade is actually worth per year
Put a number on an insulation upgrade. Enter the assembly area, the R-value before and after, your heating degree days and your fuel price, and see the annual saving and the payback period.
Over the life of the work
- Fuel saved per year
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- Saving over 20 years, at today's prices
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- Net gain over 20 years
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- Cost of the R-value added
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Show the working
How this is calculated
Every assumption is listed so you can disagree with it. If a figure here is wrong, tell us and we will fix it.
The model
Steady-state conduction through an assembly over a heating season is the degree-day method, which is the standard approach for exactly this question:
annual heat loss (BTU) = area × HDD × 24 / R
The 24 converts degree-days into degree-hours. The saving is the difference between running that at the old R-value and at the new one:
BTU saved = area × HDD × 24 × (1/R_old − 1/R_new)
Note the shape of that expression: savings scale with the difference of the reciprocals. Going from R-11 to R-22 saves as much as going from R-22 all the way to infinity. This is why topping up a poorly insulated attic is the best money in the house, and why pushing an already well-insulated one from R-49 to R-60 rarely pays.
Turning BTU into money
Saved heat is divided by the efficiency of whatever produced it, then priced in that fuel's own units:
- Natural gas — 100,000 BTU per therm, divided by AFUE.
- Heat pump — 3,412 BTU per kWh, multiplied by the seasonal COP. Enter COP directly (a cold-climate unit averages roughly 2.5–3.5 over a season); HSPF divided by 3.412 gives you the equivalent.
- Electric resistance — 3,412 BTU per kWh at an efficiency of 1.0.
- Heating oil — 138,500 BTU per gallon, divided by AFUE.
- Propane — 91,500 BTU per gallon, divided by AFUE.
What this leaves out
Air leakage is not modelled here, and in an older house it is frequently the larger loss. Sealing the attic plane before insulating it routinely beats the insulation itself on payback, and insulating over unsealed gaps buries the problem where you cannot reach it. Thermal bridging through studs and joists is also ignored, so a stud wall's effective R-value is lower than its cavity R-value — use a whole-assembly figure if you have one.
Summer cooling savings are excluded, so in a hot climate this understates the benefit. Degree-day models also assume a constant indoor setpoint; deep overnight setbacks will beat this estimate.
Sources
The constants and default values above come from these public sources. Every one is free to read and none of them are ours — check the figures against them.
- DOE — Insulation Recommendations by Climate Zone Target R-values per assembly and zone, behind the guidance shown under the R-value inputs.
- NOAA — Heating Degree Day Data Look up the HDD figure for your own location instead of using our regional preset.
- EIA — Natural Gas and Heating Oil Prices Current residential fuel prices for the price input.
- DOE — Air Sealing Your Home The loss this calculator does not model, and which usually beats insulation on payback in an older house.
Built by Vano Gelashvili
Vano Gelashvili is a software developer who builds the calculators on this site. He is not an HVAC engineer, and says so on every page: what he brings is the arithmetic, worked openly against published data.