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Insulation Savings Calculator

Enter the area, old and new R-values, degree days, and energy price to estimate savings.

How it is calculated

The heat that no longer escapes in a year is Q = A × 24 × HDD × (1/Rold − 1/Rnew) in BTU. A is the insulated area in square feet, R the US-unit R-value, HDD the heating degree days per year (°F-days), and 24 turns days into hours. A therm is 100,000 BTU, so therms = Q ÷ 100,000. The money saved is therms × price ÷ (efficiency ÷ 100), with price in dollars per therm and efficiency being the heating system's efficiency in percent: a furnace that is less than 100% efficient burns more fuel than the heat that is saved.

The heat-loss reduction is also shown as (1/Rold − 1/Rnew) ÷ (1/Rold), a percentage. The new R-value must be higher than the old one, and area, both R-values and efficiency must be above zero. It is a rough estimate: it ignores air leaks, framing and any change in how you heat the home.

Worked example

1,500 sq ft of attic improved from R-11 to R-30, 5,000 degree days, gas at $1.20 per therm and a 90% furnace. The difference 1/11 − 1/30 = 0.09091 − 0.03333 = 0.05758. Heat saved: 1,500 × 24 × 5,000 × 0.05758 = 10,363,636 BTU/yr, or 103.64 therms. Cost: 103.64 × 1.20 ÷ 0.90 = $138.18/yr. The reduction in heat loss is 1 − 11/30 = 63.33%.

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Common mistakes

  • Entering metric values, such as an area in m² or an RSI/U-value, in the US fields. The tool expects square feet and R in US units; a metric RSI is about 0.176 times the US R-value.
  • Entering the total thickness of the new layer as R-new. The new value is the R-value of the whole assembly after the upgrade, old material included.
  • Mixing up the degree-day base. The result is only as good as the HDD figure for your location and base temperature, and HDD in °C-days would give an answer about 1.8 times too small.
  • Entering the price per kWh in the therm field. The price must be dollars per therm; for electricity, convert it first (1 therm is about 29.3 kWh).
  • Forgetting efficiency or typing 0.9 for 90%. The field is a percentage, so 0.9 means 0.9% efficiency and makes the savings 100 times larger.

About this calculator

Heat loss through a building surface follows Q = A × (1/R) × ΔT, so raising insulation from an old R-value to a new one reduces the heat-loss rate by the difference between 1/R_old and 1/R_new. This calculator multiplies that reduction by heating degree days (HDD) — a standard measure that sums, over a heating season, how many degrees and days the outdoor temperature stayed below a reference point, typically 65°F/18°C — to estimate total heat energy saved, then converts it to therms (gas heat) and multiplies by your price per therm, adjusted for your heating system's efficiency.

Homeowners weighing an attic or wall insulation upgrade, energy auditors, and contractors quoting insulation jobs use this kind of degree-day-based estimate to put a real annual dollar figure on a project before committing, since R-value alone doesn't tell you what the upgrade is actually worth — a climate with few heating degree days sees far less payback from the same R-value jump than a cold-climate home, which is why building energy codes and programs like ENERGY STAR set different recommended R-values by climate zone.

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  • Editorial team

    Extra insulation pays back less each time. Going from R-11 to R-30 cuts heat loss by 63.33% and saves about 138 a year with the defaults. Adding more, from R-30 to R-49, cuts it by 38.78% and saves only about 31 a year. Heating degree days also vary widely by location. Tell us the HDD figure and the savings you compared against your own bills.

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