Insulation R-Value Calculator

Enter the area in square feet. Enter the current R-value of the whole assembly, not zero for an attic that looks bare. Then enter the target R-value, your location's annual heating degree days (base 65°F), your furnace's AFUE rating, and your fuel price per therm.

Worked example: 2,000 sq ft, R-11 to R-49, 4,500 HDD, 80% efficiency, $1.20 a therm.

Current heat loss is 2,000 × 24 × 4,500 ÷ 11 = 19,636,364 BTU a year. After the upgrade it is 2,000 × 24 × 4,500 ÷ 49 = 4,408,163 BTU. The difference is 15,228,200 BTU, a 78% cut.

The furnace has to burn more fuel than the heat it delivers. So the saving is 15,228,200 ÷ 100,000 ÷ 0.80 = 190.4 therms, worth $228.42 a year.

The result panel shows heat loss rounded to millions of BTU: 19.6M before and 4.4M after.

Q = A × 24 × HDD ÷ R, where:

• Q is annual heat loss through the surface, in BTU per year.
• A is the surface area, in square feet (ft²).
• 24 converts days to hours (h per day).
• HDD is heating degree days, in °F·days per year: each day's average temperature below 65°F, summed over the year.
• R is thermal resistance, in ft²·°F·h/BTU.

The units cancel out: ft² × h/day × °F·days ÷ (ft²·°F·h/BTU) = BTU. Since 1/R is the U-factor, the same formula can be written Q = U × A × 24 × HDD.

BTU saved = Q at the current R − Q at the upgraded R. Therms saved = BTU saved ÷ 100,000 ÷ (AFUE ÷ 100), where one therm is 100,000 BTU. Annual savings = Therms saved × Price per therm. Percent reduction = BTU saved ÷ Q at the current R × 100.

The calculator treats any R-value below 0.1 as 0.1, and caps efficiency between 1% and 100%.

Take 1,200 sq ft going from R-13 to R-38, with 6,000 HDD, a 92% efficient condensing furnace, and gas at the same $1.20 a therm.

Current loss is 13,292,308 BTU a year and upgraded loss is 4,547,368 BTU. That saves 8,744,939 BTU, a 66% cut. At 92% efficiency, that is 95.1 therms, worth $114.06 a year.

This home has a third more heating degree days, yet it saves only half as much as the first example. Three things work against it. The area is 40% smaller. The starting R-13 already stops more heat than R-11. And the more efficient furnace burns less gas for each BTU saved. Climate matters, but area and starting R-value matter at least as much.

To see diminishing returns, hold the house fixed: 1,000 sq ft, 5,000 HDD, a 90% furnace, and $1.20 a therm. Then step the R-value up.

• At R-13, heat loss is 9,230,769 BTU a year.
• At R-30, it is 4,000,000 BTU. Going from R-13 to R-30 saves 58.1 therms, or $69.74 a year, a 57% cut.
• At R-49, it is 2,448,980 BTU. Going from R-30 to R-49 saves only 17.2 therms, or $20.68 a year, a further 39%.

The second step adds 19 points of R, more than the first step's 17, but saves less than a third as much money. Doing both steps at once (R-13 to R-49) saves $90.42 a year, a 73% cut.

Price each step against its own installed cost. The first inches usually pay back quickly. The last inches can take decades on heating savings alone.

Common mistake: entering R-0 for an attic that has no insulation. The calculator treats it as R-0.1, so upgrading 1,000 sq ft to R-30 shows $15,946.67 a year, which is not realistic. Rafters, drywall and trapped air add some resistance even with no insulation. If the same attic starts at R-3, the saving is $480.

ENERGY STAR, the efficiency program run by EPA with DOE, publishes recommended insulation levels for retrofitting existing wood-framed homes. It states they are based on Table R402.1.3 of the 2021 International Energy Conservation Code. Its attic recommendations are:

• Zone 1: R-30 in an uninsulated attic, or add R-25 on top of 3 to 4 inches already there.
• Zones 2 and 3: R-49, or add R-38.
• Zones 4 through 8: R-60, or add R-49.

Floor recommendations are R-13 in zones 1 and 2, R-19 in zones 3 and 4A/4B, R-30 in zones 4C, 5 and 6, and R-38 in zones 7 and 8.

Use the table to pick your target value, then use this calculator to see what it is worth. In mild zones the step above R-38 may save little, as the R-30 to R-49 example shows.

This is conduction through one surface during heating season. It leaves out air leakage, which insulation alone does not stop, and heat gain in summer. It also ignores thermal bridging: studs and joists conduct heat around the insulation, so a wall's real R-value is lower than its batts' label.

Heating degree days describe a typical year, not this winter. Real savings move with the weather. The formula also ignores solar gain and internal heat from people and appliances, which reduce how much heating a home needs.

Poor installation, with gaps, compressed batts or damp insulation, lowers real performance. The calculator assumes the rated R-value is fully delivered. It does not include installation cost or payback. For those, use a bill-based tool.

For payback on a quoted installation, the home insulation savings calculator works from your heating and cooling bill and adds install cost. Windows lose heat by the same U-times-area-times-degree-days physics, which the double glazing payback calculator applies to glass.

Once the envelope is tighter, the heat pump savings calculator compares what the smaller remaining heating load costs with each type of system. To see where insulation sits within the whole bill, the home energy audit calculator splits spending by end use.