Insulation calculator: R-value, thickness and energy savings

This insulation calculator answers the three questions you have before you buy a single bag or batt. What R-value will the attic or wall have after you add insulation? How much insulation do you need to reach a target such as R-49 or R-60? And what will it save you each year, so you can judge the payback?

Start from a preset or type in the R-value of what you have now. Pick a material from the list; each one has a typical R per inch that you can overwrite with the number printed on the package. A thickness table shows the result for several depths side by side, and a simple materials line multiplies area by your installed cost per square foot.

US units (R-value, square feet, inches, °F heating degree days) are the default, and one click switches to metric (RSI, U-value in W/m²K, cm). Export the results to PDF or Excel for free, with no sign-up. Everything is calculated in your browser; nothing is uploaded.

R-values of layers add up. Adding insulation to an existing assembly gives:

R(new) = R(existing) + thickness in inches × R per inch

Heat flow is the inverse, the U-factor: U = 1 / R (Btu per hour, per square foot, per °F). The calculator works the same way in metric: U' = 1 / (1/U + d/λ), where d is thickness in meters and λ is the conductivity in W/mK. One US R equals 0.1761 m²K/W.

To find how much insulation you need for a target:

inches = (R(target) − R(existing)) / R per inch

Yearly savings come from heating degree days (HDD, base 65 °F):

heat saved (Btu/yr) = (U(old) − U(new)) × area × HDD × 24

Divide by 3,412 to get kWh, or by 100,000 for therms. Then multiply by what a unit of delivered heat costs you: fuel price divided by furnace or boiler efficiency.

A 1,000 sq ft attic floor has old insulation that rates about R-11 in total. You add 12 inches of blown-in cellulose at R-3.5 per inch. The home sees about 5,000 °F·day HDD a year.

1. New R-value: 11 + 12 × 3.5 = R-53.
2. U-factors: 1/11 = 0.0909 before, 1/53 = 0.0189 after. The difference is 0.0720.
3. Heat saved: 0.0720 × 1,000 × 5,000 × 24 = about 8.64 million Btu a year, which is about 2,534 kWh or 86 therms of heat.
4. If delivered heat costs you $0.05 per kWh (enter your own fuel price and efficiency), that is about $127 a year. Enter your installed cost per sq ft and the calculator shows the simple payback.

To reach R-60 from R-11 you need R-49 more: 49 / 3.5 = 14.0 in of cellulose, or 49 / 2.5 = 19.6 in of blown fiberglass. Settled depth is what counts, so check the bag chart for the installed depth that delivers the rated R-value.

Typical R per inch used as defaults (always prefer the R-value on the product label):

Material	R per inch
Blown-in fiberglass	2.5
Fiberglass batts	3.4
Blown-in cellulose	3.5
Open-cell spray foam	3.7
EPS rigid foam	4.0
Mineral wool batts	4.2
XPS rigid foam	5.0
Polyiso rigid foam	6.0
Closed-cell spray foam	6.5

ENERGY STAR publishes attic recommendations based on the 2021 IECC (Table R402.1.3):

Climate zone	Uninsulated attic	Existing 3–4 in
1	R-30	R-25
2 and 3	R-49	R-38
4 to 8	R-60	R-49

Local code can differ, so check with your building department for new work.

• Air seal first. Gaps around top plates, can lights and attic hatches leak warm air that no R-value stops. The formula assumes an airtight layer.
• Do not compress batts. A batt squeezed into a shallower cavity loses R-value.
• Framing is a thermal bridge. Studs and joists conduct more heat than the insulation between them, so a wall's whole-assembly R is lower than the cavity label.
• Savings are not your whole bill. Only the insulated area changes; windows, air leakage and hot water stay the same. For whole-room heat loss see the radiator size calculator.
• Diminishing returns are real. Each added inch saves less than the one before, which the thickness table makes easy to see.

The result is an estimate for planning, not an energy audit or financial advice.