The Dept of Energy has a page on computing payback periods for adding insulation (link below, along with an extended quote).
The inputs to the decision are simpler than you might imagine. The major determining factor in your situation (already have pretty good insulation) will probably be the cost of your heat. If you are using electric resistance heat and paying 20 cts per kWh, you could probably pack the attic full and still have a decent payback interval.
EERE Consumer's Guide: Estimating the Payback Period of Additional Insulation
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From the website:
Years to Payback = (C(i) × R(1) × R(2) × E) ÷ (C(e) × [R(2) - R(1)] × HDD × 24)
To calculate the payback, you must supply the following information:
C(i) =
Cost of insulation in $/square feet. Collect insulation cost information; include labor, equipment, and vapor barrier if needed.
C(e) =
Cost of energy, expressed in $/Btu.
- To calculate the cost of energy, divide the actual price you pay per gallon of oil, kilowatt-hour (kWh) of electricity, gallon of propane, or therm (or per one hundred cubic feet [ccf]) of natural gas by the Btu content per unit of fuel.
- To figure the price you pay per unit, take the total amount of your bills (for oil, electricity, propane, or natural gas) during the heating season, and divide it by the total number of gallons, kWh, or therms you consumed during those months. Use the following values for fuel Btu content:
- #2 Fuel Oil = 140,000 Btu/gallon
- Electricity = 3,413 Btu/kWh
- Propane = 91,600 Btu/gallon
- Natural Gas = 103,000 Btu/ccf
- or 100,000 Btu/therm
E =
Efficiency of the heating system. For gas, propane, and fuel oil systems this is the Annual Fuel Utilization Efficiency or AFUE. Typical AFUE values are 0.6 to 0.88 for oil or propane furnaces, and 0.7 to 0.95 for natural gas furnaces. Older systems are usually less efficient. Use E = 1.00 for baseboard electric systems. For heat pumps, use the Coefficient of Performance or COP for E; where E = 2.1 to 2.5 for conventional heat pumps, and E = 3.2 to 3.5 for geothermal heat pumps.
R(1) =
Initial R-value of section
R(2) =
Final R-value of section
R(2) - R(1) =
R-value of additional insulation being considered
HDD =
Heating degree days/year. This information can usually be obtained from your local weather station, utility, or oil dealer.
24 =
Multiplier used to convert heating degree days to heating hours (24 hours/day).
Example:
Suppose that you want to know how many years it will take to recover the cost of installing additional insulation in your attic. You are planning to increase the level of insulation from R-19 (6-inch fiberglass batts with moisture barrier on the warm side) to R-30 by adding R-11 (3.5-inch unfaced fiberglass batts). You have a gas furnace with an AFUE of 0.88. You also pay $0.87/therm for natural gas. Let's also suppose that you supply the following values for the variables in the formula.
C(i) = $0.18/square foot
C(e) = ($0.87/therm)÷(100,000 Btu/therm) = $0.0000087/Btu
E = 0.88
R(1) = 19
R(2) = 30
R(2) - R(1) = 11
HDD = 7000
By plugging the numbers into the formula, you obtain the years to payback:
Years to Payback = (C(i) × R(1) × R(2) × E) ÷ (C(e) × [R(2) - R(1)] × HDD × 24)
Years to Payback = (0.18 × 19 × 30 × 0.88) ÷ ($0.0000087 × 11 × 7000 × 24)
90.288 ÷ 16.077 = 5.62 years
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Other samclem notes:
1. Note that the formula requires the input of your Heating Degree Days. Most sources base this on a base temp of 65 degrees. That's fine if you realy keep your house at 65 degrees. It can be difficult to find data with a differnt temp base than 65 deg F, but this site does it. They seem to have info from airports around the world.
Heating and Cooling Degree Days Calculated to Your Specification
2. Note that the payback period is based on present fuel costs. My guess--things will get more expensive.
Happy crunching!