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Attic Insulation Payback Formula

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Posted by drydem on September 9, 2008, 12:21 am
Two years ago I upgraded my attic insulation to R38 --- recently I
looking at possibly increasing my attic insulation up to R76 - and I
wanted to know what was the payback (return on investment) - after
much surfing I found the formula below.  HTH

The following webpage coves estimating
the payback for adding Insulation to the Attic


(incase the above webpage dissappears ....)

the equation is as follows:

    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:

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.
 ( an energy star non-geothermal HVAC is typically going to have an
E=2.5 )

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.
( e.g.  for HDD for MD-DC-VA see [1])

24  =  Multiplier used to convert heating degree days to heating hours
(24 hours/day).

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 $.87/therm for natural
gas. Let's also suppose that you supply the following values for the
variables in the formula.

C(i) = $.18/square foot

C(e) = ($.87/therm)(100,000 Btu/therm) = $.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

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)      ($.0000087 =
7000  24)

90.288      16.077   =  5.62 years

district of columbia

Posted by CJT on September 9, 2008, 1:03 am

I don't see how the units work out right in that formula.

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Posted by Jim Wilkins on September 9, 2008, 11:59 am
The infrared thermometer and snow melt pattern show that most of my
remaining attic heat loss is around the edges where I couldn't stuff
in more insulation due to restricted space and protruding roofing
nails. More insulation in the middle wouldn't help much if that isn't
where the larger losses occur.

Posted by drydem on September 13, 2008, 2:02 pm
Sound like a great idea! The farmer's almanac predicts
lotsa of snow this winter for the Mid-Atlanic area so maybe
I'll get a chance to do just that....

Posted by ransley on September 12, 2008, 3:10 pm
Research how Fiberglass batts loose R value as temps drop. Depending
on where you live and winter lows you can loose 20-30% R value as the
attic gets colder. Also settling. I put in R 100-110 fiberglass batts
and it settled to R 70. Best is Foam. What is your Zone. Settling and
R value loss at -f temps increase payback.

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