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Should I add more insulation in my attic??
Old 05-27-2008, 08:26 PM   #1
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Should I add more insulation in my attic??

Reading a post about reflective barriers got me thinking...

I have one layer of 4" fiberglass approx. r-value of 12. And what used to be 12" of cellulose, now closer to 9", total r-value of approx 30. That puts me at a net r-value of 42-ish.

49 I hear is recommended here in ohio. If I do it, I'd likely add about 8",which will reduce to 6" in time, giving me a grand total of approx. r 60+

I wonder what the real-savings would be of upping my attic r-value to 60 from 40-ish. My walls are all packed with cellulose already, and windows are new.


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Old 05-28-2008, 04:35 AM   #2
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Live in Dublin so I would suggest that being sure there are no places the WIND can infiltrate the living space or into the duct work would be my first concern. After ensuring everything is air tight then I would consider adding insulation. Not sure what the payback period would be but if you had records of past utility bills with the amount of KWH and CCF (assuming gas heat) you could tell sometime after doing it. Be sure that you do not block venting air on the underside of the roof. Also you may want to wait for another tax credit for energy improvements - they expired on 12/31/2007 and hopefully will be reinstated soon (savings was about 10-15% in cost as I remember). At R-42 already IMO another R-18 may be a bit of overkill.

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Old 05-28-2008, 06:00 AM   #3
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Ditto on on RWood's comment on air sealing. It is the most cost effective. Air infiltration and leaks through cracks raises the number of air changes per hour. Each air change then must be re-heated.

Generally it is best to start on the top. Then bottom of house. Remember, temperature differentials drive air flows. Light fixture boxes have many holes, wire feed throughs in wall top sills, any piping, ductwork, etc. need to be caulked, to keep warm air in. (if it can't leak out, no cold air comes in to replace it.)

Knee walls of dormers tend to be a real pain, ofter poorly constructed, if you lift trim of baseboards often you'll find 1/4" gaps to the outside. Multiply 1/4" by the length of baseboard, equivalent hole is big.

Insulation as a rule does not stop airflow, so to prevent warm air from getting out, seal cracks. Then if you have forced air, there are duct sealing issues. But thats probably a long thread all by itself.

So get out the incense sticks hold it up to window frames, baseboards to see which way the smoke goes. This is mostly for colder climates. If air conditioning is the primary climate control, air flows are a bit different.

It is fairly tedious work,but pays off big time.
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Old 05-28-2008, 08:01 AM   #4
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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

************************************************** ****************************************
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).

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

************************************************** **
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!
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Old 05-28-2008, 10:33 AM   #5
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Oops, two other notes:

1) The above calcs only take into consideration the payback period based on heating. While Ohio has fairly mild cooling requirements, the insulation wil help out there, too. So, maybe factor that in.

2) You won't be able to make your entire ceiling R-60 due to the physical space restrictions from the roof deck. This won't affect your payback period (since you won't be paying for the insulation or getting the benefit from it in the areas of low roof, but it will impact your total heating bill.
This might not be a big deal if you've got a simple gable roof with a steep roof pitch, but it can be a more significant factor with a shallow-pitch hip roof design (which is what my house has). For example, if your roof has a 14:1 pitch and the present insulation is just at the roof deck at the edges, then you won't have room to add the full 8" of insulation you want until the roof has risen that distance. That will take a little over 9 feet.
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