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Age-old question of insulation payback — only heating costs? Why not the costs of cooling?

user-1107451 | Posted in Green Building Techniques on

I’m in lower NY state Zone 5 (just), I have R-38 batts in the ceiling and want to put plywood down after bringing insulation level up 6 inches (cellulose).

Now I’ve been looking all over Google for a payback calculator that handles both heating and cooling. I’ve seen the heating one on GBA. Or am I missing the fact that you don’t need to worry about the cooling if you have heating?

Sorry if It seems like a trivial question, but trying to figure the payback but it seems strange to me I can only find a heating payback not one where cooling in involved.

Anyone’s feedback is appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    To determine the payback of any energy-efficiency measure, you need to use energy modeling software. Once you have the software, you model the building two ways: first, without the measure under consideration, and then with the measure under consideration.

    If you do that, you'll know the savings (in kWh, BTU, or dollars) attributable to the improvement. Then (if you know the cost of the measure) you can figure the simple payback period.

    There are a great many energy software programs available that consider both heating and cooling costs. Among the available programs are HOT2000, eQuest, EnergyPlus, and REM/Rate.

    All of the software programs based on Manual J -- including Elite RHVAC, Wrightsoft Right-J8, Adtek AccuLoads, and EnergyGauge -- calculate the energy used for cooling as well as heating.

    If you don't want to learn how to use these programs, you can hire an energy consultant to perform the calculations for you.

    For more information on payback calculations, see Payback Calculations for Energy-Efficiency Improvements.

  2. user-1107451 | | #2

    Thanks will look at those programs, I'm just looking to make it a storage area after closing up the bsmt, so the attic hatch is set 6 inches higher than the ceiling so fill the R38 to R57, not 60 but close enough, and topped up in the future simply remove pieces of plywood.

    Im trying to figure numbers to see how much a 7.6KWA panels PVA would take to pay off being on my south facing unshaded roof.

    Any hints on where I can find a good calculator for PVA pay off would be great

  3. GBA Editor
    Martin Holladay | | #3

    I'm not sure what you mean by "PVA." If you mean "photovoltaic array," then the math is simple.

    1. Figure the anticipated annual output of the PV system at your location using a free online calculator called PV Watts.

    2. Research the cost per kWh of electricity for residential customers in your area. (Call up your local utility, or look at your electric bill.)

    3. Divide the cost of the system by the anticipated annual savings (in dollars) to determine the simple payback period.

  4. user-1107451 | | #4

    Full of answers
    Thanks again

  5. user-946029 | | #5


    Martin did a great job of explaining the simple payback period in #3.

    However, there are a couple of potential flaws in the simple payback logic .

    1. It assumes the system was paid for out of pocket, and not included in the mortgage. Each situation is going to have its own answer. But if you financed the PVA in the mortgage, you didn't pay the lump sum of that system upfront. So what is the monthly increase in your mortgage for adding the PVA? (Don't forget to include the incremental increase in interest cost.)

    2. The savings you create aren't mortgaged, so that is a real number. And, it's money you could earn interest on. Not much in the current economy, but there are interest-bearing financial vehicles available.

    3. The simple method also assumes static energy prices. Do you believe energy prices will remain stable? If not, your rate of savings will increase over time.

    4. Behavioral changes - This is a broad statement, and not directed specifically at you. If a homeowner experiences changes in their family (birth, death, child coming back home, parents/in-laws taken in, etc.), that will affect energy usage, and hence the savings (positively or negatively).

    Just some additional details to think about.

  6. user-1107451 | | #6

    I understand the shifting sands of payback, that why doing the savings of additional insulation, Ive air sealed the attics (holes, Ducts, top plate) and adding the extra insulation from r38 to r57 will drop my costs, The house is only 5 years old, the front of the house faces north so the whole south side of the roof is open for panels, with no shading.
    So I have 900 ft2 on the south side of the roof for panels
    Our actual payment idea is to see how much I can get away with for cash on hand, then add if I have to as time goes on, really looking for the best deal I can get and write off as much as I can on Govt rebates.
    As Im outside of NYC Im stuck with O&R as a supplier and the idea of giving them $350 a month is grating on me and since we will be here til our little kids grow we may as well bite the bullet and add solar.
    Solar systems on Ebay (7kva is $15000) add installation so should be out of it for 20K and if I can get 40% from rebates, basically the thing is costing me 12K and the PV watts is saying $1100-1400 per year so a payback of 8.5 to 11 years @15.7 cents per KWH (O&R)
    So Im trying to offset my footprint, while taking steps to superinsulate over time (when its time for the siding to be redone, redo with foam, tape seams etc
    And I must thank GBA giving me all the info I need to what I need to do (airsealing etc)

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