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A Payback Calculator for Insulation Retrofits

Calculate how long it will take for energy savings to pay back a planned investment in attic or wall insulation

Posted on Aug 24 2010 by GBA Team

Do you have a contractor’s quote for an insulation upgrade? Use our calculator, which is based on an equation from the U.S. Department of Energy, to find out how long it will take for your energy savings to pay back your investment.

1. Determine what type of insulation, and what thickness of insulation, you now have in your walls or attic.
2. Estimate the square footage of the walls or attic space to be upgraded.
3. Find a recent copy of your utility or fuel bill to determine the cost per unit of your space heating fuel.
4. Finally, have a copy of the quote from your contractor, or—if you plan to do the work yourself—the cost of the necessary insulation materials and installation equipment.

The equation that powers this calculator was developed by the U.S. Department of Energy. In addition to the details you enter, it relies on government-compiled information, including the average number of heating days per year in your region, the average efficiency of various heating systems, and the average performance of the insulation products available to builders and homeowners. All of these factors help provide a more accurate calculation, no matter where you live or what type of fuel you use to heat your home.

That said, this calculator is not perfect. For example, it does take into account fluctuations in energy costs. And it doesn't account for other factors specific to your home, like air leakage. For a more complete overview of things to consider when planning an energy upgrade, read Mike Guertin's A Complete Guide to Estimating Energy Efficiency Payback.

Aug 24, 2010 12:54 PM ET

by John Semmelhack

Here's a little feedback after using this tool for a couple of minutes:

1) Selecting "no insulation" (R-1) for the existing conditions leads to a gross over-estimate of the energy savings. I think Michael Blasnik typically includes this mistake in his presentations of why predicted energy savings are so often over-estimated, though I can't find a link at the moment. An empty 2x4 wall with exterior sheathing and interior drywall has an R-value of +/- R-4, depending on the framing factor, and thickness of sheathing, drywall and siding.

2) Northern climate bias alert! Why is there no heat pump option in the AFUE selection?

Aug 24, 2010 1:34 PM ET

by Michael Blasnik

I just tested this out for a typical older home served by weatherization (insulate walls in Ohio with older furnace) and found that the savings are over-estimated by about 1000% compared to real results.. The R-1 assumption is ridiculous -- I would actually use about R-5 for an uninsulated wall or attic. That one mistake leads to an over-estimation of savings of about 9x. The heating efficiency assumption for conventional gas is also far off. This type of calculator is far worse than the typical audit software out there that ONLY overestimates savings by 50%-100%.

Aug 24, 2010 5:09 PM ET

That was fast
by Daniel Morrison

Now do you guys think you can find my car keys too?

I'll have the uninsulated wall option increased to R-5.
I'll see about adding a heat pump to it, though that may take a little while.

Anything else while I'm at it?


Aug 25, 2010 11:47 PM ET

More feedback
by Katy Hollbacher

The idea of a simple, user-friendly tool is great--but I'm concerned that it might mislead people in its simplicity. You already mentioned that air leakage isn't addressed; maybe a default could be assumed, but allow users to enter more precise info such as location/wind exposure/whether current heating system is forced-air/etc.

Also, from what I can see, it looks like the tool assumes that a wall is entirely insulated (with no "framing factor" or voids reduction) and perfectly insulated. I'd love to see a 2x4 wall with fiberglass batts perform at R-13 (as calculator currently shows), but an accurate default is probably closer to 8 or 9. Again, how about assuming some defaults for framing factor and insulation quality, and allow the user to provide more precise info if they have it?

How about cellulose as an existing insulation option? And finally--rather than just providing the "simple payback" number (which, incidentally, I'm not a fan of and think we need to stop perpetuating) how about ROI over 30-50yrs, along with some assumption for increasing energy costs (which can wildly fluctuate, as you noted, but might conservatively be assumed to increase an average 3%... or again, maybe this could be adjusted by user)

Finally, it would be great to connect this tool to some kind of discussion about the non-payback benefits of insulation upgrades (and on a related note, who does a payback calculation before repainting or putting in a new floor?)... and OK, while I'm at it, how about a similar tool for windows or an HRV! ;) Thanks GBA.

Aug 26, 2010 7:59 PM ET

I'm beginning to think 'wiki-calculator'
by Daniel Morrison

I'll try to get the DOEi equation and the code that runs this calculator in a text file that I can add as either an attachment or insert into a wikipage. Then we can customize it the way we want to.

Maybe different insulation types can affect the air-leakiness factor of the final answer?
Maybe the answer is given in a range?
I'd love to add an option for adding the increase in energy costs -- a variable: Enter the annual energy cost increase that you want to assume. With that, the results are more concretely abstract, if that makes sense.

I like the rent or buy calculator at NY Times

The main value in a widget like this is in comparing scenarios.

Oct 18, 2010 10:35 AM ET

A Payback Calculator for Insulation Retrofits
by Savings Calculator

I read this blog & this type of calculator is far worse than the typical audit software out there that ONLY overestimates savings by 50%-100%. I like it.

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