# 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

1. | | #1

Feedback
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?

2. Michael Blasnik | | #2

really?
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%.

3. | | #3

That was fast
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?

Dan

4. | | #4

More feedback
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.

5. | | #5

I'm beginning to think 'wiki-calculator'
I'll try to get the DOE 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.

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

6. Savings Calculator | | #6

A Payback Calculator for Insulation Retrofits
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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smith

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