Exterior foam insulation: the minimum vs. “extra credit”
I’ve read Martin’s article about how to calculate the minimum thickness of exterior foam as I planned for my re-siding job in Minnesota. My 1960s home has never been re-sided and has the basic fiberglass stuffed into stud bays with no vapor barrier (a good thing.)
But, is there an easy to to calculate the payback of “extra credit”, in that I use the thickest foam that I can? The minimum R value for my zone is R 7.5. I had planned on using 4 inches of XPS to get to R20. I had planned out the number of HeadLok 7.5” screws to attach the furring strips into the studs. But then I got to thinking – why not go with 6 inches of foam? It will add cost for the extra layer of foam, sure. I’ll need to switch to Strong-Tie SDWS screws. I’ll need to buy some wider coil stock for my z-flashing, and I’ll need wider jamb extensions. But is there a way to calculate if this extra 2 inches will pay off over the expected lifetime?
Thanks.
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Exactly how worthwhile it is depends on a number of factors, like the amount of glazing, quality of the windows, and level of air sealing. If you have those figures, you can use a free modeling program like BEopt to figure out the energy benefits, and from there figuring out payback period is easy.
If the question is if it will pay off over the expected lifetime of the insulation, the answer is almost certainly yes, since foam lasts a long time. But it is important to account for opportunity cost as well. Prioritize using the money for good air sealing first, as that will make a more noticeable difference. Solar panels and a heat pump might have a shorter payback period as well, if site conditions are favorable.
DIY opinion
One approach would be to size your heating and cooling load with or without the extra insulation. That would tell you an estimate of how many BTUs you might save. You might find that your windows contribute so much load such that 6" foam makes no difference vs 4" and the money is better spent on other areas. Depending on your sewer stack access, house layout and occupancy, and water heating method, a drain water heat recovery unit might be a better investment for energy savings.
The other issue with using a heat model is that none of the freebie software I've used will accommodate that much exterior R. Perhaps BeOpt would? You might have to have a pro sizing done.
Also while I don't have a reason for you not to use foam, did you look into Larsen trusses? Once you are at 6"+, they might make more sense than a thousand expensive screws. You can also use batt insulation in them (I think).
At 4" XPS you're really only warranteed for R18, not R20 (read the fine print), and at full depletion of the climate damaging HFC blowing agents it'll be ~R16.8.
Using a double layer of cheap 2" reclaimed fiber-faced roofing polyiso (stagger and tape the seams) you'd be over R20 at a fraction of the price.
"Pay off" has a lot of cost variables to factor in- it's more than just a bet on future energy costs. As a starting point for the "will it pay off ever" question take a look at Table 2, p10 of this document, but read at least the entire first chapter for context:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
Take a look at row 6 for southern MN, row 7 if northern MN. As a crude estimate consider the fiberglass insulation, studs, wallboard, and siding as the first R10 of the whole-wall R. So 4" of roofing polyiso would bring it over R30, pretty close the financial rationality tipping point for zone 6, and 6" would be over R40, the suggested tipping point for zone 7. A layer of 2" + 3" roofing foam (R35+ whole-wall) might be the sweet spot... or not.
Using inexpensive reclaimed polyiso and sweat equity labor rather than factory-fresh XPS is cheaper, and might extend the rationality point to 6" or even more, but it might still be a better investment to put the money for the last 2" into rooftop PV solar than insulation. The price of rooftop solar has dropped more than 60% in the decade since that study was published, and the efficiency of cold climate air source heat pumps have improved 15-20% too. At the utility scale the cost of PV has dropped more than 75% since then, and midwestern wind by half, so leveraged by heat pumps the cost of energy is likely to be falling over the lifecycle of a building, not rising. To be sure building with 4" of retrofit foam is easier than 6", and the ROI on that last 2" is small.
You can run a net present value calculation on future energy savings analysis at reasonable discount rates under different energy cost scenarios if you like, to prove to yourself even at a modest energy price inflation the last 2" is not paying off soon even when getting it at 1/3 the cost of virgin-stock foam.
Dana,
I thought polyiso was a bust in Minnesota due to condensation of the trapped blowing agent below -5F or so? I'd certainly expect him to see -5F or below enough in Minnesota to wonder about it. Why not just use a different product?
I'm also not sure what the argument for air source heat pump in Minnesota would be. He likely has natural gas service and his design temperature is probably in the -13 F range, near the operational limit of the best low ambient air source heat pump. This type of equipment also seems to have high install costs (even if it shouldn't). I love the idea we are going to dump natural gas but it's still a loser in my area with 0F design temperature; I'm not sure how to reconcile it to -13F or lower.
You are definitely the expert here, not me. But it seems like he'd be better off pursuing EPS or a batt and truss system than polyiso.
Jay,
Here is a link to an article that explains all of the steps involved in your calculation: "Payback Calculations for Energy-Efficiency Improvements."
As my article makes clear, it's fairly easy to manipulate your assumptions until you get the answer you want to get. So be careful to hold your own desires in check when settling on your assumptions.
Here's what my gut says: It's unlikely that the payback associated with an increase in rigid foam thickness from 4 inches to 6 inches will be as good as the payback associated with taking the same amount of money and investing in PV.
Using payback analysis for long-term durable goods such as homes or the performance of the enclosure is not meaningful on its own; long-term investments that last the service life of the building need to be captured in the value transfer when you sell the home.
So part of the answer to is it worth it is tied to: can you get a buyer to value it? If you can, to hell with the payback on its own. If you can't, then nine times out of ten, these sorts of incremental improvements in energy performance are not justfied and made even more difficult with highly variable and unpredictable costs of energy.
See this blog for more info: https://www.buildinggreen.com/blog/capture-green-value-over-time-not-short-term-payback-analysis
Peter