GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Calculating the ROI of polyiso under siding vs. just doing PV instead

agurkas | Posted in GBA Pro Help on

My pre-war Cape is getting more and more livable and less expensive to heat (I am about 1/4th the cost from when we dumped oil and got heatpumps). I got airsealing and attic blown in with cellulose last year by MassSave. They did dense pack rafters behind knee-walls and brought them into house thermal envelope, thought those areas of the roof are now unvented, so there is that.

In a couple of weeks, I am getting MassSave contractor again to densepack all empty walls with cellulose, so we are looking at what R1 walls turning into R12 or so? 2/3 of walls are empty in 3K sq ft house, so this should make a dent in my bills. Most windows are new too.

So now I am getting to the point where I am looking at my bevel cedar siding (likely original to the house) that has 4-5 coats of paint by now and peeling everywhere. I am wondering how many years I got left in it and at which point and what I want to do about it. I am also looking into PV, since I have ditched single-pipe steam and happily enjoy our heatpump minisplits.

Knowing I will have ~R12 in my walls, I want to figure out is it worth spending $$$ on 3″-4″ of PolyISO to triple the R value, when I redo the siding. Underlayment right now is T&G boards with felt paper over it.

For house like mine would be what about ~$20K in additional materials and labor, which is what 7kW PV system would cost after tax credits etc.

P.S. Is there easy to use energy modeling software for Mac out there fairly nerdy person, like me, can use without being building scientist?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    I would suggest you use BeOpt software. Here is the link:

  2. Expert Member
    Dana Dorsett | | #2

    With both the polyiso and the PV you'd probably come close to hitting Net Zero Energy. (It may only take 3" of polyiso to get there, depending on the other insulation & window details.)

    You'd be able to figure that out with BeOpt too.

    What you can't figure out with BeOpt is just how much more comfortable R30-35-ish whole-wall houses are compared to R12. The higher R walls raise the mean radiant temperature in the rooms, which makes it palpably more comfortable.

    The other thing you can't figure out with BeOpt is how much more resilience is added by keeping the structural wood warm. When you dense pack the walls the sheathing and siding will run colder & wetter, which accelerates the rate at which the multi-layered siding paint goes down hill. Without a rainscreen for the siding and sheathing to dry into, any moisture accumulation on the exterior layers takes a long time to dry out. Moisture in the wood under impermeable paint expands and vaporizes when the sun shines, causing the paint to separate and crack, eventually flaking and peeling.

    Cedar can hang in for a long time even when wet, even when the paint is peeling. If decide to keep the cedar siding, scrape any flaking/peeling paint down and use a stain rather than paint for the new coat. Stain doesn't peel, and allows the wood to dry far more quickly than multiple layers of paint. The sections where the paint is still bonded may eventually fail (some of it surely will), but staining over that in 5-10 years or whatever can keep cedar going almost indefinitely.

    Given the vintage the paint almost certainly contains lead, so it may be worth getting rid of it altogether rather than keeping it going.

  3. agurkas | | #3

    Funny you would mention drying. The siding on East and South side of the house is warping in some places, while North side is peeling the most. West side - like brand new.

    I know outside trim had lead, so I would not be surprised if siding does too. Now it is more of a question of where to put the investment in the house. Net Zero is all fine and dandy, except achieving it just doesn't sound like worthy of investment due to ROI.

    I do a lot of DIY, but paint is one thing I hate hate hate doing and considering what painters cost in Boston (and likelihood there is lead under there), sound like I will need to redo the siding. Just need to figure out if dumping money into foam is really going to put a dent into my averaged $250ish monthly all in utilities bill enough or is modest PV system worth it more than foam.

    We run the house 68 in the winter and 76 in the summer. No complaints from the big boss yet. Except for excitement in her eyes, when she saw me take out the single pipe steam radiators.

  4. charlie_sullivan | | #4

    If you just want to calculate the difference in heat loss through a wall at R13 vs. R35 or whatever you decide to consider, it's much simpler than estimating the overall heating requirements. All the messy stuff like infiltration or solar gain or below grade heat flow are irrelevant, and you can just consider the heat flow through however many square feet of wall area with those two R-value options, and with a number of heating or cooling degree days for your location from The one messy part that remains is estimating the COP of your heat pump system, but I don't know of any software package that know how to do that, so we're left with taking a guess on that either way. The calculation is just BTU/year = HDD*24*Area/R-value for heating and the same with CDD for cooling. That ignores the second-order effect that the insulation has on the base temperature for the HDD calculation.

    Peeling lead paint worries me, not just for how to deal with it when you do new siding, but for reducing the amount that goes into the soil in the meantime. A friend's child ended up with high blood levels of lead, which they traced to vegetables grown in their garden next to a house with lead paint, some of which had gotten into the soil at some point.

    In addition to Dana's points about comfort and resilience, one other not-yet-monetized benefit is that your contribution to peak loads on the grid on cold nights would reduced with better insulation. At some point residential customers might start getting charged for that. And your ability to ride through a power outage would also be improved.

  5. agurkas | | #5

    I already de-leaded inside of the house. Took a month of nights and weekends. Honestly never want to deal with that again. In retrospect, if I had to do it again, I would have ripped out ever piece of trim and every interior door and replaced it. Live and learn.

    Siding, I hate clapboard stuff with passion. Can't wait to replace it with something that looks 100% more modern. '38 Cape is not historic (especially with '58 and '77 additions), unless you want to study how not to do things ever again.

    COP for heatpumps I calculated somewhere around 2.7 for 2014-2015 heating/cooling days. Not the 3.0 I was hoping, but hey, I am paying 1/4th of what I did before I installed them.

    Wonder if there is even simpler rule with calculations. I remember someone had calculation for insulating steam pipes where 0.5" was too thins, 1.5" was optimal ROI, and 2"+ stuff had really poor ROI.
    So Zone 5 would be R12 is X, but R"y" is optimal cost to ROI.

  6. charlie_sullivan | | #6

    A rule like that would be based on one set of assumptions about the efficiency of the heating system, cost of fuel and cost of insulation. In your case, cost of heat is low and cost of further insulation is high so you are pretty fare skewed from standard assumptions. And it exaggerates the abruptness of the cutoff. (The cuvre of heat loss vs insulation thickness is different for pipe insulation, for geometric reasons in any application, and for more complicated reasons in intermittently used DHW pipes.)
    Since it sounds like you have a good idea of COP, we can easily figure this out given the area and either HDD or location.

    I did not mean to suggest re-suggest de-leading your siding, which would silly given that you don't mean to keep it. I am just thinking about trying to keep it from getting mixed into the dirt. Possible ways to deal with it would be to simply lay a 2 foot wide strip of landscaping cloth around the perimeter to facilitatecollecting the chips, slapping some cheap paint on to temporarily stick the chips in place, wrapping in housewrap temporarily to catch chips, or flaking off the worst of them with a HEPA vac without trying to remove anything that wasn't about to fall off anyway, with plastic on the ground to catch what falls in that process. Or just plan to shovel out the top 6" of dirt immediately around the house when you do finally replace the siding, which might be a good idea in any case.

  7. GBA Editor
    Martin Holladay | | #7

    Charlie and Apollo,
    Warning to you and to GBA readers: Don't attempt DIY lead abatement unless you've taken a course and become certified in lead abatement. There are too many ways you could inadvertently poison your family.

  8. agurkas | | #8

    I wholeheartedly agree. I took the online course, read a manual on the subject, bought the special EPA-approved vacuum, approved IR paint peeler, etc. In the end - not worth the effort to save any trim or doors with lead. Took me 6 hours per door.

    In my case, I have whole dang acre around my house, there aren't going to be any kind of vegetables anywhere within 100 yards from the house. And most of the house should have 4' concrete "skirt" by the foundation, which is easy to vacuum.

    My next task is to calculate wall square footage.

  9. Expert Member
    Dana Dorsett | | #9

    The siding on the north side is always the wettest, since it gets the least amount of solar gain to help dry it out. The paint is going to fail there first/most.

    The sunnier sides are drier due to the direct solar gain, but without a rainscreen it dries unevenly, causing warping/splitting.

    There are no simple rules on ROI- energy costs vary by location (a lot), even within zone 5, and also vary (a lot) over time. The actual return is dependent on both present and future energy costs, the cost of money, etc. But code minimums are VERY financially conservative on a lifecycle basis. In most markets going quite a bit higher can be financially rational even without energy price inflation assumptions. R12 whole-wall (U-0.83) is not code-min for zone 5. According to table TABLE N1102.1.3 (R402.1.3) it needs to be U-0.057 (= R17.5 whole-wall, including interior & exterior air-films), so you're quite a bit shy.

    An inch of polyiso would get you to code min, and would be sufficient dew point control in your climate (even after derating for temp), but the labor cost for 2" is about the same as 1", and very likely to be "worth it" on a medium or long term energy cost basis.

  10. jamestimmerberg | | #10

    Dana said: "What you can't figure out with BeOpt is just how much more comfortable R30-35-ish whole-wall houses are compared to R12. The higher R walls raise the mean radiant temperature in the rooms, which makes it palpably more comfortable." Could somebody explain that further?

  11. agurkas | | #11

    Here is how I understand it in non-pro terms. You probably have been in several houses where in one 68-degree house you feel more cold than in another one where temperature seems to be exactly the same.
    In one house walls have more insulation, therefore if most object and air radiating heat are at 68F, perimeter walls might be closer to that temp too. But in a house with much less insulation (like mine), rooms feel often colder, if not much colder.
    I know exactly in my house which rooms have some insulation and where there is none. My wife complains about temperature, no matter what thermometer I show her (we have negotiated 68F being temp I keep house at). It is because walls radiate much different temps

  12. GBA Editor
    Martin Holladay | | #12

    Your skin is warm. Your warm skin is continuously radiating energy to nearby surfaces that are at a lower temperature than your skin. The mechanism is radiant heat transfer.

    If you sit near a big block of ice, your bare arms will radiate heat to the block of ice. You may want to put on a sweater in that case, to reduce the radiant heat flow.

    Similar radiation happens between your bare skin and window glass, and between your skin and the drywall on exterior walls. If the exterior wall is uninsulated, the drywall is cold, so your skin feels the radiation more than when the exterior walls are insulated and the drywall is warm.

  13. Expert Member
    Dana Dorsett | | #13

    In my antique 2x4/R14 celluose insulated house I've measured wall temperatures at about 60F, with striping at the studs in the mid-50s when it's in negative single-digits outside, at an indoor air temperature of 68F. With eyes closed under those conditions it's easy to tell the direction of the exterior walls vs. the partition walls by turning my head, by the sensed radiant temperature on my face.

    But in the rooms in my house with radiant floor, it feels warmer at 65F air temperature than the other rooms at 68F, due to the elevated average radiant temperature of the room conditioned by the direct radiation off the ~74F floor. Even though the cooler walls can still be sensed with eyes closed, it's pretty comfortable.

    The rooms heated with radiators get a similar, if less pronounced comfort boost at any given air temp due to the direct radiation. It's easy to tell where the 125F radiator is with my eyes closed, in much the same way that I can identify the exterior walls with eyes closed. The rooms served by the hydro-air zone don't have that, and need to be warmer to be comfortable.

    In a house insulated to Pretty Good House levels it takes a fairly temperature-sensitive face to tell where the exterior walls are, since the exterior wall temps are in the mid-60s, while the partition walls are at the 68F interior air temp. It's usually the windows that give it away, not the walls, since the surface & radiant temp of the windows are usually quite a bit cooler.

    The comfort factor of having the coldest walls in the room still well north of 60F when it's 0F outside would be dead obvious in an eyes-open walk-through, if one were to build a demonstration house with rooms built to code-min right next to rooms built to R30+ whole-wall, & U0.25 or lower windows.

    IIRC minimum wall surface temperature is spelled out in the PassiveHouse or PassivHaus specs (maybe both), but when it's in negative single digits F outside the coldest walls in a PassiveHouse will be at most a couple degrees below the air temp, maybe a degree or two higher than an R30 whole-wall assembly.

    So, it's hard to put dollar numbers on the value of the additional comfort factor, but mean radiant temperature (MRT) it's a well understood phenomenon. It's why I'm just fine skiing in a T-shirt at mid-day when it's 15F, sunny and calm, right up until I'm in the shadows of the trees and suddenly freezing my butt off. :-)

  14. agurkas | | #14

    Another thought, I am bit too sensitive to the smells of spray foam inside, but I have seen somewhere blog post about how house was insulated by attaching 2X4s to existing exterior sheething, then spraying foam right over the exterior (without removing old siding), and then new siding was placed over that.
    Wonder ballparkey if that would be less costly way of add Rs to the wall while residing the house.

  15. Expert Member
    Dana Dorsett | | #15

    Adding on Larsen Trusses to fatten out the R-value works, but isn't worth it unless you fattening up a lot. It takes a 5.5-6" deep truss to hit the thermal equivalent of 3" of polyiso using open cell foam or cellulose.

    Adding 2x4s and open cell spray foam is the thermal equivalent of 2" of rigid polyiso, and a lot more labor involved, and not necessarily cheaper on material cost.

    If going for big-R, say R50 whole wall, adding trusses may be easier/cheaper than a foam-over.

    Also note, in Massachusetts there are a multitude of vendors of reclaimed roofing foam, at prices well under half that of virgin-stock goods, which makes a foam-over retrofit a LOT cheaper. (The most I've personally paid for a 4' x 8' sheet of reclaimed roofing iso was $20, often available for $12-15 in decent shape. Virgin stock goods f.o.b. the yard at Harvey run about $55-65/sheet for 3".) The two biggest are Nationwide Foam in Framingham, and Green Insulation Group in Worcester, but there are others:

    If / when I'm ready to strip the 93 year old clapboards off my place that's where I'll be shopping, probably two layers of 2" (for the price of a single layer of 1" of virgin-stock polyiso). YMMV.

    Also note, if you happen to be in National Grid's operating territory you can get major rebate subsidy for a partial Deep Energy Retrofit such as adding 4" of foam to 2x4 walls. See:

    This is a pretty good program. A few years ago between using reclaimed foam and working with the Nat'l Grid DER program I talked & walked a friend who was in the middle of a full-gut rehab into Pretty Good House performance levels for about the same money he was going to spend on just the re-insulation (at below code min) and the cosmetic do-over. It's a 3-story antique now heated with a single mini-split head per floor. He's still talking to me (and his wife is still talking to him :-) ), so despite the slipping schedule nearly a year for dealing with the foam-over & subsidy, it all worked in the end. If you're only signing up for a partial DER it won't add too much, due to the narrower scope of the project. But it'll still be more time & money than slapping up new siding on your sub-code walls.

  16. agurkas | | #16


    So I calculated I am working with 1350sq ft of wall I am going from zero insulation to 3.5 of cellulose, to whatever will be on top.

    I actually got reclaimed roofing iso for $5 per sheet from roofing contractor last winter and insulated my basement. Snowstorms were coming and guy did not want to put sheets in his warehouse, so I got a deal. He let me pick them out too.

    I shot over a note to my architect. We weren't thinking just yet about siding, but if DER takes that long, might as well start the process early.

  17. Expert Member
    Dana Dorsett | | #17

    If going for the subsidy, start the conversation with Nat'l Grid early, before you've fully detailed the build out plan. If all you[re doing is a foam-over on the sheathing under new siding the paper shuffle and analysis is very well bounded, and shouldn't delay anything if you have it planned in advance rather than trying to wing-it while modifying the goals of a project already in progress (as my friend did.)

  18. agurkas | | #18

    Good advice. I will chat with my awesome architect about DER again. Blower door test is coming after cellulose insulation and something tells me it still will be not pretty. Will be lucky if they even get the place to pressurize.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |