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Pretty Good House R-Values

Mauro_Zammarano | Posted in Pretty Good House on

My rater used REM/Rate modeling software to model my future net zero house and help choosing the insulation. He concluded that with the current cost of solar panels the most cost effective way to build a net zero house in Zone 4a is to just choose R49 attic (U=0.02), R21 walls (U=0.058), R15 foundation walls, slab floor uninsulated (U= 0.365), floors R38 (U=0.028).
Now PrettyGoodHouse last book suggests whole-wall R value of 8 for slab, 15 for basement walls, 30 exposed floor, 25 for walls, 60 for attic in zone 4.
These value are based on a 2010 paper “Building America Special Research Project: High R-Value Enclosures for High Performance Residential Buildings in All Climate Zones” when solar panel prices/kW were much higher. So my question is: are these value obsolete at least in terms of cost effectiveness? Is there any other major reason why we should still use them besides cost? Like potential condensation problems, comfort, etc. In terms of comfort wouldn’t windows be  the biggest offenders anyway. But what about condensation? Any reason why I should still go with higher R values?

Another issue I am having with the book is that it reports a whole R value of 19.74 for 2×6 wall with cellulose @24 in oc.   Ekotrope online calculator for the same wall is giving me R16. Which one is right, or what am I missing? Thanks!

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  1. gstan | | #1

    Let's start at the bottom and work our way back to the beginning - the Ekotrope online calculator is correct - this is because all walls are going to be between 20 and 25 percent
    wood, the book merely listed the value for the insulation by itself. This is probably of little
    importance in figuring out what's cost effective and what isn't.
    The problem with "cost-effective" is that there are several unknown variables which must
    be replaced with your best guess. First how long will you likely live in the house, second
    what will the interest rates be during that period, third what will the total inflation be for
    the entire period, fourth what will happen to energy prices during the period. Most (raters, designers, architects, and other well meaning advisors) will tell you the "cost-effective" value
    is what they calculate using today's values. Having lived through the almost unbelievable
    inflation, interest rates, and fluctuating energy prices between 1970 and 2000 I strongly
    advise you to design and build the most energy efficient (rather than cost-effective)
    structure your budget can stand! The PrettyGoodHouse Book will give you a pretty good
    overview but when in doubt opt for more (insulation and solar electricity capacity), not
    less! Best of Luck !

    1. DC_Contrarian_ | | #2

      The cost-effectiveness equation is a little different if you're shooting for net-zero. At every point you can try to use less energy, or add more solar. So the time element is removed.

      Whether shooting for net-zero is itself cost-effective is a completely different question, and that is complicated by all the factors you list, plus questions about whether utilities will continue offering net-metering indefinitely. But with some municipalities starting to require that new construction be net-zero or net-zero-ready that question may be moot.

    2. Expert Member
      MALCOLM TAYLOR | | #3


      Yeah, it's certainly not a straightforward calculation. Our energy costs over this summer in our (not particularly efficient) all electric house were $2 a day. Come winter it will go to maybe $5. It would be insane for me to make any assumptions about how efficient I should make a house base solely on those current numbers.

    3. Mauro_Zammarano | | #4

      Thank you for your answers. I understand that future cost of energy, inflation, interest rates are unknowns but for a net zero energy most variables become irrelevant because:
      1. In first approximation by design - we assume that we will install enough solar panels in the house to produce the energy we will consume on an yearly average
      2. I am building in few months from now so let's assume that cost of energy, inflation, interest rates will not change significantly.
      With these reasonable assumptions, we have removed the unknowns and can select a net zero solution that minimizes the cost.
      Does this make sense to you?
      I would love to see some real numbers calculated by the PGH movement in all Climate Zones based on these assumptions and few typical representative house plans. That would add a much more systematic approach that I believe is much needed and would move PGH up to the next step!

      1. Expert Member
        MALCOLM TAYLOR | | #12


        I'm, not sure that argument works unless you are disconnected from outside energy sources. It relies on net metering agreements, and presupposed that utilities will buy your power at the same rate they sell to you. That may well continue in the future, but it isn't a given, so simply designing a grid-tied house that consumes as much energy as it uses doesn't make the variables irrelevant.

        Taken to it's extreme the argument would also say that a house with no insulation makes sense if enough on-site energy generation is added. That just leaves out too many other considerations.

        I also don't think gstan's points about inflation, energy costs, and interest rates are negated because you are building soon. They are unknown variables over the lifespan of the house, not one moment in time.

    4. Mauro_Zammarano | | #5

      "the Ekotrope online calculator is correct - this is because all walls are going to be between 20 and 25 percent wood, the book merely listed the value for the insulation by itself."
      The book lists an R-value of R21 for dense packed cellulose; a whole wall assembly R value of 19.74 with 2x6 studs @ 24" and cellulose (Ekotrope gives me R16.9 for the same assembly).

  2. Expert Member
    Michael Maines | | #6

    The PGH recommendation is to keep making energy improvements until they stop making financial sense, but we use Building Science Corp's recommendations for what makes a home "high performance" as a starting point. Maybe we should call it Prescriptive PGH vs. Performance PGH. If you don't want to do math, and you want a high degree of comfort, perpetually low energy costs, building durability, resilience, etc., then use those as a starting point.

    On most of my projects I run a BEopt model, starting with code-minimum specs and I run various options until I find the combination that makes the most sense on that project. It's not a perfect system but it's... pretty good. I always find a decent ROI to go above code-minimum and never find the Passive House levels make financial sense (though there are other benefits to PH).

    If you want to assume that future costs of energy, inflation and interest rates will not change significantly, that's your prerogative. I think there is a decent chance that they will change significantly in the future, and worse things might happen, so I think it makes sense to build the best envelope you can afford. A Passive House or "prescriptive PGH" in climate zone 6 will never freeze inside, and having lived through some extended power outages (that grid-tied PV wouldn't help) I like the security of an excellent envelope. But part of the beauty of PGH is recognizing that every project and every client's priorities are different, so you should do what makes sense to you. The important thing is to think about it, rather than defaulting to code-minimum (or worse) without analysis.

    We are starting to think about the next book and your suggestion to go into detail with real numbers is a good one that we will consider--thank you!

    1. Mauro_Zammarano | | #7

      Thank you Michael, I am looking forward to the second book!
      Maybe I was not clear. I am not saying that those parameters (costs of energy, inflation and interest rates) are not going to change. I am just saying that - with a net zero house - even if those parameters change (costs of energy, inflation and interest rates) they would not affect a net zero house owner with a mortgage at fixed interest rate or cash payment and no component failure in the house. These assumptions would simplify the calculations

      1. Expert Member
        Michael Maines | | #8

        Got it. Martin wrote about the conundrum here:

        We had a situation here in Maine a few years ago where our governor instituted a ramp-down of net metering, which was fortunately reversed by our current governor, but depending on which way US politics goes in the future I would not take it as a given.

        1. qofmiwok | | #9

          Idaho Power is trying to get rid of net metering; wants no netting and buy back at 1/6 the retail cost.

        2. Mauro_Zammarano | | #14

          Michael thanks for reminding of Martin's article and discussion. Well for net-metering I hope that the policy does not change in MD. I would be curious to know what is the percentage of residences in US that are in a "net-metering" area.

          1. Tim_O | | #18

            Here in Detroit, net metering was cancelled a few years ago. Current grand-fathered in users were allowed to continue for 10 years until 2029 I believe, but if they added or changed their system at all, the would be switched to the new "distributed generation" plan. Under the new plan, any energy you feed into the grid, you can reuse at a later date. However, any energy you pull back from the grid, you have to pay distribution charges, which are around $0.067/kW.

            So even a net-zero house still is going to have decently sized electric bills and be subject to inflation. Night time rates here are ~$0.05/kW (+ the $0.067 distribution), so your savings are ~40% when the sun isn't shining.

  3. qofmiwok | | #10

    1. My primary goal in building a very high performance home wasn't energy savings, it was health: air quality, mold and condensation. So yes, there are reasons besides the cost. Not only that, but the house feels great to be in, is quiet, is robust (can stay warm for days in case of power outage with no sun), etc.

    2. I didn't see any analysis of the carbon footprint. This is tricky too, but critical for the planet. More insulation has a higher carbon footprint, as does making solar panels. If done well the insulation should last for 100+ years, many years longer than solar panels. But solar panel technology keeps improving so each replacement should be significantly more efficient than the previous. I suspect there is no way to really model this since we are only guessing at what the future holds.

    1. Mauro_Zammarano | | #15

      Gofmiwok I cannot comment on point 2 but I totally agree on point1. For my project, I am worried that having a non insulated slab floor might not a good idea because might cause condensation and mold on the basement floor; but not sure if this is a real concern in zone 4a.

  4. gstan | | #11

    A little calculation talk: Insulation manufactures always list their products with the highest
    possible R-Value (namely a perfectly manufactured batch, installed perfectly under ideal conditions of humidity and pressure). This never (or almost never) occurs when building
    houses - they then assume a perfectly built wood stud wall out of perfect absolutely dry
    lumber and ignore thermal loss due to the hundreds or thousands of metal fasteners (nails)
    and then add additional R-value for things like thin air films adhering to flat surfaces like sheetrock surfaces - that's how you get numbers like 19.74 for a 2x6 wall with cellulose. The
    sensible way to do this is to assume the wood to have an R-value of about 6, the cellulose to achieve an R-value of around 3.6 per inch (there is no such thing as a perfect installation)
    and calculate based on the wood portion being 20% (or more) of the wall (carpenters
    don't frame perfectly either). The calculation then looks like this: 4 parts insulation
    (4x3.6x5.5 = 79.2) plus 6 divided by 5 = 17 for an advanced framing wall containing only
    20% wood. Running the same calc for a normal wall (with 25% wood) will give you a value
    of 16 - note round off and ignore anything to the right of the decimal (those numbers are
    useless and only shown to imply the sophistication of the author). This gives you a fudge factor which kind of makes up for some of the unknowns such as humidity in the wood and/or insulation, the value of thin air films in calm (no wind) conditions, rain gaps, etc. It is a lot more realistic than something like 19.74 - you can and should calculate these things for yourself if you doubt the figures presented or at least ask (your architect, designer, engineer
    or whomever) to run through it with you. I hope all this doesn't put you off - once again

    1. Mauro_Zammarano | | #16

      thanks for your very detailed explanation. I guess there is indeed something wrong with that 19.74 R value calculation. Not a big deal but that confused me so I wanted to be sure I was not doing some silly mistake.

  5. Expert Member
    Akos | | #13

    My way of looking at these is slightly different. The important part is the incremental cost of additional insulation and energy savings.

    So in the OPs case in mildish climate, going with any of the standard high R value wall assemblies (double stud, exterior rigid or larsen) doesn't make sense over standard construction. The cost delta is simply too large for the small change in energy savings.

    If on the other hand you look at how you can improve the assembly for minimal extra cost, the numbers start to look much better. For example there is almost no cost difference for an R60 over an R50 attic, the energy savings is small but compared to the overall cost of the house this incremental cost is noise.

    You can use similar approach for the walls. R24 batts add very little to standard build plus it is much easier to get a class I insulation install. The energy savings are small but so is the extra cost. A similar argument could almost be made for going for a 2x8 24" OC wall although that will add some actual cost to the build.

    Another example is in my climate (north edge of zone 5) code is 2x6+R5 rigid. Bumping up the R5 rigid to 2" polyiso adds minimal cost for a significantly higher assembly R value wall.

    Overall the goal should be to capture all these small changes that are easy to do, don't cost a whole lot but make for a better home in the end.

    1. Mauro_Zammarano | | #17

      Hello Akos,
      yes I agree that extra insulation in the attic would be an easy upgrade. For the walls, my builder is really pushing for wet spray cellulose due to its ability to fill all small crevices and high density. Maybe blown-in mineral wool could also be an alternative.

  6. joshdurston | | #19

    IMHO, I would use the modelling to selectively improve certain details, but not to downgrade.

    I wouldn't consider an uninsulated slab in 2020+, regardless of the energy model. Not everything boilers down to dollars and pay back and net zero. An insulated slab will deliver much better comfort even if the energy savings down justify it.

    You can keep the space at 72F with or without an insulated slab, but it will be more comfortable with in the heating season. Having solar panels might offset the energy loss of the uninsulated slab on an annualized basis, but won't offset the degradation in comfort.

  7. Expert Member
    BILL WICHERS | | #20

    I would insulate the slab for two reasons:
    1- It won't hurt
    2- It's VERY difficult to add later if you change your mind

    They way I look and insulating is that it always helps, but it's not worth overdoing. I don't try to hit any magic targets like net zero, and I don't try to optimize to the penny the best cost/benefit, either. I do look at ROI, and try to keep that under 5 years, so no R60 walls or anything crazy. What I usually find is pretty optimal is around R49-R60 in the attic, since loose fill is the cheapest insulation to put in, so it's easy to go up to pretty high R values for minimal cost. 2" of exterior polyiso on a 2x4 or 2x6 wall is a good upgrade too. I like to use mineral wool in the walls, which is a bit of a splurge since there are cheaper ways to get similar R value in stud bays, but mineral wool is so much easier to do well. I do code minimum or a little over for foundation walls. Code on foundation walls is already into the point of diminishing returns in most areas, so there isn't much point in going much over code in this location.

    I do make a big effort on air sealing though, with redundant air barriers (usually both interior drywall AND exterior sheathing). Air sealing really helps with overall energy efficiency, and it only needs some caulk, canned foam, and some care with the application. You get a lot of bang for the buck with air sealing on a new build.


  8. Mauro_Zammarano | | #21

    SLAB INSULATION: I guess there is an agreement that the slab should be insulated. What insulation would you advice in Zone 4a?
    WALL INSULATION: mineral wool batts are cheaper than wet spray cellulose and has a higher R value. Is there a blown-in mineral wool you also advice? How does density compare between high density cellulose and mineral wool?
    SEALING: I am targeting 1 ACH50

    1. Expert Member
      Michael Maines | | #22

      As far as I know, there is only one North American manufacturer currently providing loose-blown mineral wool, American Rockwool--not to be confused with Rockwool USA, the largest manufacturer of mineral wool worldwide. Owens Corning's Thermafiber is theoretically available in loose-blown but I have not heard of anyone using it. It is denser than cellulose and has a slightly higher R-value. It has a significantly higher climate impact and does not have the borate additive or hygroscopic qualities that allow cellulose to protect the framing and sheathing, but it is even more resistant to fire than cellulose and its higher density is a benefit in some assemblies.

      The Pretty Good House recommended value for slabs in CZ4 is R-8. That's based on Building Science Corp recommendations for the Building America program. The IRC building code does not require sub-slab insulation unless it's heated, in which case they require R-5. I'd use 2" of EPS.

  9. Mauro_Zammarano | | #23

    Thank you all for the great suggestions!
    So my consultant ran an energy model of the house with the R-values suggested by Pretty Good House movement. Compared to the lower R-value I reported above [R49 attic (U=0.02), R21 walls (U=0.058), R15 foundation walls, slab floor uninsulated (U= 0.365), floors R38 (U=0.028)]
    the predicted savings/year with the upgraded insulation are $84/year for heating, less than $1 on cooling.
    I was honestly surprised to see such a small difference.
    Now I am not sure what the cost of upgrading the insulation would be but consider that this is a large house relative to the Pretty Good House standard (5229 SQFT) so the cost is definitely important and if we consider financing cost (OMG that mortgage rate is sky rocketing!) with $84/year that investment does not look economically reasonable. Would you agree?
    If anyone is interested I can upload the full energy reports. Thanks!

    1. DC_Contrarian_ | | #24

      In most of the US the ground temperature is below room temperature, so having an uninsulated slab helps with cooling in the summer. It can outweigh the heating loss in the winter in some climates. It sounds like you're in one of those climates.

      1. Mauro_Zammarano | | #25

        That makes sense. So in summer the uninsulated slab helps cooling the basement and this compensates for the higher heat losses elsewhere in the house (compared to the a house with better insulation). In terms of comfort I believe that the soil temperature is about 52 F at basement depth in zone 4a. That might be a bit too cold especially with ceramic tiles.

  10. DavidDrake | | #26

    "...consider that this is a large house relative to the Pretty Good House standard (5229 SQFT) so the cost is definitely important and if we consider financing cost (OMG that mortgage rate is sky rocketing!) with $84/year that investment does not look economically reasonable."

    Pretty sure 5229 SF is a large house by any standard, not just PGH.

    A person could reduce that square footage considerably, and put the cost savings toward a higher performance envelope. Or not, and just build smaller.

    Other things being equal, larger houses consume more resources and account for more embodied energy and carbon than smaller houses. The environmental cost of super-sized housing goes beyond the current mortgage rate and affects all of us.

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