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Spray foam

Faulted1 | Posted in Energy Efficiency and Durability on

Why more is not better… or is it?

I know it is misleading but why?

Because it is only conductive?
Because it would be similar for any insulating material?

I know it not correct but why?


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  1. homedesign | | #1
  2. homedesign | | #3

    you read my mind... I was going to post that link too

  3. HomeBuilder1975 | | #4


    What thicknesses and R-value of spray foam are builders and architects using in Dallas.


  4. homedesign | | #5

    If you are asking what is common .. I would say 3" to 5-1/2" open cell(R 3.7 to 3.8 per inch) for the walls and 5" to 7" open cell at the roof deck(less than code).

    Some projects are 5-1/2" open cell walls with advanced framing and 1"polyiso sheathing.
    with 8" open cell at the roof deck = R30( just barely code)

    If it were up to me I would double the code minimum R-value at walls and ceiling and NOT use foam.

  5. GBA Editor
    Martin Holladay | | #6

    Q. "Why more is not better... or is it?"

    A. Actually, more is always better, as the graph shows. The thicker the insulation, the slower the heat flow. Eventually, however, you have to consider whether the dollars invested in thicker insulation are yielding enough of a reduction in heat flow to justify the investment.

    Q. "I know it is misleading, but why?...Because it is only conductive?"

    A. No. R-value measures heat flow by all three mechanisms: conduction, convection, and radiation.

    Q. "Because it would be similar for any insulating material?"

    A. Yes. Spray foam is no different in this regard from cellulose.

    Q. "I know it not correct but why?"

    A. The main reason that the graph is misleading is that it starts out with an illegal, uninsulated assembly as its premise -- that is, a tent. A less misleading graph would start out at the Y-axis with a code-minimum building -- because anything to the left of that is illegal.

    Doubling the thickness of any insulation material cuts the heat flow in half. The main reason that spray foam comes up against economic limits rather quickly is because spray foam is very expensive. Since cellulose is cheaper, you can justify the installation of a much higher R-value of cellulose than you can of spray foam. Of course, if you choose the thin-layer-of-spray-foam route, instead of the thick-layer-of-cellulose route, the homeowner will pay an energy penalty (higher energy bills) for the life of the building.

  6. HomeBuilder1975 | | #7

    John Brooks:

    Those are pretty much the specs we're using in Houston. Results have been quite spectacular when compared to convention insulation (fiberglass, cellulose). In talking to a client of a house I built a year ago, their monthly bills are average low $200. As you know we cool our houses about 9 months out of the year, electric bills in the summer can be several hundred$. That particular house has a pool (costs $75-$90), added appliances (wine chiller, ice makers (2), 6000 sq ft, and nightly outdoor lighting. Clients are extremely happy.


  7. homedesign | | #8

    I know you think I am foolish for trying to reduce the footprint of our(southern) Architecture.
    I suspect that the scenario you described is why we(the South) are the champions of Source Energy use.
    If the Yankees would all build swimming pools, install multiple icemakers and light up the night-time sky...
    Maybe they could reclaim the title.

    The South Rules!

  8. Mike Eliason | | #9

    insulation overload!


    R-60 roofs for dallas seems like extreme overkill to me based on our experiences working on a passivhaus for new orleans. we are looking at R-22 walls/roof, R-36 floor for a small building (~1100 sf) but with lots of surface area for outdoor spaces/bounced light. double pane, low SHGC windows w/ no gas.

    when i move the project to dallas and can utilize a concrete slab, my floor insulation drops to a near-code minimum R-15. our wall assembly is an uninsulated 2x wall w/ 3" polyiso at exterior (based on the BSC-preferred assembly for new orleans), which allows wall cavity to actually be useful for MEP runs, prevents thermal bridging and manages inward solar vapor drive.

    that's not to say it's the perfect set up, but that it is possible to have low levels of insulation in cooling dominated climates and still achieve extreme levels of energy efficiency. it's possible to get away with a passivhaus in the midwest without R-40 walls and R-60 roofs, so why would it be necessary in the deep south?

  9. homedesign | | #10

    Code wall for Dallas is R-13 ... R-26 wall is not-so-outrageous for a Low Energy Home.

    True that R60 ceiling/roof would be outrageously expensive if choosing a foam product.
    Upgrading R-30 to R-60 Blown cellulose is relatively "cheap"

  10. homedesign | | #11

    As for Slab insulation in Dallas...
    my understanding ....
    Perimeter and slab edge is the only place where it would be be beneficial
    ....but we have termite issues with foam
    Last I heard BSC and Building America were looking at the problem for one of their sponsored builders... but have not come up with a solution.

  11. Mike Eliason | | #12

    i'm not sure about actual measurements, but PHPP shows a jump in heating load AND cooling load for phoenix, dallas and new orleans if you remove all sub-slab insulation (thereby leaving only perimeter), even compared to R-10. i would imagine on a less-than energy efficient house, that jump would be even higher.

    2 termite resistant options for sub-slab insulation are foamglas and perlite... both have relatively higher embodied energy numbers than EPS... also more expensive unless oil continues to climb.

  12. homedesign | | #13

    Mike, when I say perimeter I may not be using the correct term.
    I visualize "perimeter" as a zone several feet from the outer edge of the slab?
    what are you considering "perimeter"?

  13. homedesign | | #14

    You are probably familiar with Chris Mile's project
    Bill Peck theproject Architect is the one who told me they are still looking for a satisfactory solution for slab edge /perimeter insulation.

    have you seen any slab edge or slab perimeter insulation done in Dallas?
    If so I would like to hear more about it.

  14. Expert Member
    ARMANDO COBO | | #15

    You have plenty of solutions in Dallas or any other place with termites. You can use Thermomass Wall Systems, you could build a concrete insulated “sandwich” with a pour concrete or CMUs, you could put foam on the outside and use cement coating on the foam (similar to stucco), you can use precast insulated walls, you can do conditioned crawl spaces, etc, etc… you may pay more for it, but there are plenty of solutions. You could use rockwool insulation also.
    I forgot to mention Durisol ICFs as an alternative. Perimeter is the outside face of the foundation all way around.

  15. Expert Member
    ARMANDO COBO | | #16

    Yes, I'm familiar with Chris' project. Jim Sargent has done several projects with conditioned crawl spaces and durasol. I'm designing a house with insulated foundation, but dont have the details yet.

  16. homedesign | | #17

    Jim Sargent told me recently that he was still looking for a slab solution.
    Have you seen anyone doing slab foundations with insulation?

  17. Expert Member
    ARMANDO COBO | | #18

    No. But I have often thought that if I needed one over a Slab-P&B foundation, I could be to lay down a layer of rigid foam over the slab and then a 2"-4" high strength concrete on top of it; kind of doing a Thermomass "Floor" system. I think it can be expensive and I have not talked to an engineer.
    I think doing a Thermomass wall system, Durisol or a conditioned crawl maybe less cost. Just a guess...
    I have done insulated slabs under basements and slabs on grade but never under a PTP&B foundation like here in NTX.

  18. Expert Member
    ARMANDO COBO | | #19

    Interestin article on BSC and Greencraft, I wonder how long a go. I think if I design a 8"-12" concrete wall on top of a wider footing, which is over piers (doing all the PTP&B loads), then install 2” of rigid foam next to it and then a 2”-4” second wall to completely incapsulate the foam, as in a Thermomass wall, I’ll bet an engineer would be all over that.

  19. homedesign | | #20

    I think what PHPP is telling us is that a climate like Dallas "needs" slab edge insulation and under slab insulation( at least for a certain distance from the perimeter).
    This is consistent with what some of my energy geek friends have been telling me.
    But then we have the "termite problem" what BSC calls a technology barrier.

    So if you remove the slab insulation from your PHPP model does your wall approach R26?
    Didn't Katrin Klingenberg suggest up to R-30 for Houston Walls?

  20. Mike Eliason | | #21


    Yes, PHPP is showing me that having both edge (or perimeter) and under-slab insulation are needed in order for lower heating and cooling demands. One of the things I've seen in EU is similar to a rat slab, where they just drop an ultra thin layer of concrete over compacted dirt - I believe this has two functions, provides a level construction surface as well as acting as a vermin deterrent.

    Katrin's 2011 NESEA presentation still shows envelope R-40+ for Houston, which is really high, like double what I'm seeing as necessary for walls on a somewhat compact project.

    But she also recently stated this after the New Orleans PH competition, "In a cooling climate, the delta T is much smaller than in a heating climate, and due to the internal heat gains insulation actually starts to work against you at a certain point. I think anything up to R30ish is doable/defendable for all components, including roof and suspended floor. Beyond that it will only add to your cooling problem. "

    I think this is due to growing realization that PH in cooling-dominated climates isn't terribly difficult outside of the airtightness issue, it's really more about managing internal heat gain than anything else. Add in the ginormous amounts of sunshine in the south, and PH + PV looks like a great way to not just carbon neutrality - but carbon negative houses.

  21. homedesign | | #22

    I have seen the Katrin Quote you posted and I have been meaning to ask you about this.

    I understand if you consider a house in Chicago... how the "insulation can work against you" during the cooling season.
    I don't agree for a climate like Dallas or New Orleans.

    If I take REM/Design and Model a house in Chicago... the cooling cost does increase with more insulation... just as Katrin predicts

    If I take the same model to Dallas or New Orleans .... Increasing the Insulation does NOT work against you...even at high levels

    I realize that REM/Design is simplistic and not as sophisticated as PHPP.
    What happens when you crank up (extreme) the wall & ceiling R-value for a PHPP model in New Orleans?

    Does PHPP really show the cooling expense to go up?

  22. Mike Eliason | | #23


    Yes and no, it depends on the design of the house (compact or not, orientation, shading, etc) and location. If the house is designed as low mass (e.g. stick frame, less resilient to temp swings) and has windows that let in a bit of solar gain (even with low SHGC glass) in summer - then yes, we have seen one or two instances where increasing the insulation can increase cooling load. However, I think this is fairly rare - but it's not something we've really looked at.

    For our New Orleans project, pretty much all the glass is 100% shaded outside of winter. Increasing the insulation significantly doesn't increase our numbers - they all drop (heating demand, cooling load and demand, primary energy demand), but not by much. We were shooting for a relatively optimized envelope, and to show that high R-values weren't needed to achieve Passivhaus.

    We ran across this phenomenon a few times in training and I remember thinking that seemed off, but that's what PHPP showed.

  23. HomeBuilder1975 | | #24

    John B:

    Regarding my question about Dallas spray foam specs and the excellent results I've gotten on similar specs, you replied:

    "I know you think I am foolish for trying to reduce the footprint of our(southern) Architecture.
    I suspect that the scenario you described is why we(the South) are the champions of Source Energy use. If the Yankees would all build swimming pools, install multiple icemakers and light up the night-time sky... Maybe they could reclaim the title. The South Rules!"

    I am looking for practical results from other contractors or architects like yourself who are using the specs as you described, in hot-humid climates. Do you have any feedback from houses insulated as you described?

    The reason I ask this question is according to Martin we are not meeting basic code with these specs, yet the results are far superior to our past methods of using higher R value insulation with fiberglass and cellulose.


  24. GBA Editor
    Martin Holladay | | #25

    You wrote, "According to Martin we are not meeting basic code with these specs, yet the results are far superior to our past methods of using higher R value insulation with fiberglass and cellulose."

    I don't doubt that your homes insulated with spray foam have lower energy bills than the homes that you insulated with fiberglass batts.

    Have you tested both types of home with a blower door? If you have, I'll bet that the fiberglass-insulated homes were much leakier.

    Although your homes with spray foam may not be code-compliant, it's equally possible that your fiberglass batt homes were not code compliant. For many years, the IRC has required air sealing, but these air sealing requirements are almost never enforced.

    To get the very best thermal performance -- resulting in energy bills even lower than your current houses -- builders need to achieve very low levels of air leakage and insulation levels that are above code minimum levels. My guess is that if you build such a home, the results will be better than any you have achieved so far.

  25. HomeBuilder1975 | | #26


    The air barrier aspect of spray foam does seem far superior to fiberglass and cellulose, and this is one of the benefits of it. We are now required to do door blower testing on all houses, I've done it on the last few homes I've built. But I cannot compare to other types of insulation.

    I guess the bottom line is I don't see the need for high R values when moderate R values, coupled with other benefits of spray foam AND improving HVAC techniques, better windows, and other things create houses that have very low utility usage and costs. At least in my climate. I am starting to think that high R values are a bit overated, at least in hot humid climates. Maybe I'm wrong, but initial results from the handful of houses I've built and feedback from other builders seem to say I'm not wrong.


  26. Expert Member
    ARMANDO COBO | | #27

    The issue about code or not code has to do whether you choose the prescriptive or the performance side of the code. If you choose the prescriptive code, you must apply enough insulation to meet code; i.e. R30, R38 or whatever is in your climate zone. The bad issue is “what you can get away with” when one chooses the performance, because you can apply compensating factors, like better windows, foundation insulation, ES lighting, and so on, to comply with an energy efficient target, as in a HERS score.
    The foam industry, to provide competitive bids, has done a great job selling the fact that if you spray 5.5” of open cell under the roof, it would be “enough” to do the job of an R30-R38 of FG batts or loose fill because of its air sealing properties, but there is the issue that reduces the thermal insulating capacity required for your CZ, and allows a much colder sheathing, which can increase the chance for condensation in northern CZ3 and colder CZs. Winter condensation is not the issue in Houston, but it can be a problem in the hot, humid summer with air conditioned.

  27. homedesign | | #28

    oops meant to post at another thread

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