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Diminishing Returns on Continuous Foam Insulation?

WilliamC | Posted in General Questions on

I am remodeling my exterior and taking the opportunity to fix the insulation in my 1978 house. I am in climate zone 4a, very near 3, and I’m trying to decide what R-value continuous insulation to add to the house. I will have R15 in the walls. There, of course, is a cost difference between r3, r5, and r7.5 in my area however the cost difference is not outrageous. The R7.5 is 70% more than the R3.

I believe R3 is enough to keep the sheathing warm and avoid condensation. I believe R13 + 5 is 2018 IECC and R13 + 10 is 2021 IECC for my climate zone (Although most of my state goes by 2009 codes which states R13 only).

Any thoughts on what might be an Rvalue that balances benefit with cost would be appreciated. I seem to remember coming across a document at some point that indicated what R value was the point at which there was limited value in going past for each climate zone but I can’t seem to find that document. Any thoughts would be appreciated.

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  1. paul_wiedefeld | | #1

    There are many variables involved, but broadly speaking there is no payback.

    The variables that favor adding more insulation would include the following:
    1. High energy costs
    2. Low install costs
    3. cheap cost of capital
    4. long time horizons
    5. opportunities to use smaller/different heating equipment
    6. Cold climates

    If you can check most or all of those variables, it starts to make more sense. In the attached example, these are pretty favorable inputs, and the energy saved by insulation is $83/MMBtu. I pay about $15/MMBtu for heat with a lower carbon footprint.

    1. WilliamC | | #5

      Thanks for the information! I'm not necessarily looking to recover the actual monetary cost. Mostly what I meant was, is there a point at which more insulation is insignificantly impacting the comfort inside the home and the ability with which my HVAC can keep a constant temperature in the house.

      1. paul_wiedefeld | | #8

        Oh I see. Not much insulation is needed to keep the temperature constant.

        1. WilliamC | | #10

          Thanks Paul. As I mentioned below I think I need to simplify my question and goals again. Maybe third times a charm.

          Mostly I want to know if adding R5 CI to a 2x4 house with R15 in the walls is likely to make a noticeable difference in comfort and energy consumption at all or if I'm wasting time and money by installing r5 CI.

          1. paul_wiedefeld | | #11

            That's at least a few questions, one is easy and one is hard:

            I think it's easy to say there is not an appreciable difference in energy consumption and it is certainly not monetarily worth it.

            Comfort is subjective and so is the value of comfort, so harder to answer for anyone but yourself. Have you ever spent time in a house with R-15 vs. R-20+ walls?

        2. WilliamC | | #13

          That makes sense. I am a little surprised that it wouldn’t make an appreciable difference in energy consumption seeing as how the IECC codes are recommending CI, ostensibly for energy efficiency.

          I have not spent time in a home that I knew had r15 or r20. Code in southeastern Tennessee is r13 so I assume that’s what most of the homes around here have.

          1. paul_wiedefeld | | #15

            It’ll reduce consumption. The price of the energy saved will just be a lot more than the price of energy bought.

        3. WilliamC | | #16

          That makes sense. Would it be reasonable to assume that by reducing energy consumption it may prolong the life of my AC/Heating unit?

          1. paul_wiedefeld | | #17

            I wouldn’t expect that. It would also be 20 years down the road, maybe.

  2. Expert Member


    One thing that may inform your decision is that the complexity of installing and detailing exterior foam changes once you go over 1" of thickness.

    1. michael_obrien | | #3

      I am in zone 6 (Orillia ON) building on Lake Simcoe.

      My builder wants to use closed cell spray foam in 2x6 walls, with 1" continuous foam insulation on the exterior - no thicker because of the complexity of exterior finishing.

      I have seen different combinations of exterior foam/interior batt insulation but will that matter to me if it is all foam?

    2. WilliamC | | #4

      That's a good point. I have thought about the modifications to windows that would have to happen if I added foam but I wasn't sure if foam could be added in any depth without increasing the complexity of the installation. If going over 1" is that threshold I may stick with 1". Especially since that would give me R5 which is the 2018 IECC and I am pretty much as southern as you can be in 4a without being in 3. Thanks for the help.

  3. walta100 | | #6

    Michael, I see large amounts of spray foam as a huge red flag in new construction plans for a poor design work by a lazy designer who has zero regard for your budget. I think if you ask for a lower cost option with the same or higher R value, they can come up with a better design If you will allow a thicker wall.

    To my ear Michael question is about meeting his code requirement at the lowest cost and William’s question is more about what will give him the best return on his investment.

    The ROI question is very complex with many variables not all of which can be know today and the math will give you very different answers depending on your guess at say the future price of your chosen fuel. The only real answer would require building a computer model of your home with your variables the program is free but takes 20 hours to learn and enter the data.


    1. WilliamC | | #9

      Thank you for the help. My question is ROI but I think my question is more simple.

      Mostly I want to know if adding R5 CI to a 2x4 house with R15 in the walls is likely to make a noticeable difference in comfort and energy consumption at all or if I'm wasting time and money by installing r5 CI.

  4. michael_obrien | | #7

    Sorry, I should have posted in a different thread as I strayed from the original point.

  5. walta100 | | #12

    If the question is can a human sense the difference in surface temperature between a R15 and a R20 wall from 6 feet away I say no, if they laid a hand on the two wall maybe.

    “I'm wasting time and money by installing r5 CI.”

    This question is back to ROI there are no simple answers there are literally of variables. The biggest one is likely will you still own this building 20 years from now If not CI is unlikely to ROI. CI is also unlikely to ROI if your contractor does not want to do it and over prices the job the dissuade you.


    1. WilliamC | | #14

      Hey Walta. Thanks for the clarification. I do plan on owning the home in 20 years. I am doing the work myself so it is added effort for myself and about $1000 extra to add CI, which in the scheme of things is not much. But if it’s not accomplishing anything I’m not sure why I would do it. IECC codes lead me to believe CI accomplished something. But maybe it doesn’t.

  6. moe_wilensky | | #18

    I doubt it will change the overall economics, but if you are performing an impact analysis, you should use the effective assembly r value (or the assembly U-Value).

    90.1 gives the u-value of an R15 + 3ci wall (2x4 16" oc) as 0.064, R15 + 7ci wall as 0.050. From those values you can use (U1 - U2) x HDD x 24 x Area to get a rough approximation of annual impacts in heating energy requirements in Btu and dividing by your heating system efficiency gives approximate annual energy use.

    1. WilliamC | | #19

      Thank you I will look into this. I'm curious why most of the energy consumption formulas/conversation center around heating days only? Does AC not consume an impactful amount of energy or does insulation not help keep the house cool, or some other reason? In my location in the south we have far more cooling days then heating days.

      1. moe_wilensky | | #22

        I think that's just personal/regional bias, you could do the same analysis with CDD.

        1. moe_wilensky | | #23

          Also, in light of your cooling dominated climate, (assuming you are also replacing your windows) depending on the amount and orientation of your glazing you may find you get a bigger bang for your buck with spectrally selective low-e coatings optimized to reduce solar heat gain.

          1. WilliamC | | #24

            Thanks for the suggestion. I am planning on changing the windows as well. The ones I’m looking at are low e and have a heat gain of .29. To be honest I don’t know how good, or not good that rating is. I’ll have to dig into that a bit more.

  7. walta100 | | #20

    In many heating locations the outdoor temps fall well below zero and if the indoor temp is 75° the 95° difference is much larger than any heating climate. Also, the heating season tends to be longer.

    You should understand that insulation is useless in a wall that allows air to bypass the insulation. There are a lot more dollars to be saved with air sealing leaky walls than upgrading from R12-R20.


    1. WilliamC | | #21

      Thank you that makes sense.

      I do understand that an am trying my best to address that as well. My house's masonite siding was attached directly to the studs and the cavities are filled with some kind of old spray foam that has completely deteriorated. So in replacing the siding I am going fill the cavities with Rockwool, sheath with Zip, do my best to seal everything well with tape and sealants. The R5 CI is an additional component that I am trying to decide if it would be beneficial or reasonable to add.

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