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Closed-Cell Spray Foam and Diminishing ROI

jetmonkey | Posted in General Questions on

I’m insulating an old roof deck. Attic conversion. And I’ve come across conflicting advice regarding closed cell spray foam installation. It boils down to ideal R values vs the diminishing returns of closed cell foam. At 3-4″ of foam, data shows that we get 90+% of our conductive heat flow resistance, which I believe measures BTU. (Is this identical to R value?) But if we consider that an inch of closed cell rates around R7, then four inches gives us R28. One school of thought says, anything beyond 4″ is a waste of money with minuscule gains. While another school of thought says we should shoot for a modern R49 which if using only closed cell, would obviously require more foam. (And in my case impossible to achieve).

I have 5.5″ of rafter bay depth in my attic roof deck. I initially assumed I should max this out (pay for a 6″ install then shaved back for drywall), but then the diminishing returns question arose. My contractor provided data from the foam manufacturer which prompted the contractor to agree with it being OK to factor in diminishing returns once I inquired about it.

On the flip side, we have the school of thought for maximum stated R value as is summarized in Martin’s reply to a similar question here in 2018.

So I guess my simple question is: if the diminishing return data is correct (and we’re only using closed cell to insulate), how do we reconcile achieving maximium R values while factoring in diminishing returns?

In my case, reducing a an inch or two (achieving 4″ depth) throughout the install could save 1-2k. Not monumental, but not nothin’ either!

(image uploads are from the foam manufacturer via contractor )

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  1. brendanalbano | | #1

    Two approaches to answering this "is it worth it" question related to diminishing returns:

    1. Assuming you are not required to fully fill the cavity, you can calculate the difference in energy loss through the roof with the foam at 4" and 5.5" over the course of a year, compare that to the cost difference between 4" and 5.5", and decide for yourself, based on whatever criteria you deem appropriate if it's worth it. My understanding is that shaving closed-cell foam is really hard, so realistically this is going to be more like 5" of foam since you won't be able to fully fill the cavity.

    2. Where I work, if you are insulating an existing cavity that is not sufficiently deep to achieve the new construction code minimums, the code requires you to fully fill the cavity. This makes the question of whether or not it's worth it moot, because it's a requirement either way! Because foam is expensive (and not particularly green), a common approach is to do the minimum thickness of foam required in your climate zone for moisture control, and then insulate the remainder of the cavity with fiberglass or cellulose (aka "flash and batt").

    The flash and batt approach described in option 2 is outlined in option 5 of this article:

    1. jetmonkey | | #6

      Good advice, thanks! I previously moved on from flash and batt as an option due to the depth of the rafter bay (5.5"). Based on the linked article and elsewhere, I wasn't sure there was enough space for the batt to be as effective as simply more foam. But I'm going to review that some more.

      1. brendanalbano | | #8

        What climate zone are you in? This will make a difference when comparing 4" of foam, 5" of foam and flash-and-batt.

        For example, if 2" of closed-cell foam + an 3.5" R-15 batt will work in your climate zone, you get a few benefits:

        1. you'll probably save a little money compared to 4" of foam (get your contractor to quote both options to make sure it's actually a savings)

        2. The assembly will have very slightly better performance than 4" of foam (similar total R-value in the insulation, but increasing the thickness will reduce the effects of the thermal bridging at the rafters marginally. This may not be significant, but it's a freebie). It will be slightly worse performance than 5" of foam. But the differences are probably fairly marginal in all scenarios.

        3. You will fully fill the cavity which is most likely what your local building code requires (whether or not that is actually enforced is another question).

        The calculations Paul Wiedefeld is demonstrating in their comment are how you would quantify the value of increased insulation and balance the value of more insulation vs diminishing returns, and if you're excited about learning to really optimize this thing, give it a go! But if not, you're not going to go too far wrong with any code-compliant way to fully fill your 2x6 cavity, whether that ends up being 5" of foam (close enough to fully filled), or a flash-and-batt approach.

        All this said, if you have the headroom to fur down your ceiling and add even more insulation, that may be even better, but this is often not an option in attic renovations.

        1. jetmonkey | | #10

          Again, very helpful. Climate zone 5, which I assume reduces how useful batts are in the winter versus more closed cell(?), since greater R value is required for colder winters.

          In your #2 benefit, when you mention increasing the batt thickness, do you mean beyond the 3.5"? I'm interested in how batt insulation in general improves upon thermal loss. My understanding of thermal bridging mostly focuses on direct surface contact of air temperature to conductive materials. Once an insulated assembly gets say 1/2" drywall sheathing, would batting do more for thermal loss versus 4 or 5" of spray foam? I definitely plan on filling the bay with material.

          Edit: there's no reply button available for your comment below, but THANK YOU!

          1. brendanalbano | | #15

            Closed-cell spray foam has a greater R-value per inch than fiberglass batts, so for a given thickness, you'll always get more R-value from the foam than from the batt. However, due to other factors like thermal bridging and whether or not you can fully fill the cavity (depending on if the contractor is willing to shave closed-cell or not), the greater R-value of the closed-cell may be diminished to the point where it is not worth the cost beyond the minimal amount to make the assembly safe from condensation (more on that later) and you're better off filling the rest of the cavity with something cheap, like fiberglass or cellulose.

            In #2, what I was trying to say is that when you compare 4" of foam to 2" foam + 3.5" batt, for a total of 5.5" of insulation, at the rafters, instead of your thermal bridge being 4" of wood, its now 5.5" of wood, which has a greater R-value. This was just to say that it is advantageous to fully fill the cavity, regardless of what you fill it with.

            Regarding the flash-and-batt, the advantage is that you can potentially save money and have a similar performance. The affect of the thermal bridging of the rafters is such that the advantage of the greater R/in of the spray foam is diminished.

            Using the calculators here:

            Accounting for the thermal bridging of the rafters, the R-value of your roof with 5" of closed-cell spray foam is R-24.4, while the R-value of 2" foam + 3.5" batt is R-24.3.

            The reason that climate zone affects the ratio of closed-cell foam to batt insulation is due to the colder weather creating a greater risk of condensation. The important part is not just the r-value of the foam, but also that it is an air barrier and a vapor retarder. It needs to be thick enough for your climate zone that it prevents condensation. The language in the building code describing these rules is fairly convoluted, but it can be found in section IRC R806.5.

            The table in R806.5 will tell you that in zone 5, the minimum R-value of your spray foam is R-20. It can be reasonably interpreted that that is R-20 for an R-49 roof, and the important thing in a renovation situation like yours is the ratio. To be safe from condensation, you need 41% (20/49) of your total R-value to be in the sprayfoam. 2" spray foam + 3.5" fiberglass results in about 47% of your r-value being in the spray foam layer, so you are safe from condensation risks.

            If you want to go deep on these hybrid assemblies, this is a good article:

  2. nynick | | #2

    You need to watch this video. I've watched a bunch from this guy and he's really informative. One of the things he doesn't mention in this video but in others is the dramatic ability of CC foam in help sealing the building. He also addresses why CC foam R values are not relatable to batts etc. because of this.

    FWIW I'll be using CC in my renovation. 2" on the basement walls and 3" in the rafters. On the roof deck I'll be using 2" of Polyiso as well to meet code and address thermal bridging. Spray foam is expensive and not exactly green. I'll use it, but within reason, including ROI.

    1. jetmonkey | | #4

      Thanks for posting that video for added context. It was actually one of the resources that lead me to this question. He doesn't mention thermal loss (via thermal bridging) which is something that the 2018 thread I posted brings up by a commenter. My circumstances prevent me from furring things out or adding layers above the roof sheathing to combat thermal issues. Not sure how to exactly factor in the thermal loss. Advice elsewhere suggested to expect no more than 40% loss on conventional wood framed roof deck. In my case 5" nominal rafters 24" with skip sheathing to plywood sheathing. But 40% is a lot and perhaps suggests maximizing insulation depth. But this contradicts the advice in the video, as I understand it.

  3. paul_wiedefeld | | #3

    R value is the inverse of U-value, so 1/R=U. You can back of the napkin it by taking heating degree days for your location*24*U value and compare the two. For example, using 4500HDD *24*(1/28-1/38.5) ~ 1,000 btu/sqft/year saved on heating only, so a similar calculation can be performed for cooling. If that costs you an additional $.5/sqft, you're paying about $28/MMBtu if you use 3% and 25 years in your present value calculation. Obviously, all of these inputs will need to be adjusted for your situation.

    1. jetmonkey | | #5

      Thanks :) Not going to pretend I can back of the napkin it without deciphering some terms first, but greatly appreciated.

  4. exeric | | #7

    The diminishing returns argument is false. The reason it's false is because it is related to percentages and not to occupant comfort. They are using the hidden assumption that because the percentage increase in retaining BTU is getting less and less with more insulation it also means that a person will not feel that difference in temperature. You definitely will. Those smaller BTU gains with each increment of insulation doesn't make how it will feel go away. You will feel it as cold and drafty until you get the insulation value close to what is prescribed in standard codes for building. That is a fact. The argument is completely bogus.

    As has been said "There are lies, damn lies, and there are statistics". They are using diminishing return statistics to deceive both themselves and you. Their argument agrees with BTU statistics but the intuition of the statistics disagrees with the reality of how a house with insufficient insulation will feel to you. In reality the person will just turn up the heat (or cold) and use more energy. The insulation minimum requirements in the code books are there for a reason.

    1. exeric | | #9

      There probably is also a hidden aspect to the reason closed cell foam guys came up with this reasoning. Closed cell foam cannot be shaved like open cell foam can. It means that in a 2x4 or 2x6 wall the foam thickness cannot be filled out completely. They always have to estimate a 1 to 1.5 inch reduction in thickness. And it's an art and not a science to put in enough and not too much insulation in its non expanded state.

      Never underestimate the ability of human beings to create "motivated" reasoning to come up with a false conclusion. It's only my own theory but I think the imprecision of installing CCF so as to not overfill the stud bays has worked on their minds. They've turned a bug into a feature.

      1. jetmonkey | | #12

        Makes sense, and I've encountered exactly 50/50 of contractors who are willing to shave closed cell no problem (at least verbally), while the others are like "hell no." That discrepancy actually surprised me.

    2. jetmonkey | | #11

      Interesting, and you're describing my core question, I think. And apologies if this was already spelled out on this thread, but if the chart says we're getting say 95% "heat flow resistance" (which I understand to be "Thermal Resistance" which also relates to R value) with 4" of foam, but that only gets us to about R30, and code wants us to get R49, then we're measuring different things I gather? (R vs U value?) And in general, I echo your point about statistics easily obfuscating things.

      And/or are you saying that a 3% difference seems small on paper, but in reality we FEEL that 3% (even though we compensate by using more energy)

      1. exeric | | #13

        To be honest, I'm not sure which scenario more accurately describes where the error comes. I'm sure that that there is a logical error though, and it would be helpful to know where it originates from, just as you are suggesting. Maybe others here can contribute to understanding that. I'm sure I trust the years of research that went into creating building code rules on insulation than the research of someone trying to sell

  5. jetmonkey | | #14

    Appreciated :)

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