Insulation riddle resolved

I spent a lot of time last October and November researching appropriate insulation options. In the post “Insulation – how much is needed?“, I described the SPF (spray polyurethane foam) phenomenon of diminishing returns.

I somewhat understood the concept of this effect, but had no information on the actual process or cause. So I asked whether anyone had more information on this.

Lo and behold, I received a comment on the post from a gentleman called R. Tom:

“… a report that illustrates a scenario that uses Fourier’s steady-state heat flow equation to evaluate the performance of a typical wall area with a prescribed R performance value. The results are quite profound… it indicates, basically, that the first inch of [SPF] insulation represents 80% of the heat flow reduction in the system, the next inch 9%, the next inch 3%, the next inch 2%, the next two inches 1% each and the next four inches only 1%! So in the first 2″ you are getting 89% of the total performance realized in your [SPF] insulation assembly.”

The report R. Tom mentions is a publication by Icynene Inc., describing the testing process, providing the math and plotting the results in various graphs.

My first thought was: “Is this for real?” I forwarded the report to a mechanical engineer I respect and asked for his opinion. He agreed with the rationales and results of the report.

Here is what my sleepy little brain cells retained. There are three types of heat flow:

  1. Conductive heat flow
  2. Convective heat flow
  3. Radiant heat flow

The R-value is a measure of the conductive heat flow resistance through a material, but ignores the influence of convective and radiant heat flow. SPF insulation has an R-value of 3.6 per inch thickness (as per the report). If properly applied, SPF can eliminate air infiltration (or convective heat flow), and thus delivers up to 89% of the total performance in the first 2 inches. I think I sort of get this.

My second thought was: “How does this impact our project? Do I need to rethink our insulation strategy?” The answer is yes. Now that I understand the effectiveness of the first two inches of SPF and the decline thereafter, I would like to look again at various insulation materials and their performance, cost and environmental footprint.

PS: I found a great website that explains SPF in plain language:


About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

11 thoughts on “Insulation riddle resolved

  1. Justin, thanks for bringing this up! I was very excited about Aerogel as an insulation option, until August last year when I got an estimate. The Aerogel product Spaceloft 10 (10mm thick) was quoted to me at $3.03 per square foot. The manufacturer lists the R-value for Aerogel at R-10.3 per inch thickness.
    Closed cell SPF runs around $1.00 per board foot and is listed at an R-value of 6.3. Open cell SPF runs around $0.40 per board foot and is listed at an R-value of 3.9.
    Here is my math:
    For Aerogel, I have to pay $0.74 per R-1 per square foot.
    For closed cell SPF, I have to pay $0.16 per R-1 per square foot.
    For open cell SPF, I have to pay $0.10 per R-1 per square foot.
    The price of Aerogel may have fallen a lot in the past few years, but it is still, in my opinion, too expensive to make its way into the renovation and rehab market. And we haven’t even touched some of the moisture management issues that I have to deal with…

  2. Interesting price comparison. I would expect that the price continues to fall as this product becomes more widespread. Look at the article regarding the moisture/water vapor movement: It looks like Aerogel won’t cause any moisture problems, which is why it may work so well for masonry.

    This makes me think about the timing of green rehabbing, what with all of the current and pending materials science advances we’ve been seeing. In 5 years, I’ll bet there will be a whole new range of rehabbing products based on new “meta materials.”

  3. Justin,
    Re. moisture management in my particular case: The SPF will adhere and bond to the entire masonry wall and eliminate all cavities. The Aerogel is a sheet product that I can install against the wall, but I will be left with small pockets between the Aerogel and the masonry wall, where condensation can accumulate. And that would be a problem in my case.
    Re. timing: I think you do a great job. All the research you are doing now will serve you great whenever you are ready to for your rehab! Believe me, you can eliminate a lot of stress by getting yourself informed and educated ahead of time.

  4. Marcus de la fleur,

    I beg you to reconsider your opinion on the “efficiency of SPF”. While there is no question that under real-world conditions an airtight and convection free 2″ of spray foam will outperform a fiberglass batt of twice the r-value, the fact still remains that r-value is r-value and the more r-value the better. If you were choosing between and r-38 fiberglass batt and 2″ of closed cell foam, I would go with the foam, but if you are choosing between 2″ and 4″ of foam, go with the 4″; you will not regret your decision. In a northern climate, skimping on insulation is a huge mistake and will make achieving netzero difficult and costly. Reducing energy use with higher levels of insulation is far less expensive than the cost of producing energy on-site with renewables.

    What will your maximum cavity depth for insulation be? If you have plenty of space I would recommend adhesive spray cellulose insulation or a water blown half-pound density foam such as Icynene or Sealection 500 by Demilec. The blowing agent in 2lb density foams (closed cell) has a global warming potential (GWP) that is nearly 1000 times more potent than the steam reaction that the half pound water blown foams rely on. If you must choose 2lb foam due to restricted cavity depth, I wouldn’t lose sleep over the decision. The energy saved from the increased r-value over the life of the building will greatly outweigh the global warming potential of the blowing agent.

    I agree with you that the “space gel” is unable to provide an effective airseal and is generally an expensive and poor choice in real world residential applications.

    If you opt to use cellulose, make sure to airseal all penetrations and the the rim joist with spray foam insulation (preferably .5lb).

    If you have any additional questions regarding your building envelope let me know. I’ve enjoyed following your project on the web. You take amazing photos!



  5. Lots of great talk here about the conductive and convective portions of the equation — but what about radiant?

    Is a high reflectivity/low emissivity foil layer the only choice? Where should the foil be positioned within the wall assembly/sandwich? Positioned toward the outside, you would be including the insulation in the radiant envelope — but this doesn’t seem smart since insulation isn’t going to effectively increase the conditioned thermal mass. My guess is that applying to the inside (e.g., immediately behind the wall’s furring strips) would reflect the most heat into the living space and best improve perceived warmth.

  6. Chris, a radiant barrier would indeed be a nice addition and add to the energy performance. My problem is that all radiant barriers that I have seen are basically also performing like vapor barriers – and that is something that would be counterproductive in our wall assembly. I need to have adequate vapor diffusion capacity to assure good the long term performance of the wall assembly.

    If there is a radiant barrier that allows for vapor diffusion (i.e. with a high perm rate), I definitely would be interested to learn more about such product.

  7. Marcus,

    Some quick Google research has turned up a couple types of products: paints and perforated foils.

    Some paint-based products, aka “Interior Radiation Control Coating”, that I turned up include:
    – HeatBloc-75
    – LO/MIT
    – HeatBLOC-Ultra

    These all say they’re “permeable” to water vapor, but don’t have specific perm ratings [1].

    I didn’t spend much time searching for perforated foils, but this one at Amazon claims a 70 g/m²/24hr rating — which seems pretty good to me.

    Probably worth a bit more research, no?

    [1] I had to look up the units for permeability. A good source of info is here:

  8. Take a look at this:

    Most of it talks about a different problem – spray foam mfrs/installers (specifically Icynene) playing games with “equivalent insulation” versus poorly air-sealed structures to weasel around code requirements and make their expensive (and potentially very good) product look better compared with the underwhelming norm. “Our well sealed R-20 is as good as a badly sealed R-38.” Uh, maybe, but the IRC calls for R-38 AND good air sealing.

    But down in the comments, there’s some talk about this “mind bending” stuff about getting the “first 89% of the insulation value with 2 inches of foam.” It appears to be some good old advertising statistics. These numbers like “80% with one inch, and 89% with two inches” are in comparison to what, exactly? One of the commenters points out that it’s in comparison to a wall with an R value of 1.

    This isn’t quantum physics – it’s good old, straightforward low-energy physics. Yes, convective heat loss (air leaks) is very important, and radiant heat loss also plays a role. But most of a house’s heat loss is conductive. When you plug through the simple math, R values are R values – there’s no mental gymnastics required. R-40 means half the conductive heat loss as R-20, and R-80 would mean half as much again. No “wrapping your head around it” required.

    Where the style of “one inch equals 80%, two inches equals 89%, three inches equals 92%” numbers come in might be some sort of “inverse of heat loss versus cost of insulation” trade off, (assuming that 2″ of insulation costs twice as much as 1″, and that you’re only looking at conductive heat loss.) Interesting how on one foot, the SPF folks talk about how important convective heat loss is, but then jump right over to the other foot and totally exclude it when it’s useful…

    Yes, it’s fairly nuts to put R-60 insulation or more in a roof, but if you’re going for net zero energy, then those are the types of insulation levels you’re looking at to make the “system” work for the house. There’s no “magic” that allows you to get away with only 3 or 4 inches of open cell foam under your roof and not run your boiler during a Chicago winter.

    When you get down to the simple, bare-bones physics of heat loss calculations, you see that R-20 is R-20 and R-40 is R-40. Don’t get confused by possibly disingenuous advertising malarkey. A certain number of BTUs get put into the building in the winter and a certain number of BTUs are shed out into the cold exterior. Sealing the cracks is critical, and some heat will be lost as infrared insulation, but most of the heat is lost at a rate of “so many BTUs per hour per square foot of wall/roof area per degree of difference in temperature between the interior and the exterior.”

    Now that you’ve installed your boiler and sized your radiators, the building envelope must resist the flow of heat out to the exterior at a slow enough rate that your heating system can keep up. If it doesn’t, you’ll need to get that Icynene ad writer to come down from Canada every winter with a large pile of Tim Horton’s donuts and do jumping jacks 24/7 to generate the extra BTUs inside the house to keep the pipes from freezing.

    Don’t get me wrong – I’m going to use a mix of open and closed cell spray foam in my place, and I recommend it to clients. But when the code and the modeling calcs call for R-21, them I’m spec’ing 3″ of closed cell, and when the roof is required to have R-39, then I’m calling for 11″+ of open cell, plus what’s required to compensate for the framing. Spray foam is expensive, and I don’t let the sales guys “spin” the project into under-insulating.

  9. Sorry if I’m beating a dead horse here, but I re-read the Icynene material from your link above:

    This is the source of the “89% in the first 2 inches of insulation” claim. I think I can explain how they are getting to these in-obvious numbers. Let me start at the end:

    At the end of the first paragraph on page 2, they say, “Stated another way doubling the insulation thickness (R-value) and cost; only provides a modest 2% increase in heat flow reduction. Based on this observation, it is very difficult to justify the additional cost of adding insulation beyond 6″ in thickness.” Hmmmmm…..

    I think that the above sentences were the whole goal of the exercise, and they had to do some gymnastics in order to create a starting point, no matter how odd, that would get them to that goal. The cost of spray foam insulation is it’s main competitive disadvantage, and this exercise is intended to deflect from that disadvantage. This comes at the potential cost of under-insulating a building, resulting in increased heating and cooling costs for years to come. (With the associated carbon release.) As with the discussion I linked to in the above comment, it appears that Icynene may be quite willing to make some “extraordinary” arguments in order to benefit their product/business.

    In reality, doubling the thickness of the insulation reduces the ACTUAL HEAT FLOW by 50%. But what the above sentence from the brochure claims is that RELATIVE TO their odd starting point, those second 6″ of insulation only saves 2%, which is technically a true-ish statistic, but not necessarily useful.

    OK – let’s jump back to their starting point: “Based on the assumption that the outside air film at R-0.2 and the inside air film at R-0.7, the total R-value before the application of any insulation is 0.9.” Yep – they are starting with a magical “R-0” wall. I guess this could be an inflated bubble of visqueen. That’s how far they have to go in order to get to that claim where an actual reduction in heat flow of 50% becomes a “relative” reduction of only 2%.

    Using their numbers of this magic “R-0″ wall with interior and exterior air films, and a temperature differential of 40 deg F, the starting point is loosing 44.4 BTU per hour per square foot. When you add the first 6″ of open cell spray foam, the R-value goes up to 22.5, and the heat loss goes down to 1.78 BTU/sf/hr. Add another 6″, and the R-value goes up to 44.1, and the heat loss drops to 0.91 BTU/sf/hr. So, 1.78 – 0.91 = 0.87, which is 3.9% of the original 44.4 BTU/hr/sf. (Technically, that’s 4%, not the 2% they claim, but that’s splitting hairs.)

    Try the math yourself, but start with a more realistic wall assembly. A Chicago 2 or 3-flat with 12” brick, no insulation and plaster on lath might have a starting R value closer to 5, compared with Icynene’s contrived 0.9. The resulting “relative reduction in heat loss” is quite a bit greater for those second 6″ of insulation.

    They have a good point overall – piling insulation on top of insulation has a diminishing return. That’s true for all types of insulation, not just spray foam. If your goal is cost-effectiveness, then it doesn’t make sense to spend too much on huge amounts of insulation. You don’t need advertising spin to realistically understand that issue, though.

    But the other way to look at it is, “what is my target for energy usage to heat and cool this building, and what level of insulation is needed to achieve that level of energy use?” When your goal is net zero energy use, then your starting point allocating energy for heating and cooling will be quite low, which will translate to very high levels of insulation. The relatively high cost of the insulation will be partially offset by reduced energy costs, but overall, it’s a trade-off where you gain the goal of zero net energy, but at the cost of cash.

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