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:
- Conductive heat flow
- Convective heat flow
- 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:
Tags: economics, insulation
I have something else to throw into the mix: Aerogel! Have you heard of it? It sounds like it’s being made more widely available/affordable for residential home usage. See the following article:
http://news.cnet.com/8301-11128_3-10445362-54.html
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…
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.”
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.
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!
Cheers,
Skylar
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.
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.
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.
http://www.amazon.com/ARMA-Foil-Radiant-Strength-Perforated/dp/B000GPU68K
Probably worth a bit more research, no?
[1] I had to look up the units for permeability. A good source of info is here: http://www.numericana.com/answer/gas.htm#perm
Marcus — you should add this discussion to my previous post, if possible:
http://www.greenbuildingtalk.com/Forums/tabid/53/aff/22/aft/59724/afv/topic/Default.aspx
Even though it’s about attics, I think that it’s contains a very informative lesson about how radiation/convection/conduction affects the performance benefits of radiant barrier, cellulose, and fiberglass.