Thermal conductivity is an extremely important measurement when it comes to insulation. In the US an R-value, or resistance value, is assigned and, simplistically, the higher the number the better resistance to heat transfer. R-value minimums required by building codes have been on the rise as we strive to build more efficient homes, i.e. those that take less energy to heat and cool. Not surprisingly, these changes have increased the demand for higher R-value products.
A standard R-value is around 3.5 per inch. This equates to R19 in a 2×6 wall cavity which was very much the norm for years. These days R21 is often a minimum acceptable level for exterior walls which is more like 3.8 per inch. For perspective, our blow-in insulation outperforms most fibrous insulation at R4.3 per inch and closed cell spray foam leads the way at ~6.7 per inch.
These figures are all calculations against the unanimously common ASTM C518 test. So what is ASTM C177 and why are we even talking about it? Below is a summary from a recent call with the ASTM appointed technical lead on C177. It is fair to say he is also an authority on C518.
It is a direct method test. The plate is heated to a desired temperature using a direct power source and the measurement is essentially how much power does it take to keep the heat level at a desired temperature. The power measurement is what goes into the power calculation (k). The test is a direct power measurement. There is no standard and there is no comparable reference except another test. ‘To get a number and not have it compared is essentially useless.’
Note: before getting to ASTM C518 it was made clear that many manufacturers use both tests but the C177 is for higher temperature rated products; for homes, testing should be done at room temp which is exclusively C518. Worth noting: a typical deviation is <3% when comparing C518 and C177.
It is an indirect test method. A transducer is used to measure temperature and microvolts (the latter considers some power). The equipment is calibrated to perform a specific function. It is a more uniform way of testing and therefore more readily used – specifically for homes.
In the spirit of full disclosure this debate kicked off because a competitor of ours is reporting an R-value of 5.8 per inch via C177 standards and 3.15 per inch via C518. First, it is virtually impossible for a fibrous insulation to achieve such levels but we are also well outside the bounds of a suggested 3% deviation. This instance is an increase of 84%.
With our curiosity piqued we reached out to the firm that conducted the tests. Not only was the gap between the two results glaringly odd but even more so was the fact that no density was listed. We’ll get to that conversation in a second but first it is worth noting what RDH Building Science had to say about the lack of density being reported. Paraphrasing: Essentially, the report is completely useless as there is no way to understand how much material was used to achieve the results. Real or not, there is zero applicability of these scores.
The last conversation, so far, was the most interesting. The executive director of the testing firm thanked us for bringing it to his attention that there was no density listed. He further promised to list those ‘things’ going forward. In the interim, he suggested we rest assured that every test they’ve conducted would have hand written notes detailing crucial measurements!
And before hopping off the call he offered the icing on the cake: an anecdote that has us losing sleep at night. Taking time to highlight his first job as an engineer at an acoustics laboratory he spoke of early experiences as follows: Let me tell you, I used to work for an acoustics company as an engineer. I did my measurements against the guide logs and reported findings to our clients. My boss saw the work and said where’d you get the numbers? I said from the guide books. He said, no don’t use those numbers, I’ll give you the correct figures to measure against. Since those days I’ve seen it all!!!
It is unclear if this is a straight up admission of guilt, but for sure we will not be testing with this lab.
So, what’s the outcome? ASTM C518 is how you should measure resistance value for home insulation. Are we going to get a C177 test? Yes, from a reputable lab. And we will happily follow up with the results.
In closing we’d reiterate a theme you hear from us quite a bit. Vigilance is paramount in making decisions in the built environment. This post is simply a bit of insight as to why. There are individuals and companies out there that struggle with the truth. If your instincts are telling you something is off you are probably right. And if test scores don’t tell the whole story, ask the tough questions and don’t give up until you are informed and can feel good about whatever decision you make.
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