Replies

1. GBA Editor
   Martin Holladay | Feb 11, 2014 05:09pm | #1

   Jonathan,
   Manufacturers' claims for the R-value of polyiso are all over the map. If you calculate R-6 per inch for the long-term performance of the polyiso, you'll be pretty safe -- except for cold temperatures, for which R-4.5 or R-5 is probably safer.

2. 
   jchwang | Feb 11, 2014 06:32pm | #2

   Using the same product, so interested to hear your experiences. If you want to connect offline, feel free to ping me at [email protected]

3. Expert Member
   Dana Dorsett | Feb 12, 2014 05:44pm | #3

   The R value to use depends on the application, and where it's located in the R-stackup. Freshly blown polyiso ALL runs about R8-R10/inch on hour-1, day 1, but falls off over a few years, which is largely a function of the blowing agent seeping out through the facers. Heavier metal-clad panels like the Vicwest AWIP could hang onto that higher R for long enough to matter, or maybe not. If they're selling it as a roofing panel rather than sheet insulation they can make some pretty optimisitc claims without running afoul of the FTC the way they would if they were selling it as insulation. (ICF and SIP vendors often have some interestic flights of fancy too.)

   The aged fully-depleted R-value at 75F mid-foam temp is typically between R6-R6.5. If it is installed at a layer in the assembly that averages anywhere between 55F and 80F on both sides of the foam during the relevant seasons you can pretty much go with R6, even long term.

   But if it's the outermost scabby inch on a high-R assembly in a zone 6 location that averages less than 15F in the winter, that last inch might only be good for R3 for an average number. Figure on at least R4.5 for an inch that averages 45F through that inch of foam, maybe even R5.5.

   Zone 5 locations aren't really all that cold, with January mean outdoor temps in the mid-20s, despite single-digit or even negative digits outside design temps. A 2" layer of iso on a 2x4/R13 studwall wall will have an average temp through the foam layer in the mid-40s F, and will deliver an average winter performance of better than R10 even if only performing at R7 during the temperature extremes. The same 2" layer on the exterior of a 2x6/R20 wall may only have a mid-January performance of about R8, but a seasonal average still above R9- lower than in the 2x4 case because the average foam temp is lower.

   Not all polyiso is manufactured in exactly the same way, and some DO have a climbing R for awhile, but all fall off when the mid-foam temps are in the 30s and lower. See the variation in measured derating curves in Figures 1 & 2 of this document:

   http://www.buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-value

   Note that manufacturer sample D's performance PEAKS at 40F center-foam temp whereas the sample from manufacturer B has already slid into the R4.5 range at that temperature.

   See Figure 3 for the R-performance of a 2" iso samples assuming an interior side temp of 72F plotted against outdoor temp. You'll see that if the polyiso is the only insulation in the assembly, it's performance for most samples increases as outdoor temps fall toward the mid-30s F, then fall off with temperature thereafter.

   So, bottom line, it's performance really DOES depend on the application, and you can't use a single R-value number that's relevant to every stackup or location.

4. 
   jonathanforddalton | Feb 15, 2014 03:25pm | #4

   Interesting, thanks for the answers. I'll likely go for 8" thick plus a flash coat of spray foam which should land me between 45 to 55 real life R value.

   I assume the code requirements (R-48 for the roof here) are based on specified R-value? Not that anything up there is / was to code, but I make sure whatever work I do is.

   Jerry - I will keep you posted on the results. I'm using this for a roof. Interested to know what your deal is as well.

5. 
   user-1055444 | Mar 25, 2014 08:49pm | #5

   Jonathan, hoping you might check this and be able to reply offline. Interested in asking you a couple follow-up questions about this--if so, please email me at jonathannagarre gmail.com.

   Thanks!