Replies

1. 
   matt2021 | Jan 26, 2023 06:23pm | #1

   Howdy,

   Some quick questions:

   - are you sure you’re looking at the correct zone map? I may be wrong, but “5b” recalls the zones for plants. You should look at the climate zones for insulation:

   https://www.energystar.gov/campaign/seal_insulate/identify_problems_you_want_fix/diy_checks_inspections/insulation_r_values

   - Are you acquainted with the ratio rule regarding hybrid roofs? See here:

   https://www.greenbuildingadvisor.com/article/the-ratio-rule-for-hybrid-roof-insulation

   - Why XPS, which is not environmentally friendly, and will give you an R value of 20 at 4”, while you could use polyiso boards (there are specifically food roofs), which have a much, much smaller global impact warning for the way they are produced, and have an R value, at 4”, closer to 24.

   Others way more competent than I am will chime in, I am sure. Good luck with your project!

1. Expert Member
   Michael Maines | Jan 27, 2023 10:39am | #5

   This post explains the climate zones for buildings: https://www.greenbuildingadvisor.com/article/all-about-climate-zones. 5b is basically west of the continental divide. "B" is dry; "A" is moist.

2. 
   thefinn2003 | Jan 26, 2023 06:55pm | #2

   i believe i have the right climate zone. yes, 4" xps and r19 inside gets me on the right side of the ratio. and 2" polyiso is strangely not available locally. also, i don't believe manufacturer claims for polyiso. my research indicates that it's comparable to xps.

1. Expert Member
   Akos | Jan 26, 2023 07:17pm | #3

   You need to check commercial roofing suppliers, around me fiber faced roofing polyiso is the best $/R, way cheaper than XPS. Even in zone 5 will not get cold enough for long enough time to worry about poyiso cold weather performance, rest of the time it will outperform XPS and EPS. Plus lot of the manufacturers have now adjusted their formulation to fix the issues.

   EPDM can be installed directly over polyiso which saves some cost. There are cover boards you can add but those tend to cost about the same as a layer of iso so might as well have a layer of sacrificial insulation, in your case 3" +1" layer might make the most sense.

3. 
   onslow | Jan 26, 2023 07:30pm | #4

   thefinn2003,

   Not sure what you are putting the R-19 up between. For my own un-vented roof the crew used steel pins stuck to the underside of the roof deck. The batts were pushed over the pins and discs onto the pin to keep them tight to the underside. I had trusses, so pretty much the only way to ensure they stayed put.

   I did need ice and water shield under my metal roof areas per local code. I did not use ice and water shield under the 8" nail base panels. I do not recall which synthetic roofing material was used on the primary roof deck. I don't think you could adhere EPDM to it anyway.

   If you happen to be in Colorado, the new lower GWP XPS is available at Home Depot from the Foamular people. I priced it for a now dead project and it was very close in price to the old stuff.
   The other brand available was twice the price and slow to deliver.

   Not sure how you plan to keep the foam pinned to the deck, but be sure to pin it real good. The winds at my altitude create a rather surprising amount of uplift when the skim over my 1:12 roof areas. My membrane is not glued and some areas show a disturbing amount of uplift.

   As to the 4" to R-19 ratio, yes you will be okay most of the year. Being in the B part like I am means the humidity fight is a bit easier. Sun loads might make you want to consider a bit more R top side, but that's your call and wallet. The non penetrated ceiling surface will be a big boost to keeping the risk factor low as well.