Replies

1. Expert Member
   Michael Maines | Oct 23, 2019 11:09am | #1

   Gary, because concrete is a very good conductor of heat, in a situation like yours it's probably best to create a "bathtub" of foam inside the existing concrete. The IRC requires R-10 under a slab in your climate zone, and that's a good minimum target--at least 2.5" of EPS or XPS. You'll get lower energy bills, increased comfort, and an under-appreciated benefit: a low risk of mold and fungal growth.

   If purchased new, EPS is a much better choice, due to the extremely damaging blowing agents in XPS. If you have a source for used XPS it's a viable option--just understand that the aged R-value is 4.2 to 4.4/in, not the R-5/in advertised.

1. 
   gary__b | Oct 24, 2019 06:07pm | #2

   Thanks for the input Michael. I hadn't thought of a foam "bathtub." It'll create an interesting 12" tall bump-out at the bottom of the walls, but definitely an option I'll consider.

2. Expert Member
   Dana Dorsett | Oct 31, 2019 04:44pm | #4

   >"The IRC requires R-10 under a slab in your climate zone, "

   No, it doesn't.

   The IRC requires R10 at the slab edge-only, down to either the footings or to 2' below grade, whichever is less. There is no under-slab requirement. See TABLE N1102.1.2

   https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency

   In zone 4C (a heating dominated climate) it's still "worth it" on a lifecycle energy cost basis to install ~R8 (2" of EPS) over the entire slab. The deep subsoil temperatures in Portand OR are in the mid-50sF, which isn't exactly warm, but not super frigid either. The much longer/deeper heating season benefit far outweighs any summertime "free cooling" benefit. If it's possible to retrofit the code-min R-10 onto the exterior side of the stemwall that's worth it too.

   For a second opinion, see Table 2, p10 of this document:

   https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

   For a slab-on-grade it would be fine to use 1.5" of polyiso rather than 2" of EPS under the subfloor, but not for a basement slab where flooding is possible or likely.

   For a shorter term "payback" it may only make financial sense for 1" polyiso under the subfloor unless using used or factory-seconds goods at a steep discount. The greenest foam insulation is reclaimed foam or factory blems that would otherwise be wasted as scrap in a landfill or worse. There are several used-foam vendors in OR, some of whom advertise here from time to time:

   https://portland.craigslist.org/search/sss?query=rigid+insulation

   With used foam it's important to check thickness variations when laying it down for a slab-top application, since the nominal thickness is sometimes compressed a bit over time in commercial roofing applications.

1. 
   gary__b | Nov 01, 2019 12:08pm | #6

   Thanks so much Dana! One follow-up: you said the "exterior side of the stemwall." It's possible, but much more difficult than the inside of the stemwall, and I think less effective (because of the sidewalk outside, the stemwall insulation wouldn't go below grade, so there's a heat loss path from the soil to the slab inside). Did you specify outside stemwall for a reason, or are you onboard with Michael's "bathtub" approach?

2. 
   gary__b | Oct 31, 2019 04:06pm | #3

   Just to circle back on insulating the floor/stem wall. I plan to go forth with the insulation as recommended. Maybe not quite to code, but close (targetting 2 inches, so depending on the type...). So then any reason not to do polyiso in this situation? I know in the winter it's going to suffer from cold weather performance, but I'm thinking at the ground temperatures here it's still going to have higher-R than EPS. If the slab is dry, it should be fine, right?

1. Expert Member
   Michael Maines | Oct 31, 2019 05:36pm | #5

   Gary, IF the floor never floods, polyiso should be fine. But it absorbs liquid water so it's safer to use EPS. Or recycled XPS.