Subslab insulation
I’m getting close to installing subslab insulation in my Climate Zone 6 build. I’m trying to adhere to BSC’s 60/40/20/10 guidelines, where Joe Lstiburek is recommending R-10 under slabs. My interpretation of this is the recommendation is for basement slabs, but not sure if this is a similar value for frost protected slab on grade or if more is needed?
My foundation is more like a shallow ICF stem wall, with about R-8 on the outside and R-12 on the inside. I double stud wall will sit on top of the ICF and the interior insulation is providing a thermal break to the slab perimeter. I had planned on R-20 underslab, but reclaimed insulation isn’t an option due to my remote location, and XPS insulation will be a large expense. I’m trying to balance cost/performance.
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I think the 2015 IRC indicates either edge or subslab insulation to R10 in Zone 4 and above. But I'm just going by what I see online. Let's let someone with more real-world experience weigh in.
How remote is your location?I bought about 2000 square feet of reclaimed 4" xps and paid for it to be shipped about 150 miles and it still cost less than half of what new insulation would have cost.
The ICF-like will do a better job at protecting from frost heaves if you put the R12 on the exterior and R8 on the interior.
XPS is expensive and unnecessary for use under the slab. Type-II EPS is about 2/3-3/4 the cost of XPS, and just fine for sub-slab but not under the stem wall footing, unless the engineering on the footing & foam has been done to ensure it can take the load. EPS is also greener due to the dramatically less damaging blowing agents used, and it's performance is stable over time, whereas XPS loses performance over it's lifecycle as the blowing agents leak out.
@Stephen,
I'm in south-central Montana. I have looked at all the insulation recyclers I could find online and they are all at least 1000 miles away. There are some more local, but the insulation they receive is mostly from roofs and are polyiso.
@Dana
I have my two rows (24" height) of the ICF's placed, but not secured. I had planned to put the R-12 on the outside of the wall, but it seemed to align with the wall bottom plate better with it on the inside. I can get it on the outside with some additional custom cutting of blocks and may still go that route if there is enough benefit.
All the normal suppliers here stock either Type I EPS or XPS, but I'll check with some specialty places. In price comparisons I've done here, EPS is cheaper per sheet, but almost identical on $ / R-Value.
It's a bit of a haul (still less than 1000 miles away, even round-trip) but this outfit in SLC seems to be dealing in used EPS:
https://saltlakecity.craigslist.org/mad/d/eps-rigid-foam-insulation/6217596831.html
These folks in Henderson CO can probably hook you up with EPS too:
https://fortcollins.craigslist.org/mad/d/used-poly-iso-insulation-sold/6213342528.html
http://repurposedmaterialsinc.com/
@Dana
Thanks for the resources, but the EPS in SLC is only Type I. great price though, if one wants thick foam on the walls.
EPS is easily and commonly custom ordered. These folks have a location in SW Montana:
http://univfoam.com/how-to-order
Here's another:
http://www.bigskyrcontrol.com/
thanks Charlie
I just thought I'd finish this off by letting everyone know that even after much searching, I was still finding it difficult to get good pricing on rigid foamboard, even EPS. I was thinking I was stuck buying new and the day I was going to pull the trigger on new stuff, I checked Craigslist and found some leftover, but new, XPS insulation at 33% less than what I was going to pay.
Craigslist to the rescue! Craigslist * materials is so much fun to browse...
@Ethan,
no lie, Craigslist has been good to me. I have gotten a complete package of quality windows, 4 rolls of Tyvek stuccowrap, lifetime guaranteed shingles, bathroom vanity, kitchen cabinets, and now subslab insulation. I guess I am building a low-buck Pretty Good House, Craigslist-style.
I'm surprised you got good windows... PGH level windows? R-5?
Ethan,
The windows are a bit less than BSI's recommendation of R-5 in the 5/10/20/40/60 model for Zone 6. They are Milgard's top of the line PVC, double-pane, mostly Argon filled . R-Values vary a little depending on treatments, but they are all around R-3.5 to 4. Of course, it's a compromise and I will make the energy difference up by exceeding the slab, wall, and roof R-value recommendations and maybe add some thermal blinds for winter comfort, if needed. I looked at new triple-pane (always special ordered for around here) and also the least expensive imported Euro units (very high shipping cost). Either option was at least quadruple my Craigslist finds and payback was over 100 years. With the difference, it'll pay for a PV system that will provide a realistic payback.