Question about insulating foundation Superior Walls
We are planning our new home in Illinois (border of Zone 4 and Zone 5). The entire exterior will be EIFS. The majority of the basement will be finished. The east wall of the basement will be a walkout, which we plan to have wood-framed. We plan to use the Xi Superior Insulation walls for our foundation, which adds 2.5″ of Dow Styrofoam to the original Superior Walls for R-12.5. Here’s a note from their website about the walls:
“Provided with more than double the insulation of the original R-5 Superior Wall system, the Xi has 2.5 inches of DOW¯ Styrofoamâ„¢ for superior warmth and
Nearly everything is insulated in the Xi, including corners, studs and bond beams. Special engineering makes the wall thermally isolated and insulated from the exterior elements. The new wall is monolithically poured (poured all at once) with high-strength concrete for solid structural integrity and reinforced with steel rebar and polypropylene fibers.
The Xi wall’s special high strength, low-water concrete mix also requires no additional damp-proofing, benefiting from technology developed for the original R-5. And the Xi meets or exceeds energy conservation requirements from both the 2009 IECC and the 2009 IRC in most climate zones. The Xi has a striking modern look before drywall installation, due to the galvanized steel stud facings that have replaced treated wood nailers. The high-tech engineering and numerous advanced features of the Xi make it the foundation of the future and the only choice for selective homeowners.”
My question is if we should add additional insulation into the cavity of the Superior Wall. We will have radiant floor heat in the basement slab and plan to put 2″ of rigid foam under the slab. Thank you!
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Q. "My question is: Should we add additional insulation into the cavity of the Superior Wall?"
A. To comply with the 2012 International Residential Code, you need at least R-10 of basement wall insulation in Climate Zone 4, and R-15 in Climate Zone 5 -- so the answer (at least from a code perspective) depends on your location. Clearly, an R-12.5 basement wall doesn't quite meet the requirement for R-15.
Of course, your local codes may differ from the 2012 IRC. In general, green builders often try to exceed minimum code requirements for insulation rather than barely meet them.
You could go with the XiPlus system which has an insulated "footer beam" and R-21 walls. Otherwise, you will be limited on what insulation you can use in the cavity unless you want to drywall the basement right away do to the needed fire barrier.
We will be finishing the majority of the basement right away.
The 2.5" of XPS in the XI walls is not continuous, so it's not really an R-12.5 wall....it's R-12.5 between the studs. The true whole-wall R-value is closer to R-9 when the thermal bridging of the concrete "studs" is accounted for (even with their 1" EPS wrap).
For a basement in CZ 5, I WOULD add insulation in between the studs, but you won't get the full performance of the added insulation. If you add an R-21 batt, the whole wall R-value will go up to about R-19.