Exterior insulation of rubble basement
First, wonderful website. It’s truly the “marketplace of ideas” for buildings.
I’m looking for thoughts on this approach to insulating a rubble basement wall.
I’m familiar with the interior approach BSC recommends below, and it is what I will do should this seem like a bad idea. https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations
I’m thinking of using 2″ CCSF (HFO or water blown) to insulate the exposed ~24″ of rubble wall on the exterior, covering it with Durock and finishing with stucco.
I can only assume this would be much less expensive (2 feet vs. 8 ft of wall), but would also be a lot less work for me (no moving electrical, water lines, no stud wall to build, etc.)
There is already interior drainage to sumps. It takes quite a bit of rain for the sumps to be used. There is concrete walkway around the entire perimeter. Interior/exterior tuckpointing is in good shape. I would parge the interior side for additional air sealing. House is located in St. Louis, MO – cool edge of CZ4.
There is some amount of energy lost through the underground portion of the wall, of course. My hunch is that CCSPF is dollar-and-carbon expensive enough that addressing the sub-soil losses are not “worth it.” That said, I don’t want to skimp on durability.
Does anyone have any thoughts on whether or not this assumption is accurate? Are there any other disadvantages that come to mind?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Hey Bryan.
You are right that "CCSPF is dollar-and-carbon expensive." However I have never seen any modeling or heard any experts suggest that insulating just the above-grade portion of a basement is effective. Maybe another GBA member has experience with this partial approach and can provide some feedback.
I have seen a bunch of old stone foundation successfully air sealed and insulated with closed-cell spray foam. It is one of the few situation where using closed-cell may be worth the trade-offs.
EIFS is a good way to go (I've seen some pretty lumpy rubble foundations finished with it). I would just go with thicker foam under the EIFS and skip the SPF.
Most of basement heat losses are in the above grade section, but I would still insulate a foot or two bellow grade (or do bellow grade insulation wings).
EIFS basecoat and finishes are not rated for ground contact. They're generally supposed to be held 6" above the soil line. You can use cement board if you want, but Durock and Hardiboard also warn against ground contact. Freeze/thaw cycling is hard on cement materials. There are a couple of fiberglass board products that can work to cover the insulation below grade.
Bryan,
NorthernStar and Building America piloted an approach with poured closed-cell spray foam and excavationless trenching for exterior rubble foundation insulation. Might be worth a look.
Video: https://basc.pnnl.gov/videos/umn-northernstar-doe-building-america-exterior-foundation-insulation
Case study: https://www.nrel.gov/docs/fy14osti/62557.pdf