Insulating basement with partially cast-in-concrete joists – GreenBuildingAdvisor

Hi GBA Community,

My 1960s Calgary bi-level (Zone 7A) has partially cast-in-concrete floor joists. I’ve read the threads below but want feedback before finalizing my approach.

 References:

– NRCan Fig 6-18/6-19 (air-sealing & insulation details): https://natural-resources.canada.ca/energy-efficiency/home-energy-efficiency/keeping-heat-section-6-basement-insulation-floors-walls-crawl-spaces

– GBA Q&A on embedded joists: https://www.greenbuildingadvisor.com/question/insulating-at-the-sill-when-joists-are-embedded-in-concrete

– Holladay’s article on embedded joists: https://www.greenbuildingadvisor.com/article/insulating-basement-walls-with-embedded-joists

– How to Insulate a Basement Wall: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall

Situation:

– Cantilevered joists (bay windows) were retrofitted with XPS + Rockwool per GBA’s detail library, so some joist ends are proud of the foundation, others rest on it.

– Exterior insulation isn’t feasible elsewhere (asbestos stucco, attached deck), so I’m choosing the “least bad” interior approach.

– Demo revealed inconsistent joist-bay insulation (faced fiberglass/unfaced fiberglass/none) and a poly-sandwiched fiberglass wall assembly. No visible moisture behind the walls; the only mold was near an improperly sealed retrofit window. Joists show no rot after 60 years, and I don’t want to risk that.

– Original fir blocking sits against the foundation at each joist bay end, creating a ~1.5″ lip

– Using 2″ XPS (already on hand from a bulk purchase).

Risk Factors (per Holladay’s article):

– Cold foundation: Higher risk (Zone 7A, exterior insulation not practical)

– Wet foundation/climate: Lower risk (Calgary is dry)

– Poor grading/clay soil: Average risk (graded away on all sides, but clay-rich soil)

– High basement humidity: Lower risk (radon mitigation keeps basement/sub-slab dry)

– Grade-to-lowest-wood distance: Lower risk (bi-level, bottom of joists ~3.5 ft above grade)

– Sun/wind blocked by vegetation: Low risk on 3 sides, higher on north (deck blocks sun)

– Wall orientation: Higher risk (insulating north wall)

Current Plan (feedback welcome):

 Basement wall:

1. Liquid-applied vapor barrier (Redgard) below grade

2. Air-seal slab/wall junction with polyurethane sealant (Tremco Dymonic 100)

3. 2″ XPS, seams taped (SIGA Sicrall or Wigluv)

4. 2×4 stud wall, Rockwool Comfortbatt

5. Mold-resistant drywall

 Joist space:

1. Leave blocking as-is; air-seal seams with polyurethane sealant (Tremco Dymonic 100)

2. Drill hole in blocking, inject closed-cell foam to seal foundation-to-subfloor gap (per NRCan Fig 6-18B)

3. Air-seal only, leave joist bays uninsulated

Key Questions:

1. Do you agree with NRCan’s spray-foam approach for the foundation/floor gap? What foam type, and is there deformation risk when injecting into a confined space?

2. Leave blocking in place, or move it 1-2 ft from the wall?

3. Gap between top of XPS and bottom of embedded joists to let the joists breathe, or butt XPS directly against joists and air-seal as one unit?

4. Am I being overly conservative on leaving joist bays uninsulated? If yes, how would you insulate them?

Thanks,

E.