Building Science Corporation
May 20, 2009

This construction uses 2 layers of 2" foil-faced polyisocynaurate insulation against the concrete foundation and a 2x6 framed wall with R-19 unfaced fiberglass insulation in the stud space. The slab is insulated with 2 layers of 2" XPS rigid insulation under slab and the rim joist is insulated with closed cell high density spray foam insulation.

Foundation Wall

  • Free draining backfill
  • Dampproofing to grade
  • Concrete foundation wall (over a capillary break on the concrete footing)
  • Two layers of 2" foil-faced polyisocyanurate insulation
  • 2x6 frame wall with unfaced fiberglass cavity insulation
  • Interior gypsum wall board with vapor retarder paint

Foundation Slab

  • Concrete foundation slab
  • 6 mil polyethylene vapor barrier under slab
  • Two layers of 2" XPS rigid insulation under slab
  • 4” stone pad (no fines)
  • Undisturbed/ native soil

Thermal Control

This foundation wall system has a calculated parallel path R-value of R-45.0, and a yearly heating energy consumption of 11.09 MBTus assuming R-20 under the slab and in the thermal break. This is the highest R-value foundation system in this study and is likely not cost effective unless the rest of the house is super insulated and airtight.

Moisture Control

This wall was analyzed in WUFI to predict the moisture related risk in the wall system, and it was shown that the RH at the surface of the XPS in the above grade portion of the wall is slightly elevated in the winter months (Figure 2), but does not exceed 90%. There is almost no condensation potential (Figure 3) except on the upper wall of the north orientation for two very short periods. There is virtually no risk of moisture related issues in this all system if the interior RH is controlled with a dehumidifier, and the interior drywall is well air sealed.


Figure 2: Predicted RH at the interior surface of the PIC 


Figure 3: Interior air leakage condensation potential

Constructability and Cost

This wall system is slightly more expensive than High-R Foundation 14 by changing the 2” of XPS to 4” of foil-faced polyisocyanurate. It is possible to use 2x4 framing stood out from the XPS by 2 inches, and use R-19 fiberglass batts, or blown cellulose or fiberglass. R-19 fiberglass batts should be less expensive than R-13 fiberglass batts because the manufacturing process for both R-19 and R-13 batts uses the same amount of fibreglass, but the R-13 batts require more time and effort to compact to 31/2” making them more expensive to produce.

Other Considerations

Depending on site specific conditions, and local costs, this wall is likely not economical to build unless the house is very highly insulated and airtight. 


Mitalas, G.P., Calculation of Basement Heat Loss, National Research Council Canada.