Building Science Corporation
May 20, 2009

  • Dampproofing
  • Concrete foundation wall
  • 2" XPS rigid insulation
  • R-19 unfaced fiberglass batt
  • Gypsum board with vapor retarder paint
  • 2" XPS rigid insulation under slab


Thermal Control

This foundation wall system has a calculated parallel path R-value of R-28.7, and a yearly heating energy consumption of 14.79 MBTus assuming R-10 under the slab and in the thermal break. Only if the rest of the enclosure is super insulated, and airtight, in a very cold climate will it make sense to increase the R-value of the foundation wall. It may make sense with an R-30 foundation wall to increase the underslab insulation to R-15 or R-20. This should be examined in more detail.


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 elevated in the winter months (Figure 2), and that there is some condensation potential alternating with periods of drying potential at the top of the foundation wall. (Figure 3). There is little 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 XPS



Figure 3: Interior air leakage condensation potential


Constructability and Cost

This wall system is slightly more expensive than High-R Foundation 8 and High-R Foundation 9 by increasing the depth of the framed cavity with 2x6 framing instead of 2x4 framing. 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

None noted. 




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