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High-R Foundation 15: 4" PIC, 2x6 Framing with Fiberglass Batt

By Building Science Corporation    Created: 2011/01/15

  • Dampproofing
  • Concrete foundation wall
  • Two layers 2" foil-faced polyisocyanurate 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-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. 

 

 


Reference

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