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High-R Foundation 09: 2" Polyisocyanurate Insulation, 2x4 Framing with Cellulose

By Building Science Corporation    Created: 2011/01/15

  • Dampproofing
  • Concrete foundation wall
  • 2" foil-faced polyisocyanurate rigid insulation
  • R-12 cellulose insulation
  • Gypsum board with vapor retarder paint
  • 2" XPS rigid insulation under slab


Thermal Control

This wall system has an installed insulation R-value of R-25 which is only slightly lower based on the parallel path calculation method which accounts for the wall framing assuming 24” on center. This basement combined with R-10 under the slab and R-10 thermal break results in an annual predicted heating energy loss of 15.45 MBtus.


Moisture Control

The water vapor diffusion and capillary wicking are controlled by 2” of PIC insulation assuming that the PIC is well sealed to the concrete. This wall system was not hygrothermally simulated since it will not experience any moisture related issues. The foil face on the polyisocyanurate will not allow vapor diffusion from the concrete foundation, and the increased R-value of PIC compared to XPS will increase the condensation surface temperature compared to High-R Foundation 8 and High-R Foundation 14, resulting in decreased condensation potential.


Constructability and Cost

Fiberglass batt insulation could be used in the place of cellulose to decrease the cost of the assembly.


Other Considerations

High-R Foundation 8 is one of the simplest methods of minimizing the moisture risk and saving energy which also allows the basement to be finished. It is possible to use other air permeable insulations instead of cellulose including fiberglass batt or spray fiberglass. 



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