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High-R Foundation 13: Insulated Concrete Forms (ICF), 2" XPS on Interior and Exterior

By Building Science Corporation    Created: 2011/01/15

  • Dampproofing
  • 2" XPS rigid insulation
  • Concrete foundation wall
  • 2" XPS rigid insulation
  • 2" XPS rigid sub-slab insulation
  • Gypsum board with vapor retarder paint
  • 2" XPS rigid insulation under slab

 


Thermal Control

This construction strategy has an installed insulation R-value of R-20 and has a predicted annual heating energy loss of 16.7 MBtus.

 

Moisture Control

Two inches of XPS on the interior, connected to the thermal break at the slab edge, controls the interior vapor drive and capillary wicking to the interior so there are no moisture related issues from inward vapor diffusion or capillary wicking.

 

Constructability and Cost

The interior of the insulated concrete form will require drywall or other thermal barrier to achieve the fire rating required by code. The gypsum board is very easy to attach to the plastic clips designed into the ICF. The drywall should not be painted, if it is not necessary, to allow maximum drying of the concrete. It may be easier and more practical to install a thin framed wall (e.g. 2x3 wood or steel framing) on the interior of the ICF to allow any necessary services to be run in the wall, and potentially more insulation.

 

Other Considerations

Because the concrete is installed between two vapor retarding layers, it will take several years for the concrete to dry to equilibrium. Since additional interior vapor control should be avoided, no more than latex paint should be used on the interior surface of the drywall. 

 

 


Reference

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