Building Science Corporation
May 20, 2009

This construction uses 2x4 framed wall offset 2.5" from concrete with 6" open cell low density spray foam (R-21) between and behind the stud wall. The slab is insulated with 2" XPS rigid insulation under slab and the rim joist is insulated with open cell low density spray foam insulation.

Foundation Wall

  • Free draining backfill
  • Dampproofing to grade
  • Concrete foundation wall (over a capillary break on the concrete footing)
  • 2 1/2" gap between stud wall and foundation wall
  • 2x4 frame wall with 6" open-cell spray foam
  • Gypsum board with vapor retarder paint

Foundation Slab

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

Thermal Control

Open-cell spray foam provides very good continuous thermal control. Spray foam is an air barrier, so convective looping and air leakage thermal losses do not occur. This wall system has an R-value of R-21 and a predicted annual heating energy loss of 16.3 MBtus.

Moisture Control

Open-cell spray foam is an air barrier, but is vapor permeable. The relative humidity was predicted in the center of the open-cell spray foam insulation and was found to be at safe levels (Figure 2).

Low permeance interior wall finishes should be avoided with this construction strategy.


Figure 2: Predicted RH at the interior surface of open-cell spray foam

Constructability and Cost

This solution is more practical than High-R Foundation 7 if the plan is to finish the interior of the basement.

Other Considerations

Spray foams have been improved considerably for human health and the environment. Ozone depleting substances in the process have been removed, but some spray foams use greenhouse gases that are much worse than carbon dioxide. There are options available of more environmentally friendly spray foams that release green house gases, such as water blown foams, on the market and should be considered. 


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