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 Reducing Concrete Slab Vapor Transmission

ÓHealthy Living Spaces

  Vapor transmits from the ground, through the slab and into the building envelope by a difference in vapor pressure, even in the desert. Vapor pressure depends on a variety of factors including temperature and humidity. The warmer and dryer air inside a building is relative to the ground, the greater the amount of moisture will be released by the slab. High rates of moisture emission may result in mold growth under carpet and/or musty odors under flooring and in walls. Moisture emission from the slab increases the level of humidity indoors. In the winter, higher humidity levels may cause condensation on windows. Steps should be taken during construction to minimize the potential for vapor transmission from slabs.  
  Tips for New Construction  
  • Build slab above grade. Exterior grade should be at or below level of sub grade, sloped away from wall at 5% (6 in per 10 ft).
  • Install a vapor barrier on top of 6 to 8 inch clean, dry, coarse gravel  (no fines, no p-gravel) The suggested material is polyethylene plastic. The VB should be installed under the entire slab, lapped over the foundation/footing and sealed to the foundation wall. Monolithic pours: extend VB under grade beam to act as a capillary break. Insulating materials such as polystyrene are semi-permeable. When used under a slab without a VB, they permit water to dry to the exterior if the slab becomes wet from above.
  • Do NOT put sand under or on top of a VB. The VB will still function with holes. If sand gets wet it will stay wet for decades.
  • Use a good quality, low water-cement ratio concrete. Increasing the amount of water greatly increases vapor permeability and decreases concrete strength.
  • Add control joints to concrete and cover joints with flexible caulk.
  • Cure concrete by ponding water and covering with wet burlap. Cured concrete is less porous. Control the curing process and make it as long as possible (a minimum of 3-7 days).
  • Install a capillary break membrane on top of the foundation wall (under the sill) or wrap damp proofing membrane from the sides of foundation wall onto top of foundation wall. This may act as a termite barrier also.
  • Damp proof or install a moisture barrier membrane on the foundation wall from grade level to footer.
  • Install French drains next to foundation using perforated pipes drained to a sump or below grade. Backfill with coarse gravel to grade level and place filter fabric midway. Waterproof coatings are NOT a substitute for a drainage system, as concrete and block are known to crack.
  References / Suggested Reading  
  Lstiburek, Joseph, P.Eng., Builder’s Guide, Energy Efficient Building Association, 1998.  
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Revised: July 05, 2017.

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