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Last Updated: , Created: Thursday, October 14th, 1999

MYTH: LEAVE AN AIR SPACE BEHIND THE INSULATION IN THE BASEMENT TO AVOID CONDENSATION.

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False: An air space behind basement insulation will not solve condensation problems. It can, in fact, cause condensation problems -- and create new problems to boot. The CMHC is so clear on this question that they even advise that if you are going to glue insulating panels to the wall, you do not dab the glue on the wall but apply it in a closed grid pattern that will prevent the formation of a circulating air space -- even one as thin as the glue. Here's why.

-- The concrete of a basement wall insulated on the inside will have a very large temperature difference between the top of the wall and the bottom of the wall. The top is exposed to the cold outdoors and the bottom is insulated by the earth.

-- Air in a space between the insulation and the concrete wall will become cold and heavy at the top of the wall and tend to drop to the bottom.

-- It is almost impossible to ensure that there is absolutely no space between the front of the insulation and the drywall. This space becomes the primary route for warm air to be forced up to the top of the wall by the pressure of the falling cold air in the back. Hence we find a very strong convection current that loops around the insulation.

-- This same mechanism does not happen in such a serious way with an ordinary wall totally exposed on the outside because you have an evenly cold exterior, not the large temperature differences that exist from the top to the bottom of an internally insulated basement wall.

-- The convection loop will draw moisture both out of leaks into the wall from the house and out of the lower portions of the concrete itself. This concentrated accumulation of moisture then tries to escape through the small portion of the wall that is above the ground level (and probably freezing cold).

-- Hence the above ground portion of a basement wall that has an air space between the wall and the interior insulation can easily become saturated with water. Wood in contact with this wall can easily develop dry rot -- including your floor joists. Repeated freeze/thaw cycles can cause spalling or flaking of the outer surface of the basement wall. Structural breakup of the wall could result with unsound walls.

-- Convection loops around your insulation will essentially eliminate their insulating effect, carrying the heat around the insulation to the cold wall behind. Insulation pushed directly against the basement wall (click here for cautions about moisture proofing the wall first) will effectively prevent these air convection loops. With no air currents, the only moisture that can get through the wall is what can diffuse slowly up to the top of the wall and out through the wall without causing saturation conditions.

Heavy condensation discovered behind insulation that is snug against the basement wall is a sign of either a very poor job of sealing the warm side of the wall or of a need to damp proof the foundation wall. This is one case where ventilation cannot help us.

Keywords: Concrete, Ventilation, Insulation, Wood, Condensation, Moisture, Basement, Walls, Foundation, Damp Proof, Temperature, Problems

Article 743