The Air Barrier and Your Home by the Orillia Home Inspector
In Ontario the air barrier in a home is installed under the authority of the Ontario Building 9.25.4.3. Installation of Vapour Barriers
(1) Vapour barriers shall be installed to protect the entire surfaces of thermally insulated wall,
ceiling and floor assemblies.
(2) Vapour barriers shall be installed sufficiently close to the warm side of insulation to prevent
condensation at design conditions.
Where a vapour retarder is employed, the opposite wall surface must provide a permeable surface to allow drying to occur. Thus, in hot, humid, cooling climates, where a vapor retarder is employed at the exterior, the interior wall surfaces should be permeable. Novapor retarder paints, kraft-faced insulation, or vinyl wall coverings should be used. Conversely, in northern heating climates, with interior vapour retarders, the exterior wall coverings should be vapour permeable. This simply means that in warmer climates where cooling is the main concern, the vapour barrier would be installed on the outside of the insulation.
House wraps are a permeable surface which, while protecting your home from the elements allows moisture to pass. This allows any moisture that is between your vapour barrier and house wrap can escape and evaporate rather than collecting in your wall system. The primary attribute of house wraps is their ability to operate as air infiltration barriers while not forming an impervious vapour barrier. When placed over the exterior surface of the wall sheathing, the material allows moisture vapour to escape from the frame wall cavity while reducing convective air movement in the insulation, thereby helping to maintain the composite R-value of the wall.
The Dew Point of your house is the area where warm moist air will convert to moisture. The dewpoint is a measure of atmospheric moisture. It is the temperature at which air must be cooled in order to reach saturation (assuming that air pressure and moisture content are constant). As the surface of the earth cools at night, warm moist air near the ground is chilled and water vapour in the air condenses into droplets on the grass and other objects. Dew is particularly heavy on clear nights, when the earth cools rapidly. When a blanket of cloud insulates the earth, the cooling rate is slower. The greater the difference between the temperature and the dew point, the drier the air
When warm moist air infiltrates your vapour barrier, and passes through to your exterior wall, when it meets a cold surface or void, moisture is created as it cooled to its dew point. This can cause wet insulation, frost on exterior walls and in some cases even mould.
Prevention is possible by ensuring all your insulation is covered by a vapour barrier, most codes call for a minimum of 6 mm poly, and all penetrations are overlapped and taped by Tuck tape, the red tape not the silver duct tape of Red Green fame. All electrical boxes, windows, switches, and vent penetrations should be tightly sealed.
Basements are more susceptible to breaks in vapour barriers due to the fact that most services are located there and many installers fail to replace insulation and repair holes in vapour barriers etc. This can be increased if you have laundry facilities located in your basement and an open sump hole which contains water.
Humidity is the main factor of the creation of moisture and if you live in a climate where the humidity is less than 35% this would not be an issue for you. So if you have high humidity using a dehumidifier in your basement can alleviate these moisture problems.
Written by Roger Frost, the Barrie Home Inspector