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When insulating a basement, Manitoba Hydro recommends installing a moisture barrier that extends up to just above the grade line. Please see Figure 12 taken from Power Smart Booklet No. 2 Basement and Crawlspace Insulation.
Figure 12 illustrates the recommended installation of a poly moisture barrier to the concrete wall at about 100 mm (4") above the grade line, down to the basement floor and under the bottom plate of the frame wall.
(To order a copy of Power Smart Booklet No. 2 Basement and Crawlspace Insulation, in Manitoba call 1-888-MBHYDRO (1-888-624-9376) or visit your local Manitoba Hydro district office.)
The purpose of this moisture barrier is to protect the stud wall from potential water leakage through the concrete. Water leakage through foundation walls is a constant threat and must be protected against for the life of the wall. Including a poly moisture barrier is an inexpensive way to prevent future mould and rot problems caused by water leakage.
The use of two layers of polyethylene does create a partial double vapour barrier. However since it is not continuous, its impact will be trivial compared to the potential negative consequences of omitting this moisture barrier.
An insulated frame wall has to be kept dry if mould is to be prevented, if the insulation is to work properly, and if the wood is to be kept from rotting. The purpose of the vapour barrier (the polyethylene on the inner side of the frame wall) is to reduce the movement of water vapour from the indoors into the frame wall due to a process known as vapour diffusion. The purpose of the polyethylene moisture barrier (between the concrete and the frame wall) is to protect the frame wall from soil moisture coming through the concrete foundation.
To understand the relative magnitude of these two threats we need to quantify the amount of water typically transported through the wall due to vapour diffusion versus the amount due to water leakage. While vapour diffusion is certainly a concern, it is a very weak driving force and contrary to popular belief, seldom creates major problems in houses. Water leakage (through a crack in the foundation or a leaking snap-tie hole) can easily move 100 to 1,000 times as much water as vapour diffusion.
Obviously, water leakage is the bigger threat and every effort should be made to minimize its occurrence. That means it is critical to seal any visible cracks or holes in the concrete prior to insulating. But it also means recognizing that even if all the cracks and holes are properly sealed, new ones may develop in the future. By installing a polyethylene moisture barrier between the concrete and the frame wall, any water which does leak through the foundation will drain down outside the plastic, keeping the insulated wall dry. At the base of the wall it will end up on the basement floor where it can be cleaned up or will eventually evaporate, either way with much less potential for damage. Omitting this moisture barrier could be catastrophic since wood and insulation are both moisture-sensitive materials.
As Manitoba Hydro's Power Smart Booklet No. 2 describes, the moisture barrier should only extend a few inches above grade. This leaves the above grade portion of the basement wall bare, which will allow water vapour to diffuse through the concrete to the outdoors. You will find this same moisture barrier recommendation from such sources as the Canadian Home Builders' Association, Canada Mortgage and Housing Corporation and Natural Resources Canada. This moisture barrier detail is not new and in fact has been used for many years in thousands of homes throughout Canada without problems. It is a tried and true method.
It is also worth noting that tens of thousands of Winnipeg houses are constructed with full double vapour barriers in their above-grade wall with polyethylene for the vapour barrier and plywood or OSB as the sheathing material. Plywood and OSB have a very high resistance to vapour diffusion because of the glues used to manufacture these products. Despite this, these walls show few, if any, negative effects.
For more information about materials, equipment or techniques for proper insulation installation in basements, please consult your contractor or experienced home renovations professional.
