Making a mould sandwich

If you’ve been on one of my inspections, you’ll know that water and moisture are the number one foes of your home. Quite often moisture-related problems are a result of things we do. A large chunk of a home inspection goes towards looking for moisture-related conditions, such as where the water comes from, and where it goes.

Here in B.C., poly vapour barriers have been the standard for at least 40 years. Knowing where and how to install them properly can go a long way to thwarting problems in the future.

My experience has shown that builders typically stand by the techniques or processes they've been using for years. Usually, this is because the building code, a professional they trust, or trial and error have shown them that it is the way to do it. The builders will fight tooth and nail to maintain that technique because they make money through repetition. There is only one thing that brings a change in construction technique to the masses: the building code.

A vapour barrier - more properly called a vapour retarder - is designed to stop moisture in the form of vapour that is inside the house from moving into the walls and forming condensation.  Hence it must always be located on the warm-in-winter side of any insulation.  Moisture in the room will migrate through the drywall; reach the vapour barrier and returns to the conditioned heated space of the home. Moisture will go in and out of the drywall as the humidity levels change in the house, but never affect anything deeper in the wall.

Rule 1. The first rule about a vapour barrier is that it goes on the warm-in-winter side of insulation and there should only be one.

Rule 2. Never have two vapour barriers on the same wall.

When two vapour barriers are present in a wall assembly, one would certainly be slightly or a significantly colder than the other. Since moisture will always find its way past imperfections, it can easily condense on the second barrier and/or get trapped between the two. If there is organic material inside the wall assembly between the two vapour barriers (like wooden framing members) the potential for rot and mould is significant. 

In the photo: Notice the moisture on the other side of the plastic barrier. Seen up close, mould is forming on the concrete foundation wall. Two vapour barriers were installed in this basement making a mould sandwich.

The fact that sub-surface ground temperature at various depths is frequently much colder than either exterior or interior air temperatures means that condensation can occur on the interior surface of the foundation wall when a vapour barrier is placed against the wall – especially if there is a sub-surface water problem. The interior basement insulation and the finished wall assembly are subjected to potentially significant moisture loads from vapour driven from both the exterior and interior at different times of the year. You’re making a mould sandwich.

Back to the picture in this article. Installing a plastic vapour barrier against the foundation wall where there has been known leakage is completely ineffective in retarding moisture intrusion through subsurface construction components. Knowingly installing a plastic vapour barrier against a foundation wall as an approach to deliberately encapsulate, contain, or waterproof the foundation wall is a poor practice. Waterproofing should never be conducted from the interior of a foundation wall

Since the lowest portion of the wall assembly is not completely sealed, moisture (or evidence of moisture-related conditions) will typically escape at the sole plate and below the lowest edge of the drywall or vapour barrier. This is how a homeowner might uncover their mould problem.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.