August 10, 2015

It happens every spring. Every build and building inspector knows the script: the phone call comes about the basement leak, a visit to the home is arranged and the inspection takes place. The wet insulation is visible through the 'plastic stuff'. The plastic stuff is known as the vapour barrier, and in some cases, the air barrier.

During the investigation no leaks are found anywhere. A perfectly installed air gap drainage membrane is present, weeps are fully connected to an active sump, AND there is even a capillary break placed between the footing and the foundation wall (you don't see that every day!).

Then someone suggests it might be air leakages and that "interior air with relatively high humidity was creeping past the plastic stuff and condensing on the inside of the wall". So, the plastic stuff is taped up with two kilometres of red tape.

The dehumidifier is checked. It's in great condition and running well, with the drain hose connected directly to the floor drain.

After a thorough review of all construction detailing, it is clear the home meets and exceeds the minimum requirements of the OBC.

One week later, the builder gets a call from the same home. The water is back. The head scratching begins. Unfortunately, the script will continue to replay until we accept that we cannot change the laws of thermodynamics and agree that vapour barriers in below-grade enclosures may not be appropriate in many regions of Canada.

I realize this may be oversimplifying the issue, but bear with me for a minute. It's basic science with a somewhat tricky outcome. Water in either liquid or vapour form will always move from an area of higher concentration to an area of lower concentration. Combine heat (energy) with water vapour and you are talking about this thing called Relative Humidity (RH). With RH, the science remains the same: given the opportunity, water vapour will move from areas of higher RH to areas of lower RH by vapour diffusion.

The wet insulation in basements is simple science reminding us that natural laws exist. Outdated building code rules that ignore natural laws look like this: a foundation of a new home has thousands of gallons of construction water sitting inside the small pore structure of the concrete. It will take several years for all of this unbound water to eventually dry out. For most of our Canadian season cycle, the outside is cooler and drier than the inside of a home, so the water moves (dries) to the outside. However in the spring and summer, the temperature and RH outside go up. Cooler, below-grade temperatures and air conditioning keep the basement interior cool and drier. As a result, the predominant vapour drive switches direction and moves towards the inside of the basement-conditioned space. As the water vapour moves inward, the temperature continues to drop and the RH inherently goes up. Upon coming in contact with the cooler, plastic stuff (vapour barrier), the vapour condenses and we see water! This small dehumidification cycle going on behind the plastic vapour barrier can cause substantial amounts of water to be produced and even puddle on the floor beneath.

What's the solution? Let some of the water vapour through. Not all of the water - just some of it. You can achieve this by providing material that isn't a full-stop barrier to vapour but rather retards or controls the rate of vapour diffusion through the assembly. In some areas of the U.S., this is even called - wait for it - a vapour retarder.

We know it will continue to happen. We know WHY it continues to happen. We also know a reasonable solution exists and has proven successful. We simply need the will to change what is ours: our Canadian Building Codes.

Some Background Reading:
The concept of vapour retarders has some roque roots in Canadian building science. With the 2014 NHL season closed down, there are some very informative articles to keep you entertained until the skates hit the ice this fall. A little background reading for future Building Code committees (hint, hint) - especially, the references where we see similar climates south of the border successfully adopting vapour retarders in appropriate hydrothermal regions in lieu of full-stop vapour barriers:

Understanding Basements by J. Lstiburek, Building Science Press, 27 October 2006
Information Sheet-310: Vapor Control Layer Recommendation by Buidling Science Corporation, Building Science Press, 8 September 2009
The Influence of Low-Permeance Vapor Barriers on Roof and Wall Performance by J. Straube, ASHRAE Building VIII/Attics/Roofs - Principles, 25 March 2011

By Andrew Oding, Senior Building Science Associate

Originally published in Ontario Homebuilders, Summer 2014