A poly vapour barrier in an air conditioned house is a recipe for disaster.

The practice of putting polyethylene vapour barriers in houses was intended to keep them dry, but it ends up keeping them wet. It's time to explore alternatives.

Poly vapour barriers in air conditioned houses can cause mold and rot
Poly vapour barriers in air conditioned houses can cause mold and rot © Ecohome

Much of the following was inspired by a lecture given in Barrie, Ontario, April 2014 by Joe Lstiburek of Building Science Corporation. He makes the complex seem simple, and does so with an endless stream of one-liners, some of which are just too funny not to include, not to mention that they help drive the point home. 

Why are old homes in great shape while new ones rot?

A century ago, houses were made of rocks, brick and solid wood. Early construction materials were either unharmed by moisture or put in an environment where they were able to dry. Houses were poorly or completely uninsulated and they devoured the cheap energy that was available in the past, drawing in endless fresh air and removing moisture.

Home interiors were also finished with plaster in the past, which requires about 99% Relative Humidity (RH) to accommodate mold. With paper-faced drywall it takes under 80% RH.

Even the dumbest of the three little pigs didn't build his house out of paper. Joe Lstiburek

As we insulate and make homes airtight, they don't dry as easily, and common building practices trap moisture inside wall assemblies. Adding to that problem is that we have introduced engineered wood products that are far more susceptible to moisture damage than solid lumber.

 So it should be no wonder that a leaky house with durable materials and very little insulation could remain in almost the same condition today as it was 100 years ago. While trying to stay warm we effectively kiln dried houses from the inside. It should also come as no surprise, for a number of reasons, that we cannot afford to keep building houses like that. 

So is the answer to not insulate? No, there isn't enough energy for that. Is the answer to not use composite wood materials? No, there aren't enough trees for that. What we need to do is acknowledge these changes, and design walls to handle moisture properly. As most homes are now equipped with air conditioning, the first and most important step would be to stop including polyethylene vapour barriers.

Putting a plastic vapour barrier on the inside of an air conditioned house is colossally stupid.  J.L.

Moisture-impenetrable vapour barriers work great in winter and, before air conditioning and heavily insulated walls, they were relatively harmless in summer. But as homes are now better insulated and also cooled during hot months, that vapour barrier is on the wrong side for much of the year.

If you live in a land of endless winter, an interior polyethylene vapour barrier is a great idea. While it might seem like that in most of Canada at times, there are a few hot humid months we tend to forget about. (It's easy to get off on a vapour barrier rant at this point, but I will try to stay on wood.)

It requires a lot of energy for fungus to make a meal out of solid wood, but we have made it much easier by engineering a virtual buffet of easily digestible materials. The inclusion of a poly vapour barrier in the mix creates a dining atmosphere with a 5-star rating.

Increasingly easy meals: Board lumber > plywood > OSB > hardboard > particle board > paper.
Particle board
Particle board via Creative Commons

To build the structures of our houses, we use engineered wood products that have been chewed into tiny bite-sized morsels that are held together with some tasty glues, which can be softened up in summer months thanks to the inward bound vapour drive hitting the poly vapour barrier and condensing into water droplets.

Air sealed homes without proper ventilation systems can see greatly increased interior Relative Humidity levels. The combination of that added interior moisture and cold wall surfaces caused by thermal bridges in framing puts the paper surface of drywall at risk of condensation in winter.

Even old mold with no teeth can eat paper.  J.L.

The interior environment of homes has changed significantly in the last few decades and this places new challenges on wall assemblies. Building code has failed to keep up with that. Vapour control measures that were originally intended to prevent walls from getting wet are now preventing them from drying. We know how to build walls better, but in a lot of cases we are prevented from doing so by regulatory bodies.

There was a time long in the past when doctors prescribed smoking to combat many illnesses. Being forced by building codes to put plastic on the inside walls of air conditioned homes makes about as much sense.

 Building codes are making you do stupid things. The laws of physics trump the code. Joe Lstiburek

Joe Lstiburek has left an indelible mark on the building codes of the United States, and they are much better off for it. We have not been so fortunate in Canada. His frustration with the Canadian National Building Code was evident at this conference; with luck we may see changes in the future, and housing more suited to our climate and lifestyle.