As someone who is passionate about energy efficiency and specializes in reducing the air leakage of new and existing homes, I often end up in conversations with experienced, competent builders who push back on tightening the building envelope because they believe ‘buildings need to breathe’ and ‘if the house is too tight, it will rot’. These criticisms are understandable; these builders may have experienced rot or mold due to moisture that envelope air sealing measures may or may not have contributed to. It is my hope this article will help explain why some houses built in the 70’s and 80’s experience excessive moisture, rot, and mold. It will also explain why a tighter envelope will both reduce energy consumption and improve durability.
Everyone has their own ideas on what makes the perfect home. Some prioritize high-end modern finishes while others look for a rustic experience. Some believe ‘bigger is better’ while others aim for the smallest footprint that will suit all of their needs. There are a few characteristics when thinking about the ideal home that are, and have always been, some of the most universally important aspects of a home.
A home needs to protect the occupants and their belongings from the elements.
It must be comfortable across all seasons.
The structure should be resilient and built to last.
A major consideration for what makes a home ideal and has been increasing in public awareness since the 1950’s is the energy efficiency of the home. As the cost of energy rises, people have been looking for ways to reduce the energy loads of their homes. This coupled with the climate emergency the world is experiencing has further pushed the need for energy efficient building practices to be developed and implemented. It takes a huge amount of energy to heat the air in a home during a Canadian winter. When air leaks out of the home, it carries that heat energy with it. So much energy is lost this way that Natural Resources Canada considers reduction of air leakage as the most important efficiency upgrade a homeowner can undertake.
A significant challenge in the quest for a more energy efficient house has been trying to figure out how to increase insulation in the walls of a home, to hold heat in, without allowing the moisture in the warm, humid air to condense inside the walls. In the Canadian summer, this moisture comes from the outside air permeating walls through air leaks. In the Canadian winter, cooking, cleaning, bathing, and even breathing indoors are the culprits. When this moisture is left unmanaged, it will permeate through the materials in a home and can allow for mold and rot.
In an effort to prevent moisture from reaching deep into the wall cavity, builders began using vapour barriers on the warm side of the insulation. This had the effect of significantly reducing air leakage and preventing moisture from reaching inside the wall, however in homes without mechanical ventilation or someone who diligently opened windows, the moisture was trapped inside, often increasing until mold began forming. This is the history some builders point to as reason to avoid tightening the building envelope.
Moisture lingering within a space, or collecting inside a wall, is certainly not something to be ignored. Relying on individuals to monitor humidity levels and open windows accordingly would be unreliable. Leaving buildings leaky is only an option if we want to stop insulating as well. So, what could we do? Fortunately, a solution exists, mechanical ventilation. Ventilation systems were common in commercial buildings, industrial settings, and (to a lesser extent) large apartment buildings in the 70’s and 80’s. In 1995 it became code in Canada to include a mechanical ventilation system in every new home. These systems are designed to remove stale, humid air from the space and replace it with fresh air. This exchange of air prevents the excessive buildup of moisture keeping the space healthy and the structure resilient.
We can control the exchange of air through an envelope. The next logical step to prevent heat loss from air movement is to stop any uncontrolled exchange of air through the envelope. Adding weather strips to doors, ensuring envelope penetrations are foamed, and automated whole home air sealing technologies such as AeroBarrier are all great ways to accomplish a tighter envelope. Air sealing is not only a terrific way to save energy but will also keep moisture out of the walls helping to keep your home healthy and resilient for years to come.
“Seal it tight, and Ventilate right”
-Ancient Building Science Proverb
Ben Bates
Lead AeroBarrier Technician
ben@tateengineering.com