Do’s and Don’ts for Passive Building Design
Different climates demand different strategies in sustainable design
By Natalie Leonard
As Passive House and other high performance building standards grow in popularity, it may be tempting for builders to borrow and apply building methods from disparate climate zones. While convenient, this is a risky shortcut.
An assembly that works in one climate zone can result in mold, rot and building failure in another. Understanding how the physics of a building enclosure works is crucial to build a passive house. If one of the goals is to help fight the climate crisis, we need our buildings to be durable, something that cannot be achieved without applying climate-appropriate building science to each project.
Today’s high-performance homes have loads of insulation, are airtight, and often result in several vapour control layers in their envelope assemblies.
To understand the differences, we must understand building science. Building science is a term we hear often, but what does it mean? Simply put, it is the science of how heat, air and moisture move through building assemblies.
Heat, air and moisture will always seek equilibrium by flowing from areas of higher to lower temperature, pressure and humidity.
In a high-performance house, our goal is to drastically reduce heat flow to conserve energy and promote comfort. Counterintuitively, this low heat flow contributes significantly to the moisture challenges in Passive Homes. Heat flow through the assemblies actually helps to dry out the materials — without it, moisture can easily accumulate, causing building failures.
Air flow is similar — it flows from high pressure to low pressure. Heat is lost with this air flow but the moisture it carries is a more significant issue. Moisture is the true enemy of building enclosures.
Vapour drive is also a moisture risk. This is where relative humidity (RH) comes into play — relative humidity is directly linked to temperature. The higher the temperature, the more moisture the air can hold.
Let’s look at an example in Denver, where the inside of a home in winter is 70 degrees with a RH of 30%, and the outdoor temperature is 10 degrees with a RH of 10%. In this scenario, there is a high vapour drive from inside to outside. By contrast, in a New Orleans summer where it’s hot and humid outside, and cool and dehumidified inside, the vapour drive will be from outside to inside.
Moisture is controlled by three elements: a Weather Resistive Barrier (WRB) to shield from rain and snow is placed on the outside of the assembly; an Air Barrier to control air movement can be placed anywhere in the assembly; and a Vapour Control Layer (VCL) to manage vapour drive. Depending on the climate, the location of the VCL membranes must be designed based on the physics of moisture movement.
With thick building assemblies, the moisture condition in the materials is dynamic and changes throughout the year. The point at which moisture builds enough for mold and rot is highly climate dependent. Builders rules of thumb aren’t always reliable. Dynamic moisture modelling software such as WUFI should analyze the moisture safety of any given building assembly.
We recently completed a research project, in which we designed and modelled assemblies for six different climate zones and found moisture performance varied dramatically between zones. As an example, a wall assembly that performed well in a wet coastal climate failed spectacularly in a dry cold climate.
Here are a few building science guidelines to keep in mind when building a Passive Home include.
Never borrow an assembly from another region and assume it will work well in your climate; instead, always model new assemblies for your climate before building and build assemblies with a path to dry, not a vapour sandwich.
Don’t assume the designer has it right. Instead, understand the Weather Barrier, Air Barrier and Vapour Control Membrane in your assembly and know what the “strongest” Vapour Control Layer is in your assembly.
Furthermore, never substitute a material in a Passive House assembly without comparing their vapour permeance and avoid thermal bridges in your assemblies, as they will create cold conditions for condensation to accumulate, which can lead to mold and rot.
Lastly, dry out your wood structure before insulating and sealing in the assembly. The moisture content of the materials during construction has a huge impact on the moisture risk of the home.
“Working with a knowledgeable designer to develop assemblies that perform well in your climate is key to your successful transition to building into a sustainable future.”
With the movement to high performance homes and changes to building codes, builders must adapt their construction methods. Change can be risky and result in call backs or building failure. Working with a knowledgeable designer to develop assemblies that perform well in your climate is key to your successful transition to building into a sustainable future.
Natalie Leonard is the first Certified Passive House Consultant and Certified Passive House Builder in Canada.