How to Build a Healthy House? Durable and Resilient Walls are Key
Last Updated: Apr 11, 2025As a building biologist, I am often asked, "How do I build a healthy house?" Most people expect to hear about zero-VOC paint, formaldehyde-free wood products, and other indoor air solutions. They also think they must only build with natural building materials, like straw and clay, to have healthy walls. Of course, these are important, but the ultimate starting point in creating healthy walls is to build durable and resilient wall systems.
Table of Contents
- What is a Durable and Resilient Wall System?
- What Do Durable and Resilient Walls Need to Work?
- What Does The Rain Control Layer Do?
- What Does The Air Control Layer Do?
- What Does The Thermal Control Layer Do?
- What Does The Vapor Control Layer Do?
- Bottom Line
What Do Durable and Resilient Walls Need to Work?
Durable and resilient walls have these exterior layers underneath the cladding:
- The rain control layer
- The air control layer
- The thermal control layer, and
- The vapor control layer
These are listed in order of importance: without the rain barrier, water will get through the walls. Even in Santa Fe, New Mexico, where it only rains for two weeks of the year, it rains so hard that a roof leak will make itself known right away. A raindrop is bigger than an air molecule, which is more significant than a water vapor molecule. By prioritizing the more substantial concern (rain), we drastically reduce the potential for your wall to get wet and grow mold.
The layering of walls is referred to as The Perfect Wall by Joseph Lstiburek. He is a Canadian engineer living in the US and is recognized as one of the best building scientists in the world today. The good news is that these guidelines for exterior wall anatomy work for all climates, anywhere you decide to build. Let's explore each layer individually.
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What Does The Rain Control Layer Do?
Cladding on walls, whether brick, block, siding, or stucco, needs to be appropriately installed and spaced off the wall system to allow a drainage plane behind the cladding.
Why do we need a drainage plane if the cladding is installed correctly? Because cladding has holes in it to allow for windows, doors, corners, and outlets, which are often installed imperfectly. The drainage plane allows moisture that accidentally gets back there to drain out the bottom safely. So don't fill in the weeping holes on the bottom course of your brick wall, as they are specifically designed to let water through.
What Does The Thermal Control Layer Do?
The thermal control layer is your insulation. Unless you live in the temperate climate zones where the temperature only fluctuates 10F/4C between seasons, every home needs insulation in its walls. The more extreme your weather—hot or cold— the more you can benefit from insulation. With more and more extreme weather predicted for the future, this layer can be instrumental in having a more resilient home.
While insulation is good, "outsulation" is better. What does that mean? It starts with identifying thermal bridges. Thermal bridges are found anywhere with gaps in insulation, such as wood studs or around windows and doors. A simple example of a thermal bridge is the zipper on a winter parka. Our readers from cold climates would know that the zipper gets very cold. The little flap over the zipper keeps the zipper warm. In the same way, outsulation warms the outside of wood studs or sheathing, thus reducing thermal bridging. By putting insulation on the outside of the wood studs and sheathing, we can call that outsulation. Outsulation reduces thermal bridging, a "shortcut" from hot to cold. By keeping all the building bits warmer, we reduce the likelihood of condensation and mold. So the more outsulation you can afford, the less thermal bridging you will have in your home, and the more durable and resilient your home will be.
What Does The Vapor Control Layer Do?
The vapor control layer is the last control layer and the least important. The vapor barrier is most commonly a 6-millimeter clear polyethylene plastic sheeting installed in the home. In cold climates, this is installed behind the drywall, and in most jurisdictions in North America, it is required by building code.
So why is it the least important? Because the amount of vapor that could migrate through a 4ft x 8ft sheet of drywall over a season is measured in teaspoons—not a lot to sweat about. However, suppose there is an air leak, possibly around an exterior wall electrical outlet. In that case, you can measure the amount of vapor coming through due to air leakage in pints—and that is a serious concern. With that in mind, it's essential to focus your efforts on air sealing. If your air barrier works tremendously well, you may not even need a vapor barrier—but code often requires it anyway, so it's difficult to convince a building official that it is not required. More recent vapor barriers coming on the market are "vapor/diffusion open" for certain times of the year. They work similarly to a Gore-Tex material that is breathable. They allow any moisture that becomes trapped in the wall system to migrate out safely and stop vapor drive.
Bottom Line
With durable and resilient walls, the chance of thermal discomfort reduces dramatically. The attention to detail minimizing air leakage and water intrusion means that your walls are significantly less likely to get wet and grow mold. Mold is a serious health issue, as it often goes unseen by homeowners and is challenging to remedy. Proper water management and excellent air sealing are critical for this in any climate. By focusing on these details, you will have walls that will protect you and your family for years to come.
Stephen Collette
Stephen Collette is a Building Biologist, Building Science Consultant, LEED Accredited Professional, and a Heritage Professional. Stephen is the owner of Your Healthy House and lives in Lakefield, ON with his wife and 2 daughters.
