Exterior walls of a house

How to Build a Healthy House? Durable and Resilient Walls are Key

By Stephen Collette Rise Writer
Feb 9, 2021

As a building biologist, I am often asked, "How do I build a healthy house?" Most people expect to hear about zero-VOC paintformaldehyde-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.

NIST Net Zero House
Net Zero House. Photo Credit: NIST

What is a Durable and Resilient Wall System?

A durable and resilient wall system is one that can safely and effectively handle all the elements thrown at it, including heavy wind, rain, snow, ice, severe heat, hurricane forces, and everything in between. The real challenge is that during all of those varying weather events, the occupants expect to remain at a comfortable 68 degrees Fahrenheit / 18 degrees Celsius and 45% relative humidity at all times.

Our homes' walls have a formidable job and are continually being put through significant wear and tear. Consider a deep winter in a frigid climate. One side of the wall could be 68F/18C, while the other side could be -4F/-20C. That is a tremendous strain on a wall system. At any given point, a wall could be hitting the freezing point of 32F/0C while another section is also hitting the dew point (the temperature at which air vapor turns to liquid water). Ideally, all of this flux is happening where it should and is designed to happen, and nothing goes wrong. But as homeowners, we know that the ideal is not always reality. With these challenges in mind, let's dive into the details of durable and resilient walls.

What Do Durable and Resilient Walls Need to Work?

Durable and resilient walls have these exterior layers underneath the cladding:

  1. The rain control layer
  2. The air control layer
  3. The thermal control layer, and
  4. 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.

diagram of the perfect wall assembly

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.

wood cladding
Prefinished Wood Siding. Photo Credit: Stephen Collette

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.

energy audit blower door test
Photo Credit: Stephen Collette

What Does The Air Control Layer Do?

Air leakage can bring in colder air or humid air, and that air will condense on an interior surface, like the back of your drywall. This air leakage can cause condensation, which can then lead to mold. Air leakage is, after liquid water leakage, the most damaging failure for a building. This risk is why having an energy audit on your existing home is an excellent idea for durability and resiliency. The audit must include a blower door test to measure the air leakiness of your home.

As the second most important layer, the air barrier might be recognized as "Tyvek" on new homes—a product that seems to wrap the house in plastic. There are many more modern approaches to airtight construction now, such as peel-and-stick membranes and airtight exterior sheathing with special tapes to seal the joints. As with the rain control layer, the devil is in the details of the installation. Every time there is a joint or hole (windows, doors, etc.), there needs to be a fix. After the plumbers run hose bibs, after the electricians run outdoor lights, after the windows and doors are installed, it all needs to be appropriately detailed. The good thing about air sealing is that most new construction also has airtight drywall on the interior wall that acts as a "second" air control later—further decreasing the chances of water damage.

rockwool rigid board outboard insulation
6" of Rockwool Rigid Board Insulation. Photo Credit: Stephen Collette

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.

air barrier
Delta Vent SA Air Barrier. Photo Credit: Stephen Collette

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.

Majvest Vapor Control Layer Siga
Majvest Vapor Control Layer. Photo Credit: Siga

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.

Disclaimer: This article does not constitute a product endorsement however Rise does reserve the right to recommend relevant products based on the articles content to provide a more comprehensive experience for the reader.Last Modified: 2021-02-09T20:54:42+0000