According to Green Builder Matt Hoots of Sawhorse, Inc., "The Perfect Wall” is an advanced way of insulating, air sealing, and keeping air and water vapor out of buildings in a logical manner. This efficient description supports the idea that green building is really just smart building. If we're logical and aim for best practices and conditions with great products, while minimizing material and resource use, then we’re going to get better results for builders and occupants. Fortunately, Matt and his colleagues are all about showing us how this works in practice in addition to exploring the theory.
In this video, Matt shares the specific practices and products he used to create “perfect walls” for the #1920sMakeoverATL project, a deep energy retrofit in Ansley Park, Georgia. While many aspects of construction vary by climate zone, the concepts of the perfect wall actually apply everywhere, since the idea is to control for climate regardless of specific conditions. That said, Matt explains that the amount of insulation changes by location, as temperatures and therefore the amount of heat needing control of course vary greatly by geography. Matt walks us through the 4 wall control layers for the substances or properties we most wish to manage:
Water barrier
Air barrier
Vapor barrier (moisture)
Thermal barrier (heat)
As Joseph Lstiburek pointed out in a Building Science corporation post from 2010, the perfect wall laid down is the perfect roof, and the same idea flat on the ground and flipped the other way somehow looks like the perfect slab… Because the building science of keeping the right elements out and in remains the same, and the layers don’t change even if their order might. This makes sense at a high level, and his words are worth a read if you’d like more details.
Matt walks us through the control layers of the ATL House from below the slab and on up the walls and roof. The home’s foundation was completely rebuilt on three sides, with 7 layers of foundation materials:
Compact and level dirt
Geotech mat (to prevent layers form mixing)
Gravel (allows breathing and essential for the passive radon venting system)
Insulation (below slab insulation helps keep the home at a comfortable temperature, and avoids “undermining” the other components)
Vapor control layer by StegoHome
Concrete (This layer plus the StegoHome layer also push radon back into the gravel later and mitigation system)
Shotcrete
On the walls, Matt walks us through how the team installed ZipSystem for a water resistant barrier and air control systems, as well as ROCKWOOL insulation for thermal control. He also explains how furring strips hold up the insulation and also provide support for the building’s cladding, fiber cement plus Azek.
On the roof, the ATL home has outriggers with 2 layers of insulation and insulation on the underside as well. The biggest structural risk to continuity is apparently where roofs and walls meet, due to the concept of thermal bridging, where a material like steel or a rafter is more conductive than surrounding materials and will “leak” heat, creating an energy “bridge” between building interiors and exteriors. This is one reason continuity is so important - by wrapping these potential areas for breaks and where structural materials could cause bridging, the integrity of the building is maintained.
As part of the application of the perfect wall concept, it’s preferred that the controls layers are outside the structure. Less labor is needed and the materials are not compressed which can cause a loss of insulation integrity and functionality. It’s important to maintain and protect the R-value of the insulation, which is a measure of resistance to heat flow, and really a measure of insulation strength or power. R value is well understood by building professionals, but the concept can also be confusing as it's measured per inch of material, with a higher r value meaning more insulation. This means that using more of a material provides additional insulation, so there’s often more than one way to reach the desired goal with different materials. Energy Star maintains a list of Recommended R values, and the Department of Energy provides a list of insulating values for windows, which are called U and are for some reason the inverse of R values (so, the lower the better).
One argument for exterior control layers is that simpler and seamless is better. A key concept here is the idea of continuity. Since every break, every penetration adds risk that something will seep inside or outside of a building that we don’t not want, making our controls or “proofing” continuous, if done right, eliminates this risk. Another argument in support of exterior control layers is that keeping substances out that we do not want inside the built environment is the easiest way to control them, as they never get in in the first place. As Matt reviews, everywhere there’s a break in a layer or a potential break, such as an opening or a fasteners, it must be addressed. One way Matt and his team avoid gaps is to use Liquid Flash sealer and a good amount of Zip Construction Tape.
A common way building scientists make these concepts approachable is to talk about the perfect wall as a sweater or blanket for the home. Continuous layers with no gaps wrap the house and make it comfortable and efficient, optimizing conditions for durability, energy and comfort. We don’t want moisture inside the structure, which can compromise materials and even lead to unhealthy conditions such as mold growth. And we also wish to prevent energy waste and loss. The EPA estimates that total home energy use can be reduced by 11% just by properly air sealing and adding insulation, and that 9 out of 10 homes are under insulated. This means these concepts are indeed as important to many existing homes in addition to new construction and for major renovations.
In a sense, "The Perfect Wall" is not necessarily about being perfect, but the idea that if we can’t design and build a wall to the best of our abilities and current building science, then our wall has a greater risk of failure. So, perfect is in a sense about avoiding risk or imperfections. If we do everything right, well then a wall should do its job to keep the things inside and outside of a structure as intended. We want to keep heat out in hot weather and inside in cool weather, cool out in cold weather and inside in hot weather, and we never actually want water and water vapor to penetrate our walls, or really the six sides of every building since roofs and foundations matter a lot in terms of energy, comfort and integrity and durability as well. Our homes ideally have to look great, but as Matt’s colleague, Chris Laumer-Giddens at LG Squared, reminds us in the video, the assembly you see absolutely needs to work - and last.
The #1920sMakeoverATL deep energy retrofit project, in progress.
The #1920sMakeoverATL deep energy retrofit project, anticipated!
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