In Episode 1 of this series, we defined a house, the systems of a house, the components of the systems, and a Home Inspection. If you have not read Episode 1 you can find it here. It will be the foundation and companion article for all of the articles to follow. In Episode 1, I promised that in this article we would go over the most important coalition of systems in your house that you have probably never heard of. Join me as we go over the Building Envelope.
What is a Building Envelope? A Building Envelope is the enclosure (exterior walls and roof) that defines the structure and separates the conditioned space of the home from the environment. (Note: All conditioned space is located inside the enclosure, but not all space inside the enclosure is conditioned space.)
The building envelope is an environmental separator that consists of four control layers. Let's go over these four layers in order of importance and talk about the systems and components that comprise some of them. The four layers we need to know are as follows: Water Control, Air Control, Vapor Control, and Thermal Control. Joseph Lstiburek is credited as the conceptualizer of "The Perfect Wall". He speaks all over North America, and identifies these 4 control layers of the building envelope and how critical of an effect they have towards a building's behavior, long-term performance, and viability.
I will link to one of his seminars that is vastly informative, immensely entertaining, and easy to understand. When describing the four layers and the order of their importance, Joseph is quoted saying "I know the water control layer is the most important, because you will never receive a call in the middle of the night that says, my building is leaking air". As simple as that is, it is quite profound at the same time.
While you may not have heard of a building envelope you will be familiar with most, if not all, of its comprised systems and their components, such as Roof, Siding, Doors, Windows, Vapor Barrier, and Insulation. In our future articles, we will begin talking to experts in each of these systems and talking about the products, materials, and proper installation. For now, let us go a little deeper into the building envelope and the “Perfect Wall” concept, while learning about what to look for in your current or future home.
Joseph Lstiburek’s “Perfect Wall” involves moving all of the control layers to the extremities of the structure and in the correct order. To quote Joseph Lstiburek, "If you want to keep your body warm and dry, you wouldn’t put a jacket under your clothes, or stuff insulation between your ribs. You put it on the outside of your clothes and body". In a lot of older homes, we often do not see much, if any, insulation or continuous control layers. We often refer to older homes as “Leaky”.
Water Control Layer
No matter if I am inspecting a new house or an old house, I want to ensure that the structure is dry. Older homes leak water, air, vapor, heat, and cooling. While these features do not make for an efficient structure, they allow the structure to dry. Water and moisture control is not only about keeping things dry but also to allow things to dry when they get wet. When we start to see problems in newer homes is when we build tighter, more efficient, and less leaky homes but do not incorporate a means to facilitate drying. Signs of a failed water control layer can include rot of the exterior siding or internal components, and staining or damage visible from inside the structure. On newer tighter homes I like to see a rain screen like that depicted below, courtesy of buildingscience.com.
Air Control Layer
Your second greatest living expense after your rent or mortgage is going to be your utilities. So it makes sense that the building envelope is so important. Yet, many people try to put the Air Control Layer further down the list of importance, even though we spend more time and money trying to control the air in our home than any other single aspect. According to various studies, 40%-50% of our monthly energy costs go towards the heating and cooling of our homes. Water control protects the structure from damage, the structure is what separates the inside air from the outside air. No matter if the temperature outside is -30 or 130 we all want our homes to be 70 degrees year-round, give or take 5 degrees. The air outside can be undesirable, so it is important to keep it separated from the inside air that we heat, cool, filter, and dehumidify. Air can also carry water, as we know, water can damage our structures. The picture below depicts the amount of water that can enter the home in a month through a 1inch hole.
Vapor Control Layer
Vapor is the gaseous form of a substance. There are various types of vapor and products that control them, but we will focus on water vapor. In the winter our air is dry and cold outside while warm and wet inside. In the summer the air outside is hot and humid, while our inside air is cool and dry. Again, water will travel from an area of high concentration to an area of low concentration. So in the Winter water vapor will travel from inside our homes to outside our homes, and in the Summer water vapor will travel from outside our home to inside. Water vapor is mostly made up of air and is inherently safe. Where we run into issues with water vapor is when it changes its state of matter and condensing on a cold surface. Condensed water vapor is of course water, and water vapor condensing in the walls is depicted below.
When inspecting a home's vapor barrier we want to check humidity levels. High humidity levels are indicative of a failed control layer. We also confirm that any air conditioners and dehumidifiers are properly sized and capable of drying the air inside the home. The picture below depicts the amount of water that can infiltrate the house via diffusion vs an air leak.
Thermal Control Layer
Thermal literally means “heat”. So a thermal control layer controls the structure's transfer of heat. Whether we want to prevent heat from leaving or entering the structure we do this with insulation. There are dozens of insulation options and techniques, but we rate them all by their R-Value. R-Value is a material's ability to resist the transfer of heat. The higher the value the less heat that is transferred. Thermal cameras are a big help when it comes to seeing insulation or the lack thereof. Most insulation is hidden and inaccessible, and where we do see it we often recommend more. However, It does not take a thermal camera to determine that the insulation is inadequate. Homes with insufficient insulation we experience fast thermal exchanges accompanied by long-running cycles of the Heating and Cooling systems.
Now let's look at this house cutaway, courtesy of the U.S. E.P.A, that I have modified slightly for the purposes of this article. In the picture below you will see a typical home and the control layers of today. The Blue lines represent the Water Control Layer. The Green lines depict the Air and Vapor Control Layers. The Red depicts the thermal Control Layer. With a quick scan of this picture, you can see the various layers, but also you will see that the space inside the attic and basement are not effectively protected by the thermal control and air control layers, thus affecting the efficiency of the structure. This causes issues because you have hot humid air in the attic, that reaches upwards of 160 degrees in the summer, and leaks into our living spaces through every penetration. We also see musty cool air from the basement that leaks into the living space from below, and acts as a heat sink in the winter, pulling all the heat out of our living spaces.
If we apply the concept, and principles of the “Perfect Wall” by moving all 4 control layers to the outermost areas of the structure, then we see something that resembles what you see below. The color values are the same as before, but now you will notice we have moved all the air of the structure inside all of the control layers. At the same time, we have eliminated multiple penetrations, created a continuous barrier, eliminated temperature extremes from inside the structure, and increased the square footage of the home by making the entire structure a part of the controlled air. Resulting in homes that cost less to run, last longer, and have fewer issues.
What I really love about these principles is that they are universal, they are easy to build out new using domestic products, and retrofitting is obtainable on older homes. Even though the “Perfect Wall” is not required code, they are excellent building practices. The better you can separate the outside air from the inside air, the longer your home is going to last, the fewer problems you are going to have; The healthier you will be and the more efficiently your home will perform.
As we move on in the series we will dive deeper into the systems and their components, talking with experts and learning about products like air conditioners, dehumidifiers, high-efficiency doors and windows, roofing materials, siding options, and insulation. If you are not already a member sign up for free so you can leave any comments or questions, and stay up to date on our latest posts. We do not distribute your contact information. As always you can find a local home inspector using the world’s largest trade organization of residential and commercial property inspectors, InterNACHI. You can trust that an InterNACHI home inspector will give your house a quality check-up, and provide you a list of things that can be improved.