The construction industry can be jargon heavy. I try to avoid these terms in my writing, and also in speaking with others outside of the industry. But, that can be difficult because some of these terms encompass important ideas that would be difficult to express without going into some depth. One such term is ‘Building Envelope.’ This one idea has far-reaching impacts on the cost of your home, how difficult it is to build, how much maintenance you will have to do, and how comfortable you will be in each room in your house.
The short version is that the Building Envelope is all of the outward facing elements of your home. The roof, the walls, the windows, the doors, and the floor. Each of these is a complex system with many parts that interact to achieve the design goals of the owner.
Each of these elements is comprised of 4 layers called ‘Control Layers’ that direct the flow of air and water. The layers are:
Bulk Water
Air
Water Vapor
Heat
Each layer in a wall is designed to handle one or more of these four simultaneously. How they are layered, how they interact, and how they tie together with adjacent elements are one of the central duties of an Architect while designing your home. We could go into great depth on what each is, and how to account for them, but that would be a technical manual of a thousand pages, so we’ll keep an explanation of each brief for the purposes of our conversation today.
Bulk Water
Bulk water is one of the easiest to handle and most important elements to control. The siding, the roof shingles and various seals and are intended to keep bulk water out of your home. If it does get in, it needs a way out, and designers should be sensitive to this.
Air
Heating and cooling the air inside your home is expensive. By controlling airflow into and out of your home, you limit the loss of energy to the outside and have confidence that the air you are introducing into different parts of your home has been treated.
Water Vapor
After bulk water, water vapor is the next biggest threat to the integrity of your home. If you have ever heard anyone talk about mold inside their walls, vapor is often the culprit. When vapor transmits through the wall, it needs to be able to get out. As we tighten up our homes with ever better air sealing, it becomes increasingly important to consider how vapor transmits through the wall, where the dew point is, and how the wall will dry out when vapor is present.
Heat
Last is heat. Even if the air sealing is perfect, heat will still transmit through your walls. Many designers today try to limit what we call ‘thermal bridging’ by adding a layer of exterior insulation outside of the wall sheathing to essentially give your home a jacket. Cavity insulation does a good job, but with all of the wood framing everywhere, up to one third of a standard wall has no insulation at all. Each stud in the wall is considered a ‘thermal bridge’ where heat energy more easily transfers from one space to another – either outside-in or inside-out, depending on the season.
Weaknesses in Your Thermal Envelope
In two words: Doors and Windows. No matter how well designed and built the walls, floor, or roof are, the windows and doors will transmit far more heat energy then even a code minimum wall. A good window might be in the range of 25% as thermally resistant as a standard wall. While ever bigger windows and doors are certainly in vogue, this may be one reason to limit your openings to where they are needed instead of being added to every space without consideration.
Orientation and Sunlight Harvesting
One element of the envelope has little to do with the layers, and everything to do with what is surrounding your building. In the winter, allowing sunlight to stream through your windows is an excellent way to reduce your heating costs. Conversely, ensuring the sun does not directly enter windows in the summer will keep your cooling costs down. When it comes to envelope design, the size, orientation, and shading of windows is central to keeping these elements in check.
Acoustics
If noise from outside is a concern, i.e. you are near a busy street or have a neighbor who likes to throw rowdy parties, then care can be taken in the design of the envelope to keep unwanted noise from entering your home. Like with heat, the main weaknesses are going to be windows and doors. Windows can be upgraded with an acoustic coating that can significantly reduce sound transmission. Assuming that air sealing was detailed with care, the other vulnerability will be various vents and openings in your home.
If you are embarking on your home building journey and are concerned about some or all parts of your building envelope, then get in touch so that we can help you achieve the highest quality home possible.