It might seem like the process of building a home is mainly about building walls out of lumber, putting on a roof and siding, and then adding windows and doors. Then you can move on to finishing off the interior work with sheetrock and paint and carpet and all the rest.
All those steps are part of the process, but creating a durable and weatherproof envelope or “shell” for the home is more complex than just stapling up some Tyvek and siding. In building science, professionals refer to the four control layers of a building and the job that each layer must do. These layers are listed in order of importance:
Let’s look at those in more detail and why they’re important.
This one and thermal control are probably the most intuitive. It’s easy to understand that you want a waterproof roof and water-resistant siding, but it also applies to the foundation, so that’s all six sides of a building. Under a concrete foundation, most builders will use a sheet of plastic or sheets of rigid foam. For the walls, in many cases, the water control layer is actually two layers consisting of the exterior cladding, which is the siding. Some siding choices, such as some metal siding, are pretty close to watertight. Others, such as vinyl siding, can allow quite a lot of water in during storms with high winds. The key factor is that the siding allows the wall assembly to dry out.
Behind the cladding/siding is typically a water-resistive barrier (WRB) like a housewrap. Tyvek is the big brand name for this product, but there are many others. This product is designed to stop liquid water, also known as “bulk water,” from coming into the wall from the outside. At the same time, it’s vapor permeable, so any water that gets into the wall assembly can evaporate.
Next in line is the air control layer. This layer aligns with the concept of air sealing that we mention frequently in this blog, as it’s so important for energy efficiency. Part of the issue in this layer is the potential for air moving through the wall assembly to carry water vapor, which could cause mold that’s detrimental to human health and also to many building materials. Ideally, we want to see no air or water vapor moving through the wall assembly at all.
The air barrier is most effective when installed in contact with the insulation or thermal barrier, according to the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. The air barrier may be installed on the interior or exterior of the thermal envelope as long as it’s in contact with the thermal barrier.
What works as an air barrier? A number of materials will work. Tyvek, for example, can work as long as it’s thoroughly taped at all seams and penetrations. Sheet goods like foam board insulation, drywall, plywood, and OSB will all work, as well, as long as their seams, gaps, and holes are taped and/or sealed.
“As buildings have gotten tighter over the past several decades, questions about vapor retarders and vapor barriers have confounded builders and code developers,” says the U.S. Department of Energy’s Building Technologies Program.
With the vapor control layer, building science starts to get a lot more complex and unintuitive, and you see terms like “vapor drive.” The gist of it all is, though, that there will be water vapor in the house, and you want it to be able to diffuse from areas of high concentration to areas of lower concentration so that it doesn’t collect in confined spaces like wall cavities and cause mold. This tends to happen naturally unless something is in place to stop it, like a layer of plastic sheeting, which was used in the past as a vapor barrier. Plastic is totally impermeable, though, and can also contribute to condensation. Other methods and materials are preferred, though, in some areas of the U.S., no vapor barrier is necessary, as noted in that article.
The thermal control layer controls the flow of thermal energy, or heat, through a wall assembly. For this, we use insulation, which is a material that resists that flow. Insulation is useful in both heating-dominated and and cooling-dominated climates, and saves energy in both. It would be least needed in a locale that had a year-round temperature of say, 70 degrees Fahrenheit.
Many different materials are used for this task. Fiberglass batts, those fluffy pink things you’ve probably seen, are one of the most common. They’re designed to fit between the wall studs in a home and also between the rafters in the ceiling. Denim is also available in batts that fit between the wall studs, as is sheep’s wool insulation and wood fiber batts.
Rigid foam is another popular material, and it’s available in 4x8-inch sheets in several thicknesses from ½-inch and up. Foam is also available sprayed in place, and that results in some of the best performance you’ll get with any insulation. Wood fiber insulation is also available in sheets, and like foam boards, you would install that on the exterior of your home as a continuous layer of insulation, usually followed by the water-resistive barrier, then the cladding/siding.
Creating long-lasting, energy-efficient buildings is crucial as we decarbonize our world and create a clean-energy economy. Building several million affordable homes with well-optimized control layers is one way to get there!