Six Key Components of the Thermal Boundary in Zero Energy Homes

Air barrier Minimizing air movement in and out of a house is key to building an energy-efficient home. Many of the materials used in a house as structural and finish components can act as air barriers. Sealing all the holes and seams between sheet goods such as drywall, sheathing, and subflooring with durable caulk, gaskets, tape, and/or foam sealants will reduce air leakage.

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The air barrier is extremely important to the performance of a home. Think of it like this – a house could have an awesome insulation package but if all the windows are open in the house it won’t make a lick of difference. Holes in the air barrier allow heat to escape just like an open window, rendering the insulation ineffective. To function most effectively, it’s important to have the air barrier and the insulation always be in contact with each other.

Air barriers block random air movement through building cavities. As a result, they help prevent air leakage into and out of your home, which can account for 30 percent or more of a home’s heating and cooling costs.


  • Vapor barrier: In colder climates, moisture flows by diffusion from inside-out; while in warmer climates it flows from outside-in. In most U.S. climates, vapor barriers, or — more accurately — vapor diffusion retarders should be part of a moisture control strategy for a home. A vapor barrier or vapor diffusion retarder is a material that reduces the rate at which water vapor can move through a material. The older term “vapor barrier” is still used even though “vapor diffusion retarder” is more accurate. Vapor diffusion retarders can help control moisture in basements, ceilings, crawlspaces, floors, slab-on-grade foundations, and walls. How, where, and whether you need a vapor diffusion retarder depends on the climate and the construction of your home.

In our designs, we consider the interactions among air sealing materials and techniques and other building components, including insulation, moisture control, and ventilation. This is called a whole-house systems approach.


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  • Insulation: One of my favorite insulation packages is R-20 under the slab and foundation walls, R-45 in the walls, and R-60 in the ceiling. While more insulation could always be added and sometimes is, there is a point of diminishing returns. Will adding more insulation stop more heat from escaping the house? Yes, but it may not always be worth it. For example, a 12” thick wall allows for R-45 insulation, which is ample when the air barrier is super tight, yet it also allows for common finishing materials. The thicker a wall becomes, the wider the window and door jamb materials become as a result. If the wall is kept to 12”, we can still use a 1×12 trim board, which is a stock item, instead of having to custom make jambs from plywood.

These R-values meet or exceed R-values recommended by the Department of Energy.;

  • Attention to detail: It is very easy to think that the minute details of air sealing don’t matter, but they do! Ensuring that the air barrier is as complete as possible from the underside of the foundation to the highest point of the ceiling requires absolute attention to detail. Air sealing tapes, caulking, strategies for air barrier puncture through the ceiling from can lights, bath fans, etc., take time and a willingness to do it right. Just imagine for a moment how many hundreds of holes or cracks and crevices must be sealed before a net-zero energy home is finished. This is where our workers shine. In fact, they seem to make it their personal mission to create an exceptional home worthy of the name “Net-zero.”
  • Willingness to do it right: Similar to the previous component, a willingness to do it right is critical to building a net-zero home. Our goal is to do it right the first time so nobody will have to do it over. Construction companies sometimes get a bad reputation for cutting corners, often under the assumption that it’s all about making quick money. While that may be true for some companies, “cutting corners” is not always done to make more money. For example, the weather can play a major factor during the course of a build. The willingness to endure harsh weather conditions to ensure that a wall assembly is built right or the air sealing is done to the best of our ability is extremely important. Some of this responsibility falls on the company as a whole, but most fall on the workers in the field. Fortunately, we have some of the best men on our team, and we can trust that they will take the extra time to do it right, even when no one is looking over their shoulders to ensure they do it right.
  • Thinking outside the box: Sometimes traditional framing techniques need to be altered to allow for a better air seal or insulation. For example, when possible we like to install the air barrier on the bottom of the trusses before we frame any of the interior walls. This allows for the most complete air seal possible, but framers and subcontractors such as electricians and plumbers need to be willing to install their materials differently than they always have in the past.