This is the third installment in a series chronicling the steps to building an energy efficient home based on a current project here in Flagstaff. We are building a house for a professor just north of NAU who is prioritizing energy efficiency. The first two articles covered the basics of getting out of the ground. The next few topics will dive deeper into the details.
Erecting the walls, floors, and roofs is one of the most exciting times for any building project. You can start to get a sense of what the building is going to look like and it really feels like you are making progress. But in order to get this step right, you have to understand how these structural elements will affect the long-term success of the project. Your walls need to be strong enough to hold up the roof, but engineered to keep out the cold.
We have a lot of choices these days when it comes to building walls. You can choose wood studs, structural insulated panels (SIPs), steel, brick, block, and even hay bales! With all these choices, how do you decide? Work with your builder and energy rater to select an assembly that will deliver the most in terms of efficiency, while still remaining cost effective. We like to use a hybrid of traditional wood studs assembled in a new technique called “Advanced Framing,” and overlay the exterior with a sheet of foam to reduce the thermal bridging from the studs. We have found this to be the most cost-effective assembly. “Advanced Framing” puts the studs at 24” on center instead of the traditional 16” on center, and it reduces or eliminates extra wood wherever possible. It requires a little extra planning and effort, but the benefit is less wood and more room for insulation. An “Advanced Framed” house will use 15 percent fewer wood studs, and result in a three to five percent increase in insulation effectiveness.
Once the walls are erected, you have to keep them dry and sealed off from the outside. Airflow through cracks, holes, and other imperfections in the thermal envelope of a house can account for up to a third of our energy loss, and water intrusion is the death of all structures. We can prevent the energy loss and the water from coming in with a combination of building paper, window flashings, and caulking. We use Tyvek brand house wrap on the entire exterior of a home because it provides a solid air barrier and allows water vapor but not liquid water to pass through. There are very few products that can do that. The weak spots become the windows and doors. To prevent water from coming in at the junction of the window and the house wrap, we use a flexible self adhesive flashing installed with a special technique to keep water flowing away from the home. We also make sure to caulk the top and bottom of the house wrap to the building’s sheeting. This prevents air from migrating through the tight space by the “stack effect” and bringing moisture with it.
It is always a good idea to have a totally air sealed and waterproofed house before you put on the siding material. That way you have a primary and secondary drainage plane to keep moisture out.
Applying these techniques will result in a dry sealed wall assembly that is thermally resistant to heat loss.
Now that we are vertical, our next article will focus on building insulation and interior air sealing. You can follow the progress of the home on our blog at: http://hopeconstructionaz.blogspot.com/. FBN
David Carpenter is the owner of Hope Construction, a general contracting and construction management firm with an emphasis on sustainable building. He can be reached at 928-527-3159.