This is the second installment in a series chronicling the steps to building an energy efficient home. The articles are based on projects underway in Flagstaff. We are building a house for a professor just north of NAU who has made energy efficiency a priority. The first article covered the basics of getting started, and the next few months will dive deeper into the details.
Every house has its unique set of challenges, but one of the riskiest times for any new building is coming out of the ground. That is because moving earth is one of the most unpredictable and expensive parts of construction, and the foundation is the basis for the long-term success of the entire project. Once you have formulated a plan for the foundation type, there are a few major choices that will affect the long-term energy efficiency, occupant health, and lifespan of the structure. To succeed, we need to keep the house dry, prevent heat from exiting out the bottom of the house, and keep radon gas out.
One of the most important parts of a foundation system is the waterproofing, and from our observations, it is often overlooked here in Flagstaff. The foundation wall should be sealed from water intrusion all the way down to the footing and a perforated drainpipe should be placed at the intersection of those two points. The pipe is a safety feature designed to take water away in the event that it builds up and puts pressure on the bottom of the foundation. There are many different waterproofing systems that can be used and some even incorporate insulation, which is necessary if building a slab on grade.
We lose about one-third of all heat in a house to the ground. If you are building a “slab on grade,” it is imperative that you insulate under the concrete. This is an important step that is relatively new to home construction. To meet the ENERGY STAR minimum requirement, a four-foot strip of ridged poly-isocyanurate foam is required at the perimeter of a slab. If you want to exceed the minimum and make sure you are limiting the most heat loss, plan on putting the foam under the entire slab. The insulation should be a minimum of R-10, which is typically 1.5” to 2.5” thick. The foam should be installed over the aggregate base and directly under the concrete. It is also important to insulate the stem wall (the vertical portion of the foundation) from the ground. This is because we lose heat by conduction down the stem and into the ground. The same style sheets of insulation can be utilized on the exterior of a traditional foundation or you can think about the incorporation of an ICF foundation to satisfy this requirement. An ICF, or Insulated Concrete Form, is like a giant foam LEGO that you pour concrete into and leave in place. It acts as the structure and as the insulation in one product.
If you are building a house with a crawl space, then you can treat the floor over the crawl space in one of two ways. You can incorporate it into the home by sealing the dirt floor and insulating the walls, or you can seal it off from the house and make sure that it does not communicate with the home. In either method, you will need to seal the floor of the crawl to keep unwanted moisture and dangerous radon gas out of the space, and make sure you are ventilating the crawl properly.
Radon is a colorless, odorless gas that when found in concentrations can lead to cancer. In Flagstaff, the gas is commonly found inside homes where it has accumulated form leaching out of the earth and rocks beneath the house. When building a new home, we can reduce our exposure to the gas in a couple of different ways. A simple passive system will reduce the gas beneath the home and force it outside the footprint of the home. At a minimum, a sheet of thick mil plastic should be placed on the ground of the crawl or under the slab. A more advanced system will allow for an exit path for the gas. In this method, a shallow trench should be dug around the interior perimeter of the foundation in the floor fill. A perforated drainpipe (similar to the one used for the waterproofing drain) should be placed in the trench and connected to a solid pipe that will eventually exit the foundation and out the roof of the home through a wall or other means. Then a thick mil plastic sheet should be placed on the sub-grade before the foam insulation or before the floor fill in the case of a crawl space. The plastic sheeting should be sealed to the foundation wall to keep the trapped gas from seeping around the edges. If constructed properly, the gas will build up under the plastic and exit the home via the perforated pipes.
Now that we are out of the ground, in our next article we will focus on framing, air sealing, and building insulation. 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