This is the third part of a four-part series exploring the process of building a Passive House home according to the Passive House standards. The focus of this series is to dissect the details involved in constructing a home that is built air-tight, with double-stud wall construction, ventilated properly and oriented for success. You can read Part 1 here and Part 2 here, to gain a deeper understanding of Passive House standards and how they relate to the development of a home with progressive thermal goals. In Part 3 we will explore the design development and material selection of Elk Ridge Passive House.
The intention of building Passive House is to build with longevity in mind while remaining conscious of the carbon footprint. Reducing the amount of energy our world uses while taking care of our planet is important. The Passive House standard is one way to measure how much energy a building will use. The Passive House standard is a great design tool in order to design buildings with less energy. However, this of course is a balancing act throughout the entirety of the construction process. The following factors were weighed when selecting building materials for Elk Ridge:
- Long Term Durability
- Health and Safety
- Resistance to Fire
- Does it contribute towards a more efficient structure?
- Is it available locally?
- What is the impact on the environment in its production?
The above factors are sometimes in competition with one another, and it is not always an easy choice. Sometimes a product is very durable, but has a higher up front impact on the environment. Or, it is locally available and lower impact on the environment up front, but requires a lot of maintenance and is not as durable. This selection process is ongoing and not perfect.
Using Passive House as a design tool helps inform how the structure is to be designed. Passive House standards give a measurable platform to inform on the selection of building materials and how they are best executed. Carefully considering these factors led to specific materials chosen for Elk Ridge. Here is a list of the materials used and their impact on the home’s ecological footprint.
Due to the steep hillside, goals for durability and simplicity of materials led to the selection of board-form concrete for all exterior walls. A substantial amount of concrete would also be needed in order to retain the amount of dirt on the uphill side of the building, making concrete a viable material choice. Visually, the goal was to keep the exterior look simple and cohesive, so concrete was decided on for both the below grade and above grade wall systems. Unfortunately, concrete has a high embodied energy content and the production of cement that is in concrete uses significant amount of energy to produce. However, concrete is very durable and requires no maintenance.
The only other alternative material considered for the exterior was steel. This is also a very durable product but requires a lot of energy to produce. Ultimately, concrete was decided upon based on the balancing of the criteria listed above in factors 1-8.
A concrete slab was poured for the foundation of the home. Spray foam insulation was utilized beneath the slab due to the structural density, ease of installation, vapor sealing properties and R-value benefits.
Roof Design + Soffits
One of the major considerations given to the architect Eddie, prior to the roof design, was the need for larger overhangs for shading. These overhangs would facilitate the appropriate amount of sun shade to reflect the summer solar heat gain, maintaining comfortable interior temperatures year-round. The roof also needed to have a greater thickness in order to facilitate the required insulation to meet Passive House standards. Aesthetically the house was designed with a parallel chord vaulted truss roof with a 5:12 pitch.
The Soffits will be lined in a reclaimed douglas fir from Heritage Timber. This is sourced from a local company that dismantles unwanted timber-framed structures, preserving its rich history. With all of these elements combined, marry beauty, function and sustainability flawlessly.
The walls will have a combination of Demilic Healok High Density Spray Foam Insulation and Cellulose. Since all of the exterior walls are concrete, foam insulation will be used to address the potential for condensation forming on the foundation wall. There are other ways to insulate a vapor open wall assembly, however the main determinate for Elk Ridge using spray foam was the necessity to reduce condensation. The need for controlling surface condensation was greater than allowing moisture to move through the wall and potentially condense on the foundation wall. The down-side is foam insulation is composed of petroleum based products and blowing agents that have an impact on the environment. This is where long-term durability, energy efficiency and environmental impact were not all aligning perfectly. The amount of foam that will be used will be only what is needed in order to prevent condensation, the rest of the wall will be filled with recycled newspaper (cellulose).
The roof was designed as an unvented structure (similar to the walls), so a similar insulation strategy will be implemented as the walls. A combination of foam insulation and cellulose will be used. In addition to the foam and cellulose in the cavity, there will also be a layer of continuous foam insulation on the bottom side of the roof trusses in order to minimize the thermal bridging through the wood trusses.
Selecting materials for the home was weighed against the predetermined values set going into the project. In addition, the Passive House standards served as a great navigator with each passing decision. Although the process is imperfect, Elk Ridge was constructed with careful consideration as each new scenario arose.