What a great question to get some inter-office discussions going! Though it is not possible to definitively say that one is greener than the other, we have compiled a list of things for you to consider when making this choice. As you are reading over this, consider your lifestyle and the goals that you have for your home.
- The two-story option will likely have a smaller footprint on the site, giving rise to potential opportunities regarding the following site-based factors:
- Less site impermeability (unless the site is already impermeable).
- Less disturbance of natural landscape and habitat.
- Lower contribution to heat island effect.
- More open space.
- More room for gardening, composting and outdoor living spaces.
- Higher density (which can help reduce urban sprawl).
- The two-story option has a smaller, higher roof area, creating the following potential:
- Less area for solar photovoltaics and/or solar thermal panels.
- Higher chance that solar panels will not be subjected to production losses due to shading.
- Potential for additional shading, which can limit the types of plants grown in the open space or garden.
- From an energy standpoint, a cube is going to have less surface area for the same volume, resulting in a two-story home having the following advantages:
- Less thermal envelope surface area, meaning less overall potential for infiltration and energy transfer with the external environment.
- Less insulation materials needed.
- Upper-level windows will help create natural ?stack effect? ventilation potential, which may reduce energy consumption during warmer months.
- At the same time, any ductwork for the first floor is going to be in insulated space, improving energy effiency, unless all of the ductwork was going to be within conditioned space either way. Plumbing runs can be shorter, as well, in a two-story house (using less piping and minimizing energy loss).
- A disadvantage of a two-story home is that it creates more challenges in shading walls and windows, if you are in a climate where shading is important.
- From a materials standpoint, a two-story home will require more structural strength within the first story, creating the potential for greater material consumption. If you were building with an alternative wall system like insulated concrete forms, it wouldn?t make a difference. A two-story home will have less slab ? if you are pouring a concrete slab at all ? and less roofing material, but it then adds a structural floor for the second story. So, you can enter into the concrete versus wood debate if you?d like ? but we won?t here.
- A two-story home presents accessibility issues for disabled occupants, so you want to consider universal design and generational issues when making a decision.
- Stairways take up usable space and materials, but so do hallways. Therefore, good floor plan design on a one-story house could make it more space-efficient than a two-story.
We could go on. The important conclusion I draw is that a two-story home has more theoretical advantages than a one-story in a green evaluation in most instances, but the advantages are not such that other factors couldn't easily tip the balance in a totally different direction. For instance, a one-story home in a relatively dense urban core is going to provide a much lower occupant carbon footprint than a two-story home with a long commute and dependence on automobile travel.
Home size and the quality of design and construction, as well, are going to have a larger impact than the decision of one story or two. We?re starting to experience net-zero energy Habitat for Humanity homes and another one will be built in conjunction with Greenbuild this year in Phoenix as a Legacy project for the community. I encourage you to focus on the overall potential and intent of the home and then see where that takes you with the one-story vs. two-story decision.