As I have blogged about a few times before, I am fortunate to be the architect of Cincinnati Habitat for Humanity’s (CHfH) new sustainable home model. I was on the site last week to check out the progress of the build, and I was definitely impressed! One of the biggest challenges of the home design was selecting strategies that were more sustainable than Habitat for Humanity’s current practices, which are already really quite good, and could be implemented in a cost effective way by volunteer labor. It was also very important to me to introduce some new ideas that could hopefully be implemented not only on this official “green” model but also on every house our Cincinnati affiliate builds in the future. While I hope they consider using many of the strategies I have suggested in the future, I feel confident that I was successful on at least two fronts: the use of raised heel trusses and the use of Spacejoist prefabricated floor joists for floor framing.
Raised Heel Trusses: Traditional roof trusses create a sharp, triangular intersection where the top chord and bottom cord of the truss intersect, which limits the amount of insulation you can install at this crucial point. This reduced insulation can cause unnecessary heat gain/loss and can also cause ice dams, where snow begins to melt over this uninsulated portion of the roof but then refreezes when it hits the cold roof eave. This ice build-up and standing water that occurs behind it can cause significant roof damage. A raised heel truss simply installs a short vertical piece of wood between the top and bottom truss cords, which raises the top cord up, eliminates the akward triangular intersection and offers more room to install the appropriate depth of insulation. Consequently, the raised heel practically eliminates the excessive heat/gain loss issue and the potential for ice dams. Habitat for Humanity found that switching to raised heel trusses only cost $25 for the new house model. With such great benefits and a remarkably low cost, CHfH plans to use raised heel trusses on all future builds.
Spacejoist Prefabricated Floor Joists: When I toured one of the CHfH homes that was under construction, I realized just how much time and material was being used to create soffit areas to contain ductwork and other systems infrastructure. I also realized that in a two story house, they were running duct trunk lines in the basement to serve the first floor and above the ceiling of the first floor to serve the second floor. This lead to wasted material that could be saved if the main trunk lines served both floors, plus it meant that the first floor ductwork was running in an unconditioned space. After observing these conditions, I suggested the use of Spacejoists, a prefabricated floor joist that uses dimensional lumber as the top and bottom chords of the joist and steel as the web. Using these joists, as opposed to traditional dimensional lumber framing, allowed us to easily run all of the ductwork for the first and second floors through the second floor framing, getting all the ductwork out of the basement and avoiding adding a single soffit to the house. I specifically chose Spacejoists because the steel webs allow larger opening to run ducts, pipes and wires than traditional, all wood, prefabricated joists while the steel also allows the Spacejoists to be super light, which is a real benefit on a volunteer run project. While the Spacejoists have amazing benefits, they are more expensive than plain dimensional lumber. Consequently, to control costs, we chose to frame the first floor with 2x10s and only use the Spacejoists to frame the second floor where we wanted to really benefit from integrating systems in the framing. While I am not certain CHfH will use Spacejoists on all future projects, they have been talking about it seriously.