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How to design a Passive House off-grid, and without foam
MARTaK Live/Work Passive House is an acronym of the extended family names of those who will use the space as a retreat from Boston and the Bay Area. The site is my own off-grid home in the mountains where I have lived for 20 years. I replaced a small shop and reused the packed earth and tire foundation, carefully deconstructing the rest of the building to reuse the material. The nuances of Passive House’s energy model would require a significant amount of insulation due to the small and elongated shape of the house and location. It is formed as a wedge, mimicking the beautiful local hogback hills which are dynamic sloping formations that run along much of the Front Range, and to better position the south side to maximize solar exposure.
Another strong influence is that of small homes in Japan, whose inventive spaces create harmony and unexpected experiences. Other important characteristics were to incorporate universal design for those who are less mobile and to build using typical construction materials and methodologies. At 1200 square feet the size is not tiny, but small enough to maintain easily, and with a few walls being 24″ thick, it’s sure to keep everyone cozy.
The wedge-shaped envelope is visually dynamic while maintaining a simple shape and improving solar exposure. Simplicity is highly prized in Passive House, not just for air tightness and constructability, but because it minimizes places for heat to escape by reducing the total wall area. While there are many ways to design for the energy model, in Colorado it pays to take advantage of the generous winter sun so most windows are south-facing and utilize triple-pane high-performance glass that lets in about 50 percent of solar heat.
After having the great pleasure of interviewing William McDonough and reading his books, it was clear to me one of the largest issues with high performance construction was the copious amounts of chemically complex materials involved, from solvent cured adhesives, to plastic wraps, and of course mountains of foams. Where do these all go when the home is torn down, and how do they affect health and climate change? The question of building an environmentally thoughtful home in nature seems absurd if so much of it is made with material toxic to living things. The solution is to diligently design with low, or positive-impact organic materials or what McDonough calls the nutrient cycle. Everything else is from fully recyclable components that are easily separated for their raw materials during deconstruction which is dubbed the technical cycle.
The most important material choice by environmental impact is insulation when your walls are to be R74 to R90, the floor r55, and the roof is R80. The insulation of choice is Applegate cellulose whose raw supply comes from Denver paper recycling 80 miles away. The cellulose is encapsulated on the exterior by a 2.3/8″ mineral wool board made by Roxul; a mixture of molten steel slag and rock and 4″ mineral wool bat on the interior. This combination makes for a extremely fire-resilient envelope, which is critical when building in a Ponderosa Pine forest.
Most Passive Houses use copious amounts of foam in the foundation. Looking for alternatives and after much redesigning with exotic materials like perlite and foamglas, the solution for us turns out to be much more simple: we will dig a crawlspace and top it with a 16″ I-beam wooden floor fully insulated with cellulose. This also eliminates a concrete slab, thus reducing the carbon footprint. The critical air sealing is achieved with European-imported non-toxic tapes and sealants taped on CDX plywood or a smart vapor membrane. During my materials research I found that European products were much more likely to be forthcoming about the ingredients, and typically outperformed US products that are a lot less chemically benign. Needless to say, there is a lot of room for improvement with the big US manufacturers.
The Passive House Planning Package (or PHPP) energy modeling is complex and sometimes bedevilling. It is a dance between form, solar gains, materials, thermal bridge-free connections and affordability. So even though the PHPP is not finalized, we begin construction in earnest, making sure we do not bury mistakes or make bad assumptions as we go along. The learning curve is steep because there are so many ways to lose heat in a wall; some I never would have had a second thought about. The next article will be on the construction of the super airtight envelope and the challenge of building massively insulated walls while keeping unhealthy materials off the job site.
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