(This is the second in our series of reflections on real construction decisions for one of our local projects: The Bishop Passive House)
In late 2019 - which seems like a lifetime ago already - Michael Maines and Brian Hayes teamed up for a bird’s-eye view article around designing and constructing low carbon homes, written by Kiley Jacques on the Green Building Advisor. Widely respected New England design-builders, they acknowledged lacking all the answers, yet simultaneously laid out fourteen imperative and easily-achievable tenets for low carbon home design and construction.
As I begin construction on a home I’ve designed over the last year that is aiming for Passive House certification, a fossil-fuel-free infrastructure, and as minimal a carbon footprint as is possible for our inherently destructive industry, it’s been informative and gratifying to learn that the design and planned construction meets these fourteen tenets with some minor, location-based differences.
In this article, we weigh our design against the first seven tenets, and in a following post will wrap up the comparisons to see where our home fits into the low carbon world. As this has been a redesign of a home that was originally designed two years ago but was never constructed, we’re also able to compare the two designs with some before-and-after metrics.
1. Be as small as possible. In California, in San Luis Obispo, 2,300 SF of living space on a hilltop location overlooking both the city and the most iconic hill in the county would be considered responsible. In a neighborhood of mostly 4,000 - 7,000 SF homes with putting greens in their backyards, this one might be considered dainty even. Before I took over the design, it was 20% larger than what we are building now.
2. Include PV or be PV-ready. This home will be powered by a 3.6 kW PV system with battery backup and storage. The previous home design included no PV, and would not have been required to have it under the previous California building code.
3. Be simply shaped and durable. The new design reflects a reduction of 600 SF (17%). It reduced total corners from 27 to 18 (33%) and reduced total glazing from 27 window and door units to 16. Equally important, the glazing performance was improved from the code minimum of .32 U-factor to .17 U-factor, all while aligning the home along an east-west direction. With large overhangs now included in the design, wood siding will maintain its finish and maximize life cycle while keeping walls dry from our rainy season and shaded from our sun.
4. Use wood and wood-derived products. Our insulation will be cellulose on the inside with wood fiber rigid board on exterior walls around conditioned spaces.
5. Use air-source heat pumps. Heated and cooled by Mitsubishi air-source heat-pumps, we tick that box. With the heat-pump water heater located in the garage outside the conditioned envelope (see plan above), this area will be cooled as a secondary benefit.
6. Invest in the envelope. We are building an airtight envelope with plant-based hygroscopic materials (apart from the synthetic tapes and membranes) and windows made to last - and more importantly perform for the next 100 years. These better performing windows came in at a lower cost than the commonly-used glazing in this area.
7. Be affordable, healthy, responsible, and resilient. What’s affordable in this region may (likely, will) not be considered affordable in other regions. This home’s construction costs will come in around $265/SF which is very reasonable for this area. It’s healthy via airtight construction and continuous, filtered fresh-air with heat recovery. It’s responsible with a decreased footprint and largely plant-based materials in the envelope. It’s resilient with low-energy usage utilizing well-balanced passive (i.e. free) conditioning features.
Off to a good start, we meet the first 7 tenets of Mike and Brian’s low carbon home design and construction. Next post, we’ll see how our home measures up on the rest, as well as add our own 15th tenet to their list.