PV + Electric with or Without Propane
I am the owner/designer working with a green-certified contractor on a multi-generational, green, family home (~4000 sq ft). We are wrestling with whether to take advantage of the ease of natural gas as a back-up system for our end-of-the road house or switch to all electric and improve our carbon footprint. The house will have PV but we experience many power outages (about 30 to 40 days a year, up to 3 days at a time). We plan to have a generator but easily available natural gas (propane) is tempting for back-up generator, heat and cooking given the cost of PV batteries.
THE QUESTION: should we go all electric (e.g. induction, electric fireplaces, multiple electric on demand water heaters, heat pumps) or combine electric (heat pumps) with propane (fireplaces, cooking, on demand water heaters). For a variety of reasons, geothermal is not a good option for us. Budget is an issue and the reduction in costs from not needing to buy/bury a tank or pipe the house, along with the lower costs of electric fireplaces compared to direct vent gas could be shifted into solar batteries but does not offset the increased costs. Nonetheless, the climate situation is critical and we are very committed to reducing our footprint.
LOCATION/BUILD SITE: The house is on 20 acres at the end of the road in Montana in a rural zone 6 area that gets about 300 inches of snow per year with about 60% cloudy days. The house is on a non-windy side of a ridge with trees protecting it below to increase available light and reduce wind. The 1/2 mile long driveway was put in place 30 years ago and remains in great shape. The vegetation is pristine with very few noxious weeds and abundant native vegetation. The build site is partly flat with a mild slope (~6 feet front to back) inviting a walk-out basement. However, about 3 feet down we discovered a huge bedrock seam of partly fractured, partly solid granite
BUILDING ENVELOPE: Given the difficulties with the bedrock, we adapted the foundation rather than destroy the site. Recognizing the carbon footprint issues with EPS, we still opted to use R-28 graphite-infused ICF blocks for the foundation because we could both insulate and manipulate to the uneven shape of the bedrock while minimizing construction disturbance. The underslab insulation was limited by the bedrock. All footers have at least an R-6 thermal break with underslab insulation of R21 (garage & crawl) to R4-40 (living). All of the foundation walls will be bermed except for the area immediately around the entry door and egress window for the bedroom.
The house walls are metal skinned PUR sips R-40 and the sealed roof and constructed of R-70 SIPs. The windows and doors are all triple pane, European style high efficiency situated to maximize solar gain in winter/minimize heating in the summer. There are few exterior penetrations and those that exist are grouped in sheltered areas to minimize risks for air infiltration. The entire house, foundation, walls and roof, will be wrapped in a stick-on membrane to increase airtightness and reduce the potential for water penetration and decrease the risks associated with SIP seam separation. The exterior will be clad (over a rain screen) with KWP Ecoside composite wood.
THE STRUCTURE The structure is 40 feet deep and 72 feet long with a continuous 6″ wall running the 72 feet from ridge beam to foundation. The garage sits on the main level and the house sits over 1/2 crawlspace and 1/2 walk-out basement. Engineered floor (open web) and roof trusses hang inside the building envelope and rest on the 6 inch wall so no span exceeds 20 feet. With this, the only other load bearing walls are for the stacked staircases. The garage flanks the “storm side” (north) as a buffer between the main house and the frequent weather that moves across the area. The garage is incorporated into the building envelope but there is additional insulation in the garage ceiling (under the upstairs living area) and the wall that connects the garage to the main living space.
HVAC: There is a foundation to ridge beam 3 x 3 central chimney adjacent to the 6″ wall to house the air exchange and other duct work. The house is heated by a high SEER variable speed, 10 zone heat pump along with a multi-head minisplit heat pump for the over-garage ADU. The mini split system is being used (1) because the ADU will be occupied prior to completion of the main house and (2) it simplified the ducting system dramatically. There is top-end air exchange system that ventilates the entire house, including the basement, garage and crawl. Because we need a secondary heat source in extreme cold when heat pumps go into what we call “toaster mode” when they have to heat air to circulate, we have three strategically located fireplaces (wood, gas, electric???).
The house will have a solar array easily capable of supporting the entire function of the house. It is also on the grid with our local electric cooperative. On sunny days we should be able to send power to the Coop and run the house. On stormy/cloudy days we will need to use Coop power or stored power. There will be a whole-house generator (diesel, propane or solar???) integrated into the system for our frequent power losses. We must have the ability to store power for 2-4 days if we rely on solar because of the short days and heavy cloud cover we have in the winter. For example, our solar-power weather station goes off line after a two day storm because there isn’t enough sun to produce more power. We have three years of daily, on-site solar potential data and can predict the needs from this.
Thank you in advance for your opinions and advice.
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