Maximizing thermal mass in passive solar high desert home
My partner and I are building a small (600 sq ft) house in climate zone 5B (NM/CO border). When we lived in Colorado, we had friends who built commercial passive solar greenhouses using south-facing glazing and water tanks for thermal mass. Using their formula, we calculated that we would need approximately 1500 gallons of water to meet their design specs for a self regulating high desert (7k ft elevation) house of this size.
The design specifics of storing that much water in an already very limited living space is an obvious practical constraint. As such, we were considering a couple of strategies, including:
1. A dark colored in-ground hot tub in the living room floor with a reinforced plexiglass cover. This could yield ~500 gallons of water thermal mass.
2. A dark 4″ stamped concrete slab floor over 3″ rigid foam. Since water is 4x as efficient for thermal mass, call this 1200 gallon concrete slab equivalent to an additional 300 gallons of water.
That brings the total up to 800 gallons, so to hit the target we would need an additional 700 gallons of water storage. Some potential options are:
a. Call it good enough, supplement with solar thermal radiant floor heat and a heat pump and propane water heater. I’d like to avoid the expense of a boiler. We plan to do this regardless.
b. Add another in-ground tub for rainwater storage, same design as above but with a fixed cover.
c. Add a covered rainwater cistern below the slab, possibly tied into solar thermal panels, either outdoor or built into the floor. This could easily start to get elaborate and expensive.
Another major consideration for all of the above are building code considerations. While the thermal mass is desirable, we would prefer not to jeopardize our ability to pass inspection.
We are still in the planning phase and fully open to suggestions. Any advise is very much appreciated.
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