Ideating passive solar applications for Bay Area, CA home
Hello,
This is my first post on the site. I am a beginning designer in California working on remodeling an historic home in the Bay Area.
The home has east-west orientation and full south facing views from a hillside. Being in the East Bay, the weather is fairly mild but can have stretches of very hot (>90 degrees) and uncomfortably cold (40-50 degrees) days. It is not uncommon to have warm, sunny days that cool off considerably in the night as the marine layer sets in.
I am far along on the design, finalizing plan check with the building department. At some point, I had a realization that I may have inadvertently designed good opportunities for passive solar heating in some specific instances. I’ve attached some plan views (north is up) of the areas I would like advice / feedback on. Maybe I just need someone to tell me I have it all wrong, that’s OK, too.
In the first application, I am changing what used to be a 90 degree wall into a flattened angle, because the best views today are hiding directly behind the corner of the wall. I intend to leave the existing roofline as an overhang. The triangle of flooring underneath the roof area – which will become part of the exterior – seems like one opportunity for thermal mass. I envision something like 3″-4″ of dark colored concrete acting as a “heat sink” during hot days, helping reduce solar heat gain (along with exterior solar shades). Then, in evenings, there are some days that heat coming from that triangle would be welcome when spending time on the deck or in the kitchen with the doors open. It doesn’t need to last all night – just a few hours of added comfort would be a bonus. I have no idea if such a small area will accomplish anything… it is shielded from wind which comes down a creek channel on the east side of the home.
The second application is a solarium attached to the primary bedroom. This design originated from the primary bedroom being fairly cramped and zoning rules preventing any expansion due to our location near a creek. The solarium is technically a “deck” with a “deck” above acting to enclose the space as a loophole to enlarge the usable space. To access the solarium from the bedroom, there is a sliding glass wall. My thought would be to use ~3″ of dark colored concrete as flooring, then use the sliding glass wall to regulate when solarium heat can flow freely into the bedroom. The solarium glass would have highest allowable solar heat gain, but also high performing U-value to hold heat in better (unit U-factor of ~0.20). The solarium also has operable windows on either end for a cross breeze to flush heat. The sliding glass would have a unit U-factor potentially ~0.25. I suspect the heat loss through the glass in the evenings could make this a challenging setup.
The third application is a rooftop spa area with another wall of windows (same spec as solarium) for views. Adjacent to the north is a small, lofted bedroom. My thought was to use recycled brick from our chimney (which is being removed) to build a brick wall between the spa and bedroom. It would be insulated and finished on the bedroom side, then have operable (frosted) transom windows high on the wall to again regulate when heat from the spa room can flow freely into the bedroom. The roof pitch slopes up from the spa to the bedroom, then down from the bedroom to the north face of the home (which seemed to facilitate the flow of warm air).
I read a couple of articles on the site that were more focused on whole home passive solar, which was very helpful context.
1) https://www.greenbuildingadvisor.com/article/all-about-thermal-mass
2) https://www.greenbuildingadvisor.com/article/reassessing-passive-solar-design-principles
In my case, I’m thinking I could make a few changes adding thermal mass and maybe handle a decent % of heating needs for two bedrooms and potentially help reduce heat gain where it is most susceptible while offering some early evening comfort as a bonus. These effects are backup / supplemental.
If I sound like a novice, it’s because I am.
Appreciate any feedback or advice and thanks for your time.
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Replies
The best approach is to model energy usage using software, BeOpt is free and very good, although it's a bit of a challenge to learn.
You'll never know until you model it, but generally the kinds of things you're talking about have minimal impact on annual energy usage. I don't think you'll find a lot of passive solar fans here, and longtime readers know not to get me started on "thermal mass."
This property is in a climate with less than 3,000 heating degree days (i.e. cooling-dominated) and you want to INTENTIONALLY trap more heat? I live in Texas and I could not imagine how uncomfortable an all-glass solarium would be during 90% of the year. Passive solar works by shifting (and hopefully reducing) the peak heating demand times in cold climates (at least CZ5). It would be better from an energy usage perspective to just reduce all heat flows into and out of the building in the first place (air tightness, low U-value fenestrations, insulation, etc.).
These are some good points. user-7612165, one of the key aspects to passive solar heating is the use of overhangs to block out the direct sunlight in the warmer months when heating is not needed. This would likely be key to helping prevent overheating in a house like what you are describing. Energy modeling should be able to help you determine the ideal size of overhangs to help prevent this.
The problem is that the sun follows the same path through the sky in August as it does in April.
More broadly, there's no way to insure that you get heat when you need it, and don't get it when you don't need it.
The way to analyze it is energy modeling. Obviously, you'll never know until you do the modeling, but usually it turns out that things like positioning of windows make little difference in annual energy usage.
Thanks for the replies so far. I appreciate the energy model referral - I had explored a couple that were not working well for me (a bit too complicated to use without training from the vendor).
Regarding the solarium getting too hot - definitely a valid point. I will need to model it out, but directionally I had designed the space to have external solar shades (forgot to mention that), a ceiling fan, and operable windows on either end that are designed to catch the typical wind flow which comes down the creek channel on the east of the property.
On hot days, to keep the bedroom from gaining heat, one would put the solar shades down and close the sliding glass door. To use the space during the heat of the day, you would count on cross breeze + ceiling fan + the heat sink of thermal mass (tbd if that's enough).
Thanks again for the thoughtful input.