1. Designing a Passive- Solar, Fossil-fuel- free, grid-neutral home
JPL Green Club Meeting
27 September 2012
Ann Tavormina

2. High Level Goals Return to individual research
Planetary science
Be surrounded by natural beauty
Get back to a rural environment
Truly dark night skies!
Live a healthful, sustainable lifestyle
Minimize fossil-fuel usage, direct & indirect
Home, Land Use, Transportation
Produce much of my own food, organically
Design for system maintainability
Be robust to infrastructure breakdowns
Devote more time to arts & music
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3. Near-term Objectives Design and build a home that:
Enables comfortable living while stewarding the environment
Low carbon footprint (i.e. fossil-fuel-free home)
Takes advantage of California’s abundant sun and mild climate
Passive solar design (collect excess BTUs in winter; avoid them in summer)
Energy efficient design (attend to heat loss, in all directions, through all surfaces)
Produces at least as much electricity as it uses (grid-neutral or better)
Active solar electric system
Is robust to power-grid outages and extended periods of cloudy weather
Backup heat source; backup power for critical systems
Do it cost-effectively, so others see they can do it, too
Get the word out!
Blog construction, write articles, give talks!
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4. Why focus on fossil-fuel-free?
Homo sapiens: 500,000 YA
Homo sapiens sapiens (anatomically modern humans): 200,000 YA
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5. Big-picture Considerations
Desired climate (physical and political)?
How much humidity?
How much sun?
How much winter?
How long a growing season?
How red? How blue?
Desired population density?
How much land?
How much house?
One-story or two?
How many SqFt?
Grid-tied or Off-the-Grid?
Dig a well or use community water supply?
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6. Zeroing In Kern County, Tehachapi, CA Bear Valley Springs
Building Site:
Latitude: 35deg, 11.5min
Longitude: -118deg, 41.8min
Elevation: 3,750 ft
Rainfall: ~12.5 in/yr (LCF~21.5)
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7. Start with the Sun!
From “Sustainable by Design” website, determine sun positions through the year for the building site’s latitude/longitude
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8. Passive Solar Considerations
To optimize passive solar, orient house to true (not magnetic) North
Align the long axis of the house along true East-West line
The south side of the house is the “business” side of the house for solar energy collection in winter – make sure you have GOOD SOUTHERN EXPOSURE!
Deep eaves (overhangs) provide protection from direct sunlight in summer
Clear (not low-e) glass on south side allows enough BTUs to enter during the winter to provide daytime excess to store for nighttime use
Very important to provide night-time window insulation [good drapes (or better), especially for south-facing “regular-e” windows]
Contain window area and use low-e glass on North side windows to limit heat losses at night during winter
Contain window area and use low-e glass on East & West side windows to limit morning and evening heat gain during summer
Collect local climate data, by month (ave. highs, ave. lows, sunniness, etc) to support home energy balance/thermal model calculations
27 September 2012

9. The “Solar Slab” (goal: keep the furnace off overnight)
Developed and patented by James Kachadorian in the 1970’s
built by him through his company Green Mountain Homes in Vermont for several decades
described in his book, “The Passive Solar House”, c2006
Purpose of solar slab is to provide sufficient thermal mass to store excess solar BTUs from winter days for use during winter nights.
Constructed of building materials that would be used anyway (at least in the Northeast) to build the home
trade a basement for a concrete foundation with built-in air channels that provide
Sufficient total concrete mass to store the collected excess daytime BTUs and
Sufficient surface area via hollow concrete blocks to transfer BTUs into/out of slab
Air circulation through the slab is both passive and active.
Thermal worksheets provided in book for energy balance calculations
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10. Draft Design with full-length “Solar Slab”
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11. Solar Slab Reduced by ~50% to reduce cost
Kept predicted daytime solar slab temperature rise within recommended bounds (3 deg -> 6 deg)
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12. House “Section”, after reducing solar slab to approximately half original length
Deeper-dug center portion makes room for hollow-core concrete blocks, laid north to south, with duct channel along center-line Center portion shows concrete blocks “end-on”
Right & left ends show insulated, encased duct lines under “regular” slab
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13. Key Design Trades (underscore indicates chosen option)
Ground-mounted solar electric vs. roof-mounted system
Grid-tied vs. off-grid for solar electric
Ground-loop Heat Pump/AC vs. electric FAU and whole house fan
9-ft vs. 10-ft ceilings (~10% reduction in heated volume)
Attached garage or attached pottery studio (not room for both)
Solar thermal hot water vs electric water heater
House size – had to fit budget, with allowance for margin
Solar-slab size – whole length of house vs center section only
PLUS: Get feedback on your design from friends, professionals, and others who have been down this custom-home road before you!
They’ll see things you won’t and
Will have learned lessons you don’t want to have to learn yourself!!
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14. Thermal Model – furnace should generally stay off
Kachadorian’s book provides sample worksheets to calculate heat lost/gained given home construction and building location particulars.
R-value calculations for walls, roof, windows
Home Heat Loss Calculations for day and night, using local temperature data (January is coldest month w/ average low ~36 deg; ave. high ~52 deg)
Solar heat gain, by month, using window performance values and published “sunniness” and “atmospheric clarity” values
“Excess” BTUs generated in daytime are stored in the ~800 CuFt of concrete in the solar slab (concrete heat capacity ~30.1 BTU/(CuFt*deg F)
Model predicts January daytime excess of ~143,000 BTUs
Solar slab has a thermal capacity of ~24,000 BTUs per deg F, so slab temp will rise ~6 deg during typical January day
Home will lose ~138,000 BTUs between 8pm and 7am in January, so slab temp will fall ~ 6 deg by morning and furnace will not have needed to turn on
Without the solar slab, the furnace would have had to produce these BTUs and the house might have reached higher than desired temp during daytime
Kachadorian’s worksheets were assembled into an integrated Excel spreadsheet
This allowed changes in component performance parameters to show immediately the change in home’s predicted thermal performance.
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15. Overview of Building Site – facing South
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16. Final Approved Plans!
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17. Front & Rear Elevations
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18. Eave Design (“Overhang”)
South-facing windows should be protected from direct sun in summertime to avoid overheating
From Noon sun elevation, June 21 vs Dec 21, can optimize eave height and depth for building location
Result: Full illumination at Noon on Dec 21, full shade on June 21.
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19. Eave Design Reference Link
Window “shade-drop” for eave (overhang) calculations by season
The home at this link also uses a radiant heat floor system and is built into the side of a hill for additional thermal buffering.
East Elevation showing overhang above south side windows:
27 September 2012

20. Active Solar Considerations
For roof-mounted active-solar features (solar electric or solar thermal), design roof so one of the roof faces points to true South
Optimize pitch angle of the roof (for roof mounted systems)
Steeper roofs favor winter solar production; shallower roofs favor summer
If your system is roof-mounted, plan for a way to keep the panels clean!!
Collect local weather data, in particular “sunniness” by month
To optimize for year-round total production, set the roof pitch to approximately the latitude of the building site (if roughly equivalent sunniness in all seasons)
Note: Builders use “Rise to Run”: e.g. 8’ over 12’ = ~34deg
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21. If you’re thinking of a ground-mounted solar electric system, plan to protect it!
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22. Garage Elevations (solar electric panel loc.)
Note: Deck allows hosing off panels during dry season
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23. More on Solar Electric Production
sensitivity to panel elevation angle and time of year
Based on Figure of Merit (FOM) proportional to sun position vs true perpendicular to panel, integrated 7am to 5pm, 1-hr steps
27 September 2012

24. Backup Heat – A Must! (& generally required by code)
You still need backup heat (for cloudier/colder weather than normal)
Electric FAU is first backup, but sometimes the grid goes down…
Masonry Heater provides “grid-down” back-up as well as lovely ambience
Needs to go in ~center of house – NEVER against an outside wall
Regular firewood charge burns for ~2 hrs, radiates thru the firebrick for ~12 hrs
Various sizes available for various size homes (see
Bonus: you can get these with a bake oven that opens to back or front!
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25. Additional Considerations
Building Site should have good southern exposure (did I already say this??)
Avoid shading south-facing windows in winter
Deciduous trees may be ok on south side, but not evergreens
Shade on east/west is ok and reduces heat load in summer mornings & evenings
Won’t hurt winter-time input since sun rises in southeast and sets in southwest
Avoid shading solar panels, whether ground or roof- mounted, winter or summer – it impacts output by more than the percentage of shading
Consider a separate circuit, powerable by generator, for freezer, fridge, and a few outlets for grid-down case
Find a builder who thinks your project is as cool as you do.
You need them to be invested in the project’s success!
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26. Project Progress to Date
Lots of reading, learning & preliminary designs – 2007 thru 2010
Initial plans submitted to Kern County – December 2010
House size reduction to keep within budget – Summer 2011 thru Spring 2012
Final plans turned in to Kern County for “Plan Check” – 6/12/12
Building Permit issued by Kern County - 6/28/12
Final approvals from Bear Valley Springs Home Owners Association – 7/25/12
Selected Solar Electric Installer – 7/26/12
Contract signed with Builder – 8/10/12
Grading for driveway and home site - 8/23/12
5,000 gallon Fire Tank installation - 8/31/12
Garage slab & Solar Slab (bottom-layer) concrete pour: TODAY!!
Estimated completion ~6 months from grading (~Feb-Mar 2013)
Stay tuned via:
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27. Grading Day: August 23, 2012, View to SW
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28. Grading Day: dirt from up top used to build driveway with gentle grade
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29. Home Site, View Approximately Southeast, Foundation digging underway
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30. House foundation, View to West, deeper section dug for “solar slab”, w/ 4 mil poly
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31. Solar slab foundation w/ 2-inch thick rigid foam insulation (R ~12) & taped joints
then 6 mil polyethylene
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32. Solar slab foundation: w/ 2 inches of sand
(over 6 mil poly), ready for 2-inch layer of concrete
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33. Garage foundation, view to South,
Ready for pouring (scheduled for today!)
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34. After the house is built. . .
Fence ~ 1-2 acres around house/garage to keep elk and deer out of the gardens & yard
Install car-charger in garage for planned Chevy Volt
Build greenhouse for citrus trees, winter vegetables
Build walled courtyard connecting house and garage
Plant orchard trees, berries, grapes, etc
Plant vegetable & herb gardens
Outbuildings/fencing for animals (chickens, honey bees, dairy goat, other?)
Outfit pottery studio (kilns in garage)
And, of course, monitor house performance and keep spreading the word ;-)
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35. References & Useful Links
“The Passive Solar House”, James Kachadorian, c.2006
Passive solar fundamentals, solar slab description, design concepts, good drawings, thermal modeling spreadsheets & procedures
“Sustainable by Design” website by Christopher Gronbeck
Many useful design tools: sun positions, overhang design, solar panel shading, plus USA climate data for 250 cities
Window “shade-drop” for eave (overhang) calculations by season
Lee & Anne Elson generously shared information about the passive solar home they built in the Carson Valley in Nevada.
Especially helpful was the information on their masonry stove and their “lessons learned”.
Follow progress via blog:
If you’d like more details on any aspect of this project (most aspects go deeper), please don’t hesitate to contact me:
27 September 2012