Energy efficient residential construction: a closer look at a passive solar home

Straw, Mud and Sun

A passive solar home

Today’s discussion

• Passive solar 101

• Thermal mass

• Insulation

• Roof cachement

• Greywater

• Domestic hot water

• Photovoltaics

• Embodied energy

• Soapbox

• Resources

June 22

December 22

Passive solar 101

Solar radiation – enters through south facing windows– is captured by thermal mass – held inside by good insulation

• Everyone gets fired up about gadgets • Passive solar has no gadgets• The only moving part is the sun

– Reliability!– Efficiency!

• The south face of the house is the critical design point – south facing glass– sun is higher in the summer – overhangs prevent overheating– sun is lower in the winter – south-facing glass lets in solar radiation

• Minimize other glass, especially north facing (in cold climate)• Shade west facing glass• Overhang length depends on latitude and window height• Other options: Trombe walls

Note: this is written for the Northern hemisphere

Passive solar – house design considerations

• Keep the footprint small

• Orient the house within a few degrees of true south

• Need a decent southern exposure – McMansions to the south of the site are problematic

– So are big evergreen trees

• South facing glass 7% to 12% of total floor space

• Control southern exposure with overhangs

• Exposed west facing windows can get very hot in the summer

• Slanted glass can lead to overheating in all but coldest climates

• Make good use of deciduous trees• South facing greenhouses are an option• Without interior thermal mass, none of this

works - there’s no storage mechanism

• Insulate, insulate, insulate

• Open floor plans work better

• To really go all-out, berm the north side (e.g. Earthships)

Thermal mass

• Interior thermal mass is critical to making passive solar work– Thermal mass stores heat– Radiates slowly back into the space throughout the

night– Without thermal mass, heat dissipates quickly– Direct exposure is better, but indirect works also– Usually floors and walls

• Examples of thermal mass– Concrete slab– Brick floors or dark colored tile– Adobe interior walls– Masonry fireplaces– Plaster – Planters

Insulation

So…You let in solar radiation and captured the heat in thermal mass…now you have to keep it in the house

Design points– Insulate under slab– Extra insulation in the roof– Straw bale walls– And yes, backup radiant heat

About straw bales…• Existing building code in AZ, NM, others • Low embodied energy• Excellent insulation• Thick walls• MUST be kept dry• Need to “breathe”• Wonderful in dry climates

Roof cachement

• Fresh water is an issue worldwide – get used to the idea now that the more you can conserve, the better

• Taos is a high desert– Water rights

– Respect for the land and climate

• Well vs. roof cachement• That’s not a roof, it’s a rainwater

collector• Rainwater stored in underground

cisterns (6800 gal capacity)• Three stage filtration: particle, carbon,

and silver ceramic anti-bacterial… and just in case… rough in for alternate water main (a future well if needed)

Could have dug a well, and done roof cachement for external irrigation only • Can be very low tech!• An old olive barrel = a rain barrel in Tampa

Cannot use asphalt shingle roof to catch drinking water

Greywater

Greywater is: • Drain water from bathroom sinks, showers, tubs, washing machines• NOT from toilets (blackwater) or kitchen sink • The lynchpin that makes the roof cachement system viable

That’s not a hole in the foundation, it’s a greywater filtration system

– Greywater is kept separate from sewage

– Planter is lined / gravel / filtration plants

– Recirculation/oxygenation loop (small pump w/ solar panel)

– 1500 gal holding tank (would not want a holding tank without filtration)

• Building code for greywater in AZ, NM, CA • Systems can be much simpler than this one• Create an Oasis with Greywater, by Art

Ludwig

Domestic hot water

• Lets recap for a minute – “solar energy” could mean:– Passive solar design

– Photovoltaics (generation of electricity from solar panels)

– Solar water heating (directly heating water for household use with solar heat)

• Solar domestic hot water is one of the best solar technologies around• Usually a shorter payback than PV• Can be done almost anywhere

– In colder climates, systems circulate antifreeze

– In warmer climates with no risk of freezing, can circulate water

Can also heat water for radiant in-floor backup heating

(we chose not to do so in this house)

Solar hot water panel(two skylights below – not part of system)

Photovoltaics (PV)

• 1 Kw PV grid tie system with battery backup planned• Budget constraints – rough in and roof blocking only• Planned install in about 3 years – significantly reduced costs

– PV technology keeps improving– No contractor overhead– May owner-install– Buying wind power from local electric co-op in the meantime

Things to know about PV• You MUST have good southern exposure• Grid-tie or off grid• Roof mount vs. pole mount

– Pole mounted may track the sun automatically – Panels adjusted seasonally for optimal efficiency

• Determining the angle of the panels – latitude and season• With current technology, any shade can seriously compromise system

efficiency• “Watch this space” – venture capital is being committed daily

Interesting side note:• Commercial production of electricity from solar heat rather than PV

Embodied energy

• How hard is your project on the earth?• What can you use less of? • How much energy did it take to make the materials you use? • Can you use any recycled materials?• Are the materials renewable?

Examples• Concrete is very high embodied energy – use it wisely• Straw bale is very low embodied energy – transportation,

processing• Wood is renewable if it is sustainably harvested

Resources

• The Good House Book by Clarke Snell• Books by Dan Chiras: The Solar House, The Natural House• Building with Awareness (video of hybrid house construction)• Mother Earth News magazine• Natural Home magazine (“green Better Homes and Gardens”)• Home Power magazine • Create an Oasis with Greywater, by Art Ludwig

Just to see what’s possible:• El Monte Sagrado resort, Taos, NM – amazing on-site full waste

treatment system• Greater World – earthship community outside of Taos, fully self

sustaining• Earth: the Sequel, by Fred Krupp and Miriam Horn

Final thoughts

• Building is expensive - but building a non-green house is pretty much just as expensive• Your house should work for you … not some mythical resale buyer.

– Keep your living space as small as you can

– If it works for you, it will work for others

– Don’t be influenced by neighbors, Architectural Digest, or anyone making money off of your decision (realtors, builders) – they are all slightly crazy

• Without conservation, energy technologies amount to playing with new toys• Conservation does not equal deprivation• Demand energy efficiency in construction and real estate

– You vote with your wallet

– Trends change in response to consumer demand

• Look at the whole picture in any construction project, whether remodel or build• Bring political pressure to level the playing field

– The current financial deck is stacked against energy innovation efforts

– In the US, oil, gas, and coal all have enormous subsidies and benefits to the tune of $6 billion a year (that’s your tax $$, folks)

– Need long-term policies to foster energy innovation

– In 2007 dozens of Fortune 500 companies called for a national cap on carbon emissions, including GE, Alcoa, Caterpillar, Duke Energy, Shell Oil, and all three US automakers

– Industry gets it, but if politicians don’t – inform them or vote them out

• 100% of energy from renewable, carbon neutral sources should be our new “space race”