Passive Solar Design

My house (I used to call it “the ugliest house on the block”) was purchased primarily for its passive solar potential as well as its affordability.

The south-facing windows were unfortunately shaded by a 6-foot eave, and some windows had been covered entirely with exterior siding – negating all its wonderful passive solar potential.

house front orig

I hired a handyman to uncover some of the windows and enclose the existing front porch into a sunroom for passive solar gain.  Even though the new room is only 3 1/2 feet wide, it serves as a pleasant sitting area, entry way, and room for drying food and seed starting.

On the Winter Solstice at noon (photo below), the sun now enters 90% of the windows.  (These were salvaged windows.  Ideally, they would be a few inches shorter, so the tops wouldn’t be “wasted” in the shade, but the money and environmental savings in using local salvage warrants the small additional loss of heat at nighttime.)

The daytime solar gain helps substantially to heat the house when the doors and windows are simply opened from the house to the sunroom on winter days.  Winter heating costs are also lessened by the use of a fireplace, and our winter utility bills are negligibly larger than summer’s.

winter solstice

On the Summer Solstice (below), the windows are in full shade all day.

summer solstice

I plan to upgrade the design by adding air vents between the house and sunroom at floor level and near the roof of the sunroom to facilitate the cycling of hot air without opening the front door or windows to the sunroom.

Two more upgrades will be to install solar blinds (to hold heat in the sunroom on winter nights) and medium-colored tiles on the floor.  Tiles will provide “thermal mass” – one of the three essential elements for passive solar design besides solar gain and insulation.  Thermal mass transforms light into heat and absorbs some of it, reducing the air temperature somewhat during the day and saving it to slowly release at night – moderating radical temperature swings.

There is also the possibility of removing the framing that used to be part of the front exterior of the house.  It was left in place when the sunroom was enclosed because it appears to be holding up the porch roof, but the air space inside the framing separates the sunroom warmth from the main building (except for open doors and windows) preventing the heat from moving through the cinderblock wall by conduction.  By removing the airspace (and some or all of the framing), I could improve the passive solar function by allowing solar radiation to reach the cinderblock wall directly and transfer it into the house through the adobe sculptural finish inside.  I hope to replace this airspace with more sculptural adobe.

Adding Thermal Mass to complete the Passive Solar Design

My friend and natural plaster artist Gavio helped me create these wall sculptures and finishes from natural adobe plaster.  I calculate that we brought over a ton and a half of adobe materials into the interior, which greatly stabilizes the interior temperatures summer and winter – and I have plans for another ton or more.

office fin

Both tree sculptures were crafted of tree branches covered or partially covered with adobe.  Each room has one wall with 2-3″ of adobe laid against either 14″ of solid concrete or standard cinder blocks.  Insulation on the outside of the house keeps this wealth of stored heat on the interior.



This banco (bench, below) and firewood storage were created of broken bricks and scrap lumber covered with adobe.  They both still await their final coat of “refined adobe,” for which every ingredient is sifted to a specific size, to create a smooth finish.  Both these, plus the fireplace hearth and porcelain tiles, add to the thermal mass inside the house, all helping stabilize temperatures, keeping the house cooler in summer and warmer in winter, because the heat is stored in the mass, rather than in the air where it is lost when doors and windows are opened.


This home was originally a store with extremely simple rectangular architecture.  Natural plaster obviously has wonderful potential for transforming a space!


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