Residential Solar Power III: Passive System
The passive solar system for Double Solar House is designed around three major elements:
1) Lots of glass on the south side.
Active systems can often be oriented independently of the house. Passive solar systems, however, make the most sense when a house with lots of glass can be positioned in a direction that maximizes winter sun angles, is on a lot that allows flexibility for tree-planting as a sun moderator, and has a desirable view. This lot met all three criteria.
Of course, it wasn’t just serendipity. Once our clients found this lot we looked at it with them and determined that it had good and long exposure to the south and would lend itself to both passive and active heating systems.
By positioning this house on the north side of the lot, its south-facing glass wall also faces the yard. This allows the owners to plant and prune trees to control shading of the active and passive solar systems as needed. This also gives lots of space for the yard that will be the foreground of their primary view.
2) Lots of thermal mass on the inside to absorb and release heat and cold.
Inside the house, the absorbing mass is the concrete floor, the concrete columns, and the concrete wall that divides the open south side of the house from the private bedrooms and bathrooms on the north side.
Thermal mass elements, columns and wall, shown during construction
The sun pours deeply into the house on a late winter day
3) The overhanging roof on the south side was designed to allow the low winter sun to flow into the house but provide shade from the hot summer sun.
We designed the roof to overhang the south side of the house by five feet. That allows the winter sun deep into the house and shades the interior in the summer and provides a comfortable feeling of containment given the nine-foot ceilings inside the house. This was yet another decision that meshed both the technical and the architectural in a positive way.
The concrete wall and columns pass outside of the conditioned space of the house and define the carport (see photo one) as well as the east terrace (above).
How do the sun, the overhang and the mass work together? In winter the heating capability of the passive system is pretty simple. The sun rises in the southeast. By 9 am the sun is high enough above the horizon and morning haze to blaze deeply into the house, heating the space and the concrete floor, columns, and wall. However, a space like this could become uncomfortably warm in the winter if the concrete mass were not siphoning off the excess heat.
For instance, a sunny winter day here in the Shenandoah Valley is often in the 30s or 40s. Daytime temperatures in the house might reach 74 degrees at its peak by about 4 in the afternoon when the heat generated by the sun begins to subside. Our experience suggests that without the mass to absorb heat, the daytime temperature in the house could get into the 80s!
As outside nighttime temperatures drop, the excess heat that has been absorbed by the mass during the day is then released into the living space, warming the house and reducing the need for active heating.
Curtains on the glass wall are an additional option to help retain more heat at night and provide shade if the winter sun is too bright.
Overhanging eave welcomes the sun in the winter but shades the house in summer
How does this system work to cool in summer? At night, the doors and windows can be opened to let in our nighttime air which is usually in the 60s, cooling the space and cooling the mass in the concrete floor, columns, and wall. The next day, as temperatures rise outside, that mass cools the house. The house can be left open to nature during the day, or the doors and windows can be closed to prolong the coolness.
In the case of Double Solar House, the roof overhangs all three of the outside doors so they can be left open even if it rains. And who doesn’t love sitting inside, listening to the rain drumming on a metal roof?
As the above example illustrates, living in a passive solar house naturally puts one in touch with the rhythms of the sun, the seasons, and the weather, a benefit our clients love.
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Double Solar House demonstrates that active and passive solar energy have both become truly viable sources of energy for residential use, particularly when one can aesthetically integrate them into the overall design of a house and its site.