Passive Solar Home Design

Sun Angle and Passive solar design

What is Passive Solar Home Design?

In passive solar home design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. Passive solar homes are more energy-efficient and help reduce your home heating costs. Contrary to popular belief, passive solar homes can be cool in the Summer as well as beautifully designed and aesthetically stimulating.

Although passive solar home design is simple in methodology, Greenovision Home Design in Bozeman, Montana is trained and experienced in passive solar strategies and will ensure that your home’s solar potentials are utilized most effectively. The key principles of passive solar design are as follows:

  1.  Building site properties must be evaluated and exploited.
  2.  The home must be designed so that it captures the solar radiation.
  3.  The home has a floor plan that is fairly open in order to promote convective air cycles.
  4.  Building materials must increase in mass for heat storage. i.e. masonry, concrete, stone, thick tiles.
  5.  Mass is in correct location in regards to solar aspect.
  6. Windows or apertures are at correct heights and face due South.
  7. Building overhangs to the South are deep enough to shield the sun from the interior during the summer months to prevent overheating.
  8. An aggressive strategy of cross ventilation through windows as well as an air exchange system are designed into the building.
  9. Windows are glazed with the correct type of glass. This type will change depending on its aspect i.e. facing North, East, South, West.

Every building site has its potentials and weaknesses in regards to utilizing the sun’s energy. Sites that are sunny and fairly free of tall coniferous trees, especially to the Southern aspect, have great passive solar potential. During the Summer, deciduous trees on the South side of the home help screen the sun to reduce excessive solar gain. During the colder months, deciduous trees drop their leaves, allowing the lower angled sun rays into the home.

We typically design homes that combine hydronic radiant heating with passive solar heating for maximum efficiency. With this approach, the heat is passively collected, but actively distributed throughout the home.  So in a sense, Greenovision mostly utilizes a hybrid method, which we call Sun Smart Radiant Heating. SSRH sensibly captures the sun’s energy when it shines, but has the added benefits of distributing the heat uniformly through out the home and efficiently produces heat during long stretches of cloudy days. Our new home clients report that the heat does not run in their homes on any cold, sunny day in the fall, winter, and spring- even on sunny days with temperatues as low as -20F!

For more information on passive solar home design, please see “Reduce Your Heating Costs” on the Greenovision blog. You can also read “Bring on the Sun: Homes keen to solar rising in Bozeman,” an article we wrote for The Bozeman Magpie.

Passive solar home roof overhangs

Summer Sun

During the heat and high angles of summer sun, deep overhangs cancel out interior solar direct radiant gain.

Summer solstice roof overhang

Autumn Sun

Perhaps the most ‘dangerous’ passive solar time of the year when sun angles start to come down allowing the sun’s rays to penetrate into the building, yet daytime temperatures are still high.  Roof overhangs, walls, and window heights are carefully calculated to keep solar gain under control.  Often we integrate lower semi-transparent awnings (examples here) to help filter the solar radiation.

Passive solar overheating in autumn months

Winter Sun

During the winter, lower midday sun angles are allowed to penetrate deeply into the home, heating the mass floors and walls through direct radiation.

Passive solar in the winter

An example of modelling solar gain below.

 radiant gain thermal mass surface and a view