Passive Solar Design and the Passive House

One of the principles of Passive House is the integration of passive elements.  Once such element is solar energy.  Passive solar design is simple yet complex.  Simply put, it's a set of design  principles that takes advantage of the energy from the sun.  The Pueblo peoples (in Colorado) had free heating and cooling at the Mesa Verde cliff dwellings.  They didn't have electricity, insulation, natural gas, air conditioning or modern building techniques.   The cliffs provided everything they needed and provides a basis for passive solar design.

That's where the simplicity ends and complex home design begins.  Much like the cave dwellings above, this type of home takes advantage of the natural progression of the sun's altitude downwards in the sky as winter approaches.  In the northern hemisphere the sun is always on the south side of a building.  With such specific celestial behaviour rules can be defined to define solar design.  

South facing windows are beneficial in the winter since the sun will shine straight in, even at midday.  In the summer, however, there could be a lot of over heating.  This is easily taken care of by adding a trellis over south facing windows or larger eave overhangs.   Summertime overheating and heat storage in the winter can be facilitated using high thermal mass materials like concrete.  It makes for an amazingly simple floor that looks great when polished.  

Concrete has a large thermal mass.  it can absorb heat, redistribute it, and it will continue to release that heat into the evening.  In the summer, that same thermal mass serves to help keep the inside of the house at a constant temperature.

Careful placement of windows are very important.  In the summer, the sun rises quickly in the East  and since the suns rays pass through less atmosphere then in the winter, the sun's energy starts to warm the east side of the house early in the day.  This will lead to overheating later in the day.  In addition, west facing windows will continue to heat the house late into the day when the house is already warm leading to overheating.  So minimizing the size and amount of windows on the east and west of the building will make for a more comfortable house year round.  To the north, the house is always in a shadow.  The light through north facing windows is great but they will act as an energy sink in the winter.  Minimizing the number and size is obviously important to a good design. 

Finding a lot is part of the battle for a good design....especially for passive solar.  A lot with good southern coverage will maximize solar gains if the home is oriented towards the south as shown in the picture below.  However, orienting the home up to 22 degrees away from south will still allow for about 92% gains.  This is still pretty impressive.   However in the summer, the west setting sun will lead to excess heat in the home unless some shading control is added to block the summer sun. Passive House design is is flexible.  As long as there is some south facing wall of the building, a Passive House design is still possible.  Architects and designers have some great ideas about how to achieve this and still maintain the look and feel of a building so it doesn't look like a space ship!
Maximizing this passive element is a key to minimizing yearly energy demand for heating...and all it takes are some windows with the right orientation.  The next time you are driving down a road in a  subdivision take note of where the windows are.  It never ceases to amaze me when builders decide to use massive windows on the north side of a house.  You may as well leave the wall open and let the cold air into the space.  North rooms with large windows are thermally inefficient, have no potential for gains and should be avoided if you are concerned about energy usage.  Subdivision design should be centered around house orientation.  Subdivisions are great compact areas for living.  But random orientations of the roads and shaded buildings make passive solar design challenging.  However, it is achievable.  A passive house designer would know if it can be done.


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