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Updated on:  Wednesday, February 20, 2008 02:11 PM        

The earth's axis is tilted, we learned that in elementary school.  As a result of the earth's tilt, the sun rises lower in the winter sky and rises higher in the summer sky.  In planning for passive solar in our homes, we must remember that in a Northern Hemisphere, the winter sun rises in the South East, stays low in the South sky, and then sets in the South West.  The summer sun rises more in the North East, quickly gains height to a higher elevation than in winter, and then sets back in the North West. 

So the best things to do in the Northern Hemisphere are:

• Let the sunshine in to heat a thermal mass.  Orient the house such that the major axis runs east-west.  

• Carefully place the windows.  Limit the number and size of windows on the Northern side of the house, to reduce Summer heating of the interior. Place more windows on the Southern side of the house.  Use wide eaves and tall shade trees to limit solar radiation in the Summer, yet provide plenty of solar heating in the Winter. Clerestory windows and dormers can bring winter light into otherwise dark areas of the house (minimize skylight use). Use a solar path chart to design a building so that low winter sun shines in and high summer sun is blocked by effective use of windows, overhangs and shade.

• High R values with minimal air leaks are key to any low-energy house.  Do a blower door test to verify house tightness. Invest in a high performance ventilation system; an air to air heat exchanger recovers the heat in exhausted ventilation air.

• Thermal mass, inside the building, moderates temperature swings by storing heat during the day and releasing it a night. Materials commonly used for mass include water, concrete, masonry, and earth. Keep the mass at 3-4” thick, and keep it in the direct sun for best effectiveness. Mass must be carefully balanced with glazing area to perform properly in a given climate.

• Do your homework before building.  Time spent on the building site observing sun, wind, rain, and ground water pays off in selecting the ideal location and design for the house. A solar building energy modeling software program such as Energy10 allows a solar building design optimized for the local climate.

In colder climates, it's important to store as much of the incoming solar energy during the day as possible, and then allow it to continue heating the house during the night.  The best way to store this heat is in a large thermal mass.  One popular (and patented) way to do this is with a thick slab floor.  This floor could be solid concrete, or a layer of rock.  Since the incoming sunlight will only heat the South side of the slab (via the windows) several ingenious methods have been devised to circulate air through the slab.  This air serves to moderate the temperature of the slab during the day, and help heat the air within the house during the night.