Passive Solar Shading Options

Posted October 1, 2008

In a passive solar design, you need to have shading on the south facing windows during the summer and have direct sun on the windows during the winter.  There are a few different ways to accomplish this.

One way is to plant deciduous trees and shrubs on the south, east and west side of the building.  During the summer the trees will shade the building and block the heat from hitting the house.  It also can have a cooling affect around the building due to the transpiration from the plants.  In the fall and winter, the trees will loose their leaves and will let the sun shine in, heating the house.  The advantage to using trees is that they closely track the temperature changes during the seasons, with the leaves budding in the spring once it has warmed up and falling off in the fall once the temperature has dropped.  The disadvantage to using trees and shrubs is that the woody mass of the tree will always shade the building, reducing the amount of sun in the winter.  The trees also take some time to grow to the point where they will be an effective shade.  They can also be a problem if solar panels are installed on the roof and are shaded by the trees.  This last problem can possibly be avoided by having the solar panels ground mounted beyond the shade of the trees.

A second way to shade the house is to use overhangs. You can calculate the depth of the overhang by finding the angle of the sun at the summer solstice – June 21 (90° – latitude + 23.5° = ss) and winter solstice – December 21 (90° – latitude -23.5° = ws).  Then take the distance from the bottom of the window to the bottom of the overhang (wh) and the distance from the bottom of the overhang to the top of the window (oh).  Then use the formula wh/tan(ss) to get the best overhang for the summer solstice and the formula oh/tan(ws) to get the optimum overhang for the winter solstice.  You want the overhang to be less than the winter solstice calculation and more than the summer solstice calculation.  For an example, I will use a house at 44°N latitude. The bottom of the window is 78″ from the bottom of the overhang and the top of the window is 18″ below the bottom of the overhang.

The angle at the summer solstice = 90° – 44° + 23.5° = 69.5°
The angle at the winter solstice = 90° – 44° – 23.5° = 22.5°

Summer overhang = 78″/tan(69.5°) = 78″/2.67 = 29.2″
Winter overhang = 18″/tan(22.5°) = 18″/.414 = 43.5″

So the overhang should be between 29.5″ and 43.5″.  Since you want shading for some time on either side of the summer solstice add about 6″ to the overhang.  In this case I would use a 36″ overhang, which would give complete about 6 weeks on either side of the summer solstice.  The disadvantage of a set overhang is that the temperatures are not the same the months before and after the summer solstice, which means there is more shading than you want before the solstice and not enough after the solstice.  The advantage is that you have the shading immediately after the house is built, and the shading is predictable.

Another option would be to build a trellis overhang, which would be a hybrid of the two above systems.  The trellis which would be built the same depth as an overhang would give some shading by itself, but if it is covered with a deciduous vine, such as grape, the leaves would give additional shading during the summer and in the fall the leaves would drop and give you more light before the winter solstice through the holes in the trellis.  You would have the added bonus of grapes to harvest.

Posted under Design

Rammed Earth Construction

Posted September 22, 2008

Rammed Earth Construction is the ultimate in building with local materials.  It is build from earth that is dug up from the building site.  The technique consists of building a form, similar to a form used to pour concrete, and then filling the form with a mixture of sand, clay and gravel with portland cement as a binder and then compressing it together with a tamper.

Rammed earth construction has been in use since the neolithic ages and there are archeological sites in China from 5000 BCE where rammed earth was used for walls and foundations.  In the past binders such as blood or lime were used.

Rammed earth construction is particularly good in passive solar design, as it has a large amount of thermal mass, which will even out the temperature fluctuation during the day.  It also has the advantages of being almost soundproof and fireproof.  In some sites, other materials, such as glass or shells are added to the mixture to give it additional texture.  It can also be coloured by adding pigments to the mixture.  Variations can be created by using different mixture in the various layers.  Since the rammed earth is used as the finshed wall, it is very non-toxic, as the materials are soil and cement, so it is a good construction method for those with environmental allergies.

The technique for construction is quite simple.  Forms are put in place in the shape of the desired wall and then a damp mixture of sand, clay, gravel and portland cement is placed in the forms four to ten inches deep.  The mixture is tamped down with either a mechanical pogo stick tamper, a pneumatic tamper or by hand with a plate tamper until it has been compressed to about half it’s original thickness.  Another layer is then put in and the process is repeated until the top of the form is reached.  The form is then removed and move up so the process can continue until the desired wall height is reached.

An interesting variation on rammed earth was developed in British Columbia, called SIREwall, which stands for Stabalized, Insulated Rammed Earth wall.  In their process, a piece of foam insulation of the desired thickness is placed vertically in the center of the form and the rammed earth is placed on either side around reinforcing rebar and then tamped down using custom selected mechanical tampers.  This creates a wall that has an insulated core, but has the thermal mass exposed on the interior and a durable rammed earth exterior protecting the insulation.  This results in a wall between 14 and 21 inches thick.  The SIREwall process has a protocol that controls the soil consistency, the method of mixing and curing to produce a wall with predictable, cost-effective results.

Properly done, a rammed earth construction will create a building that is weather resistant, soundproof, insect and rodent proof, inexpensive to heat and cool, very comfortable to live in and will probably last for centuries.

Posted under Techniques