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

Choosing Floor Joists

Posted September 18, 2008

Floor joists are used to frame the floor of most buildings and there are a variety of materials that have different impacts on the environment.  The joists used to frame the floor of a house fall into three general categories 1) dimensional lumber, 2) I-joists and 3) open web trusses.

From an environmental standpoint, dimensional lumber should not even be considered, as it requires the cutting down of the largest old growth trees to get lumber large enough.  The other problems with dimensional lumber is that you are limited to just over 15 feet of span for 2×12, which is the largest commonly available lumber.

The second choice is I-joists, which are made by having two pieces of wood, typically 2×3, connected by a piece of plywood or Oriented Strand Board (OSB) creating a beam shaped like the letter I when viewed from the end.  There are some I-joists where the top and bottom members are also made of laminated lumber.  This results in 60% less timber being used vs and equivalent piece of dimensional lumber.  The I-joists are commonly available with up to 16″ depth, and are manufactured with different widths of lumber for the top and bottom pieces.  With the heaviest I-joists spaced at 12″ centers, the longest span is about 32 feet.  An advantage of I-joists is that they can be cut to length on-site and many have pre-punched holes for running electrical and plumbing lines.  Holes can be cut for ductwork, but you must follow the guidelines  from the manufacturer for placement of the holes.

I my opinion the best choice for floor framing is the open web trusses which consist of a top and bottom plate which is joined by web that forms a series of triangles.  The top and bottom boards are usually 2×3 or 2×4 lumber that is finger jointed together to form the long spans and the web can be either wood or steel.  The trusses can be manufactured with depths up to 24″ for spans up to 40 feet.  The biggest advantage of the open web trusses is that there is an open space in order to run all electrical, plumbing and ductwork with ease.  The disadvantage of the open web trusses is that they are much more limited in the ability to be trimmed on site with typically only about 2-3 inches on each end being trimable, so the measurements must be fairly exact when they are ordered.

Both the I-joists and the open web trusses, being engineered products will result in a quieter floor, as you don’t have the warping and twisting found in dimensional lumber.

In my home I used the open web trusses and found them very easy to work with.  They are much lighter than dimensional lumber and having all the open webs made it much easier to route wires and pipes without having to worry about drilling holes.  Care must be taken when installing them as they have a top and a bottom and so must be oriented in the correct manner.  All the trusses also have to be aligned in the same direction, or the webs will not be in alignment and can cause problems when installing ductwork.  By using the open web trusses, I was able to avoid installing any bulkheads.  These normally would have been needed with dimensional lumber or I-joists for the installation of ductwork.  Without any bulkheads I was able to avoid any drops in the ceiling height.

Posted under Materials