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 Energy Efficient Windows

Posted September 5, 2008

When considering the windows for an energy efficient house, there are 2 things to look at. The first is the material the frames are constructed of, and the second is the type of glass in the window.

The most common type of window on the market currently are the PVC framed windows. These are constructed of an extruded PVC channel that is cut to length and then welded together to form the frames. If you haven’t seen the film Blue Vinyl you really should and then you will understand how toxic PVC really is and why it has no place in the home.  Unfortunately it is cheap and easily available so it is very commonly used.  PVC is also not very thermally stable and will expand when heated, causing problems with windows binding or frames warping in the heat.

Another type of window that you will see in some older homes is aluminum frames. These are the worst kind of window to have in a heating climate like Canada, as the aluminum is a conductor and will rapidly conduct heat to the outdoors. The only aluminum windows available in Canada currently are for commercial buildings, although they might show up in condominiums and apartments.

Another choice for window frames is wood.  Wood has the advantages that it is a renewable resource and has a very high aesthetic appeal.  The disadvantage is that it rots and will need regular maintenance in order to maximize its lifespan.  Also a lot of wood windows are treated with a chemical fungicide to reduce rot.  If you are chemical sensitive, this may cause a problem.

My personal choice for window frames is fibreglass.  The windows are formed much like the PVC windows, with the fibreglass being pulltruded into long channels that are cut to length and then connected with mechanical connectors to form the frames.  Fibreglass has the advantages of being very stable, and so will not rot.  Fibreglass is also thermally stable, so there are fewer problems with binding and warping, fibreglass is also stronger than PVC.  This all results in a stronger, longer lasting window.  Three companies in Canada that produce fibreglass windows are Inline Fibreglass, Thermotech, and Fibertec.

The next decision is the type of mechanism for the window. The choice of mechanism makes a difference to the amount of air infiltration that you will get through the window. Remember that air infiltration equals heat loss. The best type of window is a fixed window with no mechanism, but in most cases you will want a window that can be opened for ventilation. Of the windows with an opening mechanism, a well sealed hopper style window has the lowest air infiltration, followed closely by casement windows. Of the more classical window styles, single hung windows will have a better performance than double hung.

Next is the glass. There are a lot of choices regarding the glass for windows. In Canada windows are rated with an Energy Rating (ER). A positive ER window, in a south wall, will gain more heat than it loses. If the ER is not available, look at the Solar Heat Gain Coefficient (SHGC) which is a number between 0 and 1. The higher the number, the more heat a window will allow in as radiant heat from the sun. The other factor to look at us the U-value. This number is the inverse of the R-value commonly seen in insulation, and indicates the amount of heat a window will lose. The lower the number the better. A double pane window with an low-e coating will have as SHGC of around .45 an a U-value of .35. A triple pane window with a low-e coating will have an SHGC of .35 and a U-value of .25. Another factor is the spacer at the edge of a double or triple pane window. Most newer windows come with plastic spacer that has a lower conductivity than the aluminum spaces that were common in the previous generation of windows.

Typically a triple pane window will have lower U-values, lower SHGC and higher ER values and so will lose less energy, but the price can increase substantially over double pane. You will have to do a heat loss calculation in order to determine whether or not the extra cost is justified. When I built my house I went with double pane low-e windows on the south, west and east side of the house and triple pane on the north facing windows. I did the heat loss calculation for my house and having triple pane in all windows would save about $100/yr in heating costs, but would cost $8000 more, for a payback of 80 years. Even with higher energy prices, in my home the triple pane was not justified. If you are in a colder climate, like Winnipeg, then the triple pane becomes more justified. If you are concerned with having the lowest energy use possible and have no budget constraints, then triple pane, or even quad pane is the way to go.

Posted under Design