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Cob
Building Tutorial
Chapter 10: Passive Solar Considerations
If your goal in building a cob home is to maximize
efficiency while minimizing environmental impact, you’ll want to use
passive solar heating and cooling for your home. Passive solar just
means making the most of the energy in sunlight.
While cob is a rather poor insulator (a two-foot cob
wall has an insulation factor of about R-15), cob is high in thermal
mass. You can take advantage of this thermal mass to maximum effect by
orienting your home properly towards the sun. With the use of proper
orientation, your passive solar system will have no operating costs, low
maintenance costs, and no greenhouse emissions. With careful planning, a
passive solar system can greatly reduce the need for home heating and
cooling.
The theory behind passive solar design is to take
into account the different positions of the sun in the sky during
different times of the year, and to design your home accordingly. For
example, the sun is higher in the sky in the summer, and lower in the
winter. You’ll want as little sun as possible coming in through the
windows during the summer months, but you’ll want as much sun as
possible during the winter months, to help heat your home.
If you live in the northern hemisphere, and your
building site has a south-facing slope, consider yourself lucky! You
have the ideal place to build a home. Of course, if you live in the
southern hemisphere, you’ll need a northern-facing slope. Since I live
in the northern hemisphere, the rest of this chapter will assume that
you do as well. If, however, you live in the southern hemisphere, simply
reverse compass directions for anything else I say in this chapter.
If you don’t have a south-facing slope on your
building site, make the best of what you have. The worst-case scenario
would be a north-facing slope that completely blocks the sun. In such a
case, the best thing to do would be to build as high up on the hill as
possible. The best way to tell exactly how much sun you’d be getting on
your home is to face due south, then spread your arms at a 90-degree
angle to each other. The space between your fingertips is the space
where the sun will travel across the sky. In order to take maximum
advantage of the sun during the winter months, you should have a major
wall of your building aligned to this angle, perpendicular to due south.
Some people incorporate a solarium on the southern side of the home to
maximize winter sunlight.
 Figure 10A gives you an idea of how to orient your
building to take advantage of the sunlight. Once you’ve positioned the
walls at the correct angle relative to due south, the next step is to
find the maximum and minimum heights of the sun. The reason you’ll want
to know this is so that you can build an overhang that will block out
the summer sun while allowing the winter sun to come into the building
to heat it. Since the sun is higher in the sky during the summer, such
an overhang will block the summer sun to a greater degree than the
winter sun.
The maximum angle that the sun achieves from summer
to winter varies depending on your latitude. If you have the luxury of
time, you can determine the exact angle to construct your overhang by
measuring the difference in the Sun’s positions during the summer and
winter solstices, then splitting the difference. Here’s an easy way to
do that: Go outside on the summer solstice (usually around July 21-22 in
the northern hemisphere…the first day of summer). Now drive a stick into
the ground. This stick will have to stay stationary for six months,
until the winter solstice, so maybe you’d better make it a stake or a
pole instead, just to be safe. If you already have
a fence post that faces south, you can use that. Wait until noon, then look for the shadow of the top
of the post. Drive a stake into the top of the shadow, and plan to leave
it there until the winter solstice. Wait six months, then go outside on
the winter solstice (the first day of winter, around December 21-22).
Wait until noon on the winter solstice, and find the top of the post’s
shadow again. Drive another stake in the ground at the top of the
shadow. Now split the difference between the stakes. For example, if
there’s six inches between the stakes, find the halfway point at three
inches, and drive a nail into the ground there. Tie a string to the
nail, and then stretch the string to the actual top of the post. Measure
the angle of the string relative to the ground, using a protractor. This
angle will be the angle you’ll want to use when constructing an overhang
for your south-facing wall (see Figure 10B).
If all of this seems like too much trouble, or if you don’t
have the time to wait six months to measure it all, there’s a simple
formula to calculate the proper size overhang. First, determine the
latitude of your building site. Write this number down. Now, subtract
18.5 degrees from the latitude. For example, if your latitude is 35
degrees north, subtract 35-18.5. This will leave you with 16.5. This
number is the angle that should be between the bottom of your windows
and the overhang of your roof. See Figure 10C for a clearer picture.
 So which rooms should you place on the south side of the house
to catch all that sunlight? A rule of thumb regarding such placement is
that generally the living quarters (living room, den, study, etc.) are
placed on the sunward side of the home, and the bedrooms are placed on
the side of the home opposite the sun. If you’ve ever had a morning
person pull aside the curtains, and been hit in the face with a sunbeam
early in the morning while you’re still half-asleep, you can probably
see the wisdom of this room arrangement! Rooms that generate their own heat, like the kitchen
and the bathroom, can be placed on the north side of the home, near the
bedrooms. In addition to using the heat generated by cooking and bathing
to also heat the home, you’ll gain an insulating buffer from these rooms
as well.
Trombe walls can be an effective part of any passive solar
design system. A trombe wall is a thick wall made of some sort of
material that has a high thermal mass (can you say, ‘cob’?). The
sun-facing side of this wall is painted a dark color, usually black.
 Outside of this wall, on the sunward side of the
house, is a large glass window. Between the cob wall and the glass is
several inches of air space. Above the trombe wall is an overhang set at
the proper angle to block the summer sun, but to allow the winter sun to
strike the glass. There are vents in the top and the bottom of the wall
itself to allow for circulation of the air. These vents can be simple
PVC pipes embedded in a cob wall. One variation is to place mylar flaps
over the vents so that the air may only flow in one direction, thus
keeping the air moving in the direction you want it to go in…hot air out
of the house in the summer, and cool air out of the house in the winter
(see Figure 10D).
Another consideration when using passive solar design
is the placement of windows. In order to maximize the cooling potential
of those summer breezes, you’ll want to have windows on opposite sides
of the home that can open, so that you can generate cross-currents.
Spend some time on the property before building your home. Notice the
prevailing wind patterns. Use them to your advantage by placing windows
in the path of the prevailing summer winds.
 Did you know that you can use sunlight to heat your water?
Placing a solar collector on a south-facing wall can help you to cut
down on your water heating bill. Such a solar collector can be made
fairly easily at home using 1” PVC pipe spray-painted black. The pipe is
fitted together, using U-joints, in a zig-zag fashion (see Figure 10E).
This assembly is then placed inside a plywood box (also spray-painted
black on the inside) that has a plate-glass face. This is then placed on
the south side of the house, on the roof. Pipes lead up to the
collector, and down to the water heater. The sun heats the pipes, which
are full of water. The hot water then naturally circulates to the water
heater. The only drawback to this type of collector is that if you live
in a place where the temperatures drop below freezing, you’ll have to
drain the collector during the winter months. An alternative method to
heating your water during the winter is to put a similar collector, but
made of copper pipe, in close proximity to your heater. If you have a
rocket stove or a fireplace, it might even be possible to embed such a
piping arrangement into the cob wall behind the heater.
A final consideration for passive solar design is the use of
geothermal flooring. Geothermal flooring is just plastic piping filled
with water. The piping runs underneath the floor, then outside into the
yard. This works especially well
on a south-facing slope. The sunlight
hitting the buried pipe in the yard circulates into the cooler home to
warm the floors. The process is reversed in the summer. The outside pipe
is buried below the frost line so the water inside doesn’t freeze. Some
systems add antifreeze, but if this seems like too much of an
environmental hazard for you, you might try more environmentally
friendly alternatives like alcohol.
This chapter is designed to be only a basic overview
of some of the concepts of passive solar design. There are many good
books on the subject, so if you’re interested, check your local library
to expand your knowledge. Another great source of information is
www.earthshelters.com. Incorporating as many passive solar techniques as
possible into your final home design will allow you to reap the savings
in energy costs. In fact, you might be able to reduce your energy needs
to such a degree that you’ll be able to get off of the grid altogether
and get by on a small solar or wind power system!
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