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Overhangs and Oversights
In
this article I will be discussing overhangs. The
simplest thing
in the world, is to just put the correct overhang over your
south
windows. It is totally automatic, and needs no attention, winter
or
summer. The tilt of the Earth's axis gives a reliable seasonal
change of sun
angles, so your south facing windows will be fully
illuminated in the winter,
and get no direct sun during the summer.
If you bought that, you are a
victim of over simplified
passive Solar literature. It could not be further
from the truth.
First, there is no such thing as the Winter position, or
the
Summer position, of the sun. The sun moves through half
its total
noonday altitude range (23.5,° half of 47°),
during each of the winter and
summer seasons. Every day,
it rises to its noon altitude, from the horizon.
So, how high
is it on any day? In the US, somewhere from the horizon
to
straight up. Take 90,° minus your latitude, then add
23.5° to it. That is
the highest point of Solar altitudes
that you will have to deal with, for
every house, whether
you call it Solar, or think that by not using the
word,
that the sun will go away.
The sun is at its lowest noonday
point on "Winter Solstice,"
at 90,° minus your latitude, minus 23.5.°
Winter Solstice
is the first day of winter, not the middle day of Winter.
The
sun is at its highest point on "Summer Solstice," which is the
first
day of Summer, not the middle day of summer. These
are also the shortest and
longest daylight days of the year,
even though they are not the coldest and
the warmest days.
The solstices are on approximately December 21st,
and
June 21st, respectively.
At Winter Solstice, winter begins, and
the days begin to
get longer, as the sun rises and sets further toward
the
closer pole (north in the northern hemisphere, etc) and
traces a path
higher across the sky. Every day it will be
slightly more than a quarter
degree higher in the sky at
noon., so in a mere two weeks, it will have risen
nearly
4°, so you can see that Solar collector tilts might not
need to be
all that critical, but that is another myth to
debunk, at another time.
Anyway, as the sun runs its
ever longer, and higher course across the winter
skies,
it eventually reaches the last day of Winter, which
is also the
first day of Spring, the equinox.
The solstices, and these other points,
called "equinixes"
are highly unlikely to ever fall on exactly midnight, so
the
last day of one season, and the first of the next season are
the same
day. These times, when the sun's path is half
way between its lowest and its
highest, are called the
"Vernal Equinox" which some just call the
"Spring
Equinox" and the "Autumnal Equinox" which we can
also call the
"Fall Equinox" Not only is this the middle
point of the sun's height at
noon, it is also the middle
point for the length of day. Interesting? No?
Well,
let's think about that a lttle. I did not just say that it
was the
middle point for the people in the north. It
is also the middle point for the
people in the south.
That is because the equinoxes are the points when
the
Earth's axis is exactly perpendicular to the light
from the sun. Everybody
(except right at the poles),
gets exactly 12 hours of daylight, and the
sunrises
due east, and sets due west, everywhere. On the
poles, the
equinoxes are the points when the six
months of day and night change
places.
This all gets a little more interesting at the equator,
and
other location in the tropics, between the Tropic
of Cancer (north) and the
Tropic of Capricorn (south)
which set at 23.5° from the equator, which
coincides
to the 23.5° tilt of the Earth's axis, which creates the
47°
range of the seasonal sun angles, that was mentioned
above. In this region,
however, the sun can actually go
past the 90° point, and be in the northern
sky, in the north,
and in the southern sky, in the south. This changes
that
simple high/low sun altitude situation, but the equinoxes are
still
the mid-points between solstices. At the Equator the
length of day does not
vary hardly at all, and is always
basically twelve hours. At the poles, there
is six months
of 24 hour daylight, and six months of 24 hour
darkness.
Though areas like the far north, and far south have
winters
with very short daylight, they also have much longer
summer
daylight, than in more tropical climates, often with over
sixteen
hours of sunshine. All of this factors into controlling
Solar
gain through south facing windows.
Here is the essence of why
simple fixed overhangs
are not a very effective way to shade south
facing
windows, used for Solar heating,
It is cool near the end of
March, at Vernal Equinox,
the last day of Winter, yet it is warm near the end
of
September, at Autumnal Equinox, the last day of
Summer. This happens
even though the sun follows the
identical path across the sky, during both
times. Any
fixed shading will block the same amount of sunlight,
and let
the same amount through, during both times.
A fixed overhang makes no
differentiation between
these times, though the average daily
temperature,
may require considerable heating in March, and
considerable
cooling in September.
This situation does not effect just these two days
of the year.
It effects every day. The sun's angles are the same on
October
14th as on February 26th, the same on November 21st as
on January
20th, and the same on April 24th (when I just got
a few inches of snow) , as
on August 18th. None of these
sets of days have even close to the same
weather conditions,
and heating or cooling needs...yet the Solar gain, and
the shading,
will be identical with fixed overhangs. Similar inequities occur
for
every day of the year, and their matching days. There are ways
to
overcome these imbalances, but it must be first recognized
that fixed
overhangs are far from the ideal solution, that many
have claimed they
are..
Since, winter is not centered around the sun's lowest path,
nor
its shortest days, and Summer is not centered around the
sun's highest path,
nor its longest days, fixed overhangs which
are designed to totally shade the
windows on Summer Solstice,
and allow full sunlight on Winter Solstice, will
also shade much
of the windows during the coldest part of winter, and
allow
direct sunlight to enter during the hottest days of Summer. A
fixed
overhang, therefore can cause underheating in Winter,
and overheating in
Summer. The seasons lag considerably
(about forty five days) behind the sun's
movement.
This situation, of partial shading, is further
exacerbated
by the tall windows that are often used for both large
areas
of Solar gain, and for allowing sunlight to strike the floor
mass,
as presented to be so simple in most presentations
of direct gain Solar. The
same overhangs which undesirably
shade the glass, even more radically shade
the floor. With
tall windows, even a fairly poor functioning overhang
would
need to be high enough, and far enough extended to begin
to dominate
the whole design of the house. It will be high
and large enough to usually
look odd. On top of that is
the fact that the sun is not at the same height,
above the
ground, all day. An overhang that might be perfect shade
at
Solar noon, will allow the sun through in the morning
and afternoon. Without
some very strange "wings" floating
out in front, and to the sides of the
building, windows will
not be shaded in the morning or afternoon, when the
sun
is lower in the sky. Overhangs are wonderful for getting
water away
from the walls of the house, but as shading,
for Solar windows....they fall
desperately short of the mark,.
To further demonstrate the basic problem
with all fixed overhangs,
the following is NREL weather data for Sioux City
Iowa:
Average sunlight, per day, per sq ft, on a south facing
vertical
window.
March........... 1150BTUs/day
September..
1160BTUs/day
Virtually identical amounts of Solar gain.
Average
temperatures:
March ......... 35.8°F
September ...63.4°F
28°
colder in March...
Compared to the standard heating base temperature
(65°F)
March 29.2° of heating needed
September 3.3° of
heating needed
also
September 1.8° of cooling
needed
September average 1.6° of heating needed
To put this into
perspective........
To keep a house at about 72° you only need
a
temperature above 65°, because of the heat
of the people, lights, appliances
etc. The heat
from normal life could over-heat the house in
September, if
you do not open some windows
part time. However in March, when the
temp
outside averages almost 30° colder, and is below
freezing about a
third of the time, you need
your windows closed, and you definitely do
not
want them completely shaded, like you would
want in September. The
heating need in March
is eighteen times greater, than in September.
You
may need 100 times more south glass,
un-shaded in March.
Fixed
overhangs do not allow for that. They give
you the same identical amount of
glass exposed, on
both equinoxes. They can only give you a very
small
amount of desirable Solar control, and that will be just
at the
extremes of the solstices. In the between times,
they will shade part of
your windows, most of the time
when you need the heat, in late Winter and in
early
Spring. And, they will also let the sun shine in most of
the time,
when you do not want heat, in late Summer
and early Fall.. Fixed overhangs
are a poor shading
device. All they give you is a big area of exposed
glass
most of the summer, letting in heat you do not want,
and a big area
of shaded glass most of the winter,
letting out the heat that you want to
keep inside..
Even the specialty Low-E glass, which reflects
a high
degree of Summer sun, will reject as much
March warmth, as September
over-heating.
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What are the alternatives?
There are several
categories of Solar gain control strategies.
Perhaps the most basic
division is between interior and exterior.
The most effective strategies are
usually a combination of approaches.
It is possible to use movable
shading devices, which get positioned
differently for the heating and cooling
seasons. Thought must go
into these systems, to be sure they are durable and
convenient
enough that they will remain operational for the life of the
building,
with little repair or maintenance beyond what is needed by
other
building components. Many systems of this type, especially
movable
louvers, have fallen victim to ice and snow. Moveable shading
devices
might be as straight forward as boards, screens, or shades that
simply
hang from the fixed overhangs, on eye hooks, and are removed to
a
protected storage area, during the heating season. They could be
boards, that
lay on horizontal beams projecting from the wall, and
might even serve as a
balcony floor in summer, then get stacked
back against the wall in the
winter. In some cases, greenhouse
shade screen may work well. For others,
roll-down security
shutters will be the ideal solution. There are many
workable
systems, which will do the job that a fixed overhang can
never
do, as well.
My favorite external shading device is self installing, self
adjusting
to the degree of need, is long lived, and self replenishing. It
also
actually cools the area through evaporation. It even floats,
high
out in front of and to the sides of the windows, but never
looks
odd. It is vegetation, either in the form of trees, bushes,
vines,
or any combination. They know just when to bloom and grow
leaves,
and then when to drop them in the fall, just in time to
let the sun shine in,
as the cold winds begin to blow. Vegetation
is about as perfect an "overhang"
as we could ever wish for.
There are as many varieties, and ways of using
it, as there
are styles of houses. Some plants hang down, other grow
up,
other hang far above, and even others climb over the
south face. Personally,
I have a south wall shaded by an
ancient oak tree, plus ivy that covers the
whole wall in the
summer, then drops every leaf in the fall, to let the
sun
shine in all winter. Vegetation can also shade the roof,
and
non-window areas of the walls, as well as the
ground around the house, all
adding to the
effectiveness of its cooling.
The other basic category
of shading devices, work inside the
house, after the glass. Most of these
are the standard window
coverings that we are all accustomed to. Some reflect
the light
back out the windows. Some absorb it, and let the resulting
heat
rise, perhaps back out an opening in the window. Some are
the
insulating shades, shutters, or curtains that keep heat from
escaping in the
winter time. They can just a well insulate the
house from unwanted heat gain,
as from unwanted heat loss.
There are literally hundreds of variations,
including approaches
where the heat of the sun is used, like a Solar chimney,
to draw
a breeze through the house, and out the south windows.
The
combination of exterior, and interior shading can do a very
effective
job of controlling Solar gain through south facing
windows, so that it will
always be working for you, not
against you.
There is one
circumstance where fixed overhangs can
work excellently. That is when they
are inside of a Solar
sunspace*. Inside of a sunspace the overhangs can
stop
the light, but the heat can still be collected from the air
of the
sunspace. This gives the option of moving the heat
into the house, or into a
heat storage, or to heat your
water, or venting it to outdoors, or even
letting it rise out,
to draw an easy breeze through the house. These
interior
overhangs need only be shelves above the windows that
let light
pass through the sunspace, and on into the inner
house. They may even
function as "light shelves" to reflect
the light up into the house, to light
the ceilings of the rooms.
Balconies do a fine job of doing this interior
sunspace shading.
*For those not familiar with the term, a "sunspace" is
a
shallow glass room, like a big walk-in Solar collector on
the south wall
of the house. Sunspaces can be extremely
beautiful living spaces, with high
ceilings, their big wall of
glass, and sometimes a glass roof. They are very
similar
to "Solar greenhouses" but are designed to give up their
heat to
the house, and not for growing plants. In cold
climates, they are far more
efficient as Solar heaters,
when they contain very little "thermal mass" so
they
can go cold when the sun is not shining, and not
spend all their heat
back out through the glass,
in the middle of the night.
One last area
to mention, is the east and west facing
windows. Overhangs work even worse
for these,
than for the south facing ones. However, vegetation
works even
better for shading them, because by the
time the sun is around far enough to
shine in these
windows, it is very low in the sky. This means that
low
trees, or bushes, set out in front of the windows
can shade them. Obviously
this must be done in a
way that does not shroud your favorite view, but
it
is usually very workable.
There is very good information available on
where
to plant trees (or leave them standing) around the house,
so that
you get heat when you want it, and shade when
you want that. Plants will
also shelter you from winter
winds, and bring in cooling summer breezes. The
moisture
will cool the summer air, and keep it warmer into the
winter.
In summary:
It is not possible to design fixed overhangs
so that South facing
windows are fully illuminated in the winter, yet get no
direct sun
during the summer. The sun follows virtually the same path
across
the sky at the end of winter, as it does at the end of summer.
Therefore,
other provisions must be made to control Solar gain,
in a way
that will allow as much gain as is needed to heat the
house, yet also limit
the Solar gain, when cooling is needed.
-Laren Corie-
Integral Solar
Building Design
and Consultation for the
Owner-Builder, Since
1975