SHGC equation use?

Posted by Bill Li on July 28, 2008, 8:56 am

I've been going through James Kachadorian's book, The Passive Solar House,
as part of my overall house design.  In Kachadorian's book, he includes a
section talking about using "Shade Coefficient" (SC) as part of figuring out
how much passive solar contribution comes through the windows using SHGF
(Solar Heat Gain Factor) tables and the window area.

In an online search, I found the following
http://www.gard.com/ml/bldg-sim-archive/msg03634.html  which suggests that SC
is no longer in use and, for example, Thermotech only lists SHGC which
according to the post is different than SC.

So my question is whether or not anyone can point me at a useful reference
(book or net) where I can find out how to use SHGC to calculate solar gain.
By the definition of SHGC where it is a fractional amount between 0 and 1
for the fraction of incident solar radiation admitted, I think I can just
use SHGC the same as SC.  However, the Curjica's post from the link above
makes me think there is something else.

Bill

Posted by nicksanspam on July 28, 2008, 10:28 am

The shading coefficient is the fraction of sun transmitted compared to
a single pane of glass, so SC = 1.15SHGC, approximately.

Warm air rises. Why would it want to flow under the floor? Lots of warm
air needs to touch Lots of thermal mass surface to raise the slab temp
on a sunny day without overheating the house and ensure a low day/night
temperature swing. During a cloudy week, such a house gets exponentially
colder and colder, without woodstoves and so on.

Removing most of the solar glazing to a low-thermal-mass sunspace (one
that gets cool overnight and stays cool during a cloudy week) with an
insulated wall between the living space and the sunspace allows the same
or more solar gain when warm air flows between the sunspace and the house
during the day, but reduces the nighttime and cloudy-day heat loss from
the living space...

Dr. Rich Komp (author of Practical Photovoltaics and president of the
Maine Solar Energy Association) says warm hollow floors like his
(which predates Kachadorian's) aren't new. Romans built hypocausts,
hollow floors heated with warm air from hot water or fires. So did
Chinese peasants. Warm hollow floors make good homes for dust and
varmints. Rich's friend Ernie the Ermine takes care of that problem.

Living inside the heat battery, we are subject to its temperature swings,
and if there are no temperature swings, there is no solar heat storage.

We can't charge the slab up to a high temperature because we have to
live with it in the room. The same amount of thermal mass at a higher
temperature stores more useful heat than lower temp mass, and it allows
keeping a constant room temp until the mass cools to something close to
that room temperature.

Floor slabs don't usually have much insulation between themselves and
the room air, and they are difficult to insulate because of their shape.
The same amount of insulation applied to a cube with equivalent mass
lowers the rate of heatflow a lot more.

And water stores about 3X more heat than masonry by volume. It can
also be cheaper and more useful, even in sealed containers.

K's book has lots of whopping mistakes. For instance, he thinks a house
needs 2/3 ACH for health, 27X more than the 0.025 ACH Swedish standard.

Page 17 says "As you can see, the reduction in solar benefit increases
exponentially as you rotate the home's orientation away from true south."

Page 30 says:

If this combination of poured concrete slab over horizontally laid blocks
is ventilated by air holes along the north and south walls, air will
naturally circulate through this concrete radiator when the sun is out...
the south wall will be warmer than the north wall... air that is next to
or alongside the south wall will rise. Warmed air will then be pulled out
of the ventilated slab, and the cooler air along the north wall will drop
into the holes along the north wall. This thermosiphoning effect will
naturally continue to pull air through the Solar Slab.

Page 49 says "Incorporate an air lock entrance" with miniscule energy savings
except for a department store, or a house with a huge active family.

Page 53 describes "reflective" foil smack up against plywood:

The interior foil face will reflect heat back into the room, even though
it is sealed inside the thermo-shutter... The outside foil face of the
insulation contained within the wood veneers will reflect the sun's summer
heat back out the window.

Page 94 belies the natural air circulation described on page 30:

The duct shown running down the middle of the base under the poured slab
is included in all cases. It should always be used as the return-air duct:
do not reverse the air flow pattrern shown on the control diagrams. By
using the Solar Slab as part of the return-air duct system, the Solar Slab
will constantly assist the furnace by preheating the return air. Even if
the home will be heated with a woodstove and emergency electric furnace,
the return duct should be included and the air mover hooked up per the
appropriate control diagram...

Page 101 says:

2. Size of electric heating system = 9.25 kilowatt=hours,
with an annual consumption of 7.616 kilowatts,

Page 102 says:

The calculation for the electric backup option determined that
we would need 9.25 kilowatts per hour for the Saltbox 38...

Page 106 says "The theoretical minimum temperature to which a home with
a Solar Slab will drop is the ground temperature under the solar slab..."
(Yes, that will keep the pipes from freezing in most parts of the US,
in a perfectly airtight house with infinite insulation :-)

Page 107 says "it also costs more to cool air than to heat air," as if
K. is unaware of evaporation, night sky radiation, or the phrase
"coefficient of performance."

Page 137 ignores one-way passive backdraft dampers:

It may seem that a sunspace that is gathering enough heat to become
90 degrees Fahrenheit on a cold, 15-degree but sunny winter day would
be beneficial to the home. And yes, it can be beneficial. However,
the same overglazed sunspace that accumulated all that heat during
the cold but sunny day will need lots of added heat when the sun goes
down to prevent it from freezing, which means that the sunspace or
greenhouse will tend to draw heat from the rest of the house as its
flow of solar heat reverses course, back out through the glazing.

Nick