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Crestone Eagle, October 2006:
Insulation for passive solar buildings:
Keeping the heat in
by Paul Shippee
insulation is a rather mundane subject. It is also generally
understood as the most effective means of energy conservation.
Therefore, your first green dollars spent to “save home
heat” are best spent on additional insulation before
you even consider solar heating. Let’s take a look here
at a special application of insulation.
Even though the subject of insulation is not glamorous—it
works silently, invisibly, and is not expensive—it begins
to get challenging and more interesting when we consider buying
moveable insulation for windows, aka night insulation. Windows
are often treated as a decorator opportunity, but passive
solar homes have large south-facing windows designed for gathering
winter sunlight. Installing moveable insulation for these
windows can be a challenge, as well as an opportunity for
enhancing the night-time comfort as well as the thermal performance
of passive solar buildings.
In general, the recommended levels for adding insulation
to passive solar buildings are not different from the established
standards for well-insulated conventional buildings. A basic
feature of insulation, wherever it is placed in a house, is
that you only have to pay for it once—and it keeps on
working for you year after year. Of course, with heating fuel
you have to keep on paying more year after year into an unpredictable
future. So, how much is an insulation investment worth?
Imagine a house, perhaps yours, where the large glass areas
on the south side are soaking up lots of the sun’s heat
energy during a typical short and cold winter day, and then
the sun goes down. The house has been comfortable; it has
not overheated during the day due to large enough amounts
of interior thermal mass. When the sun goes down, bringing
on cold winter nights that are twice as long as the days,
those large glass areas are going to get cold. This can feel
uncomfortable as the solar heat collected passively during
the day migrates back out through the cold windows.
This excessive heat loss can spoil the comfort factor you
enjoyed during the daytime. How much heat loss are we talking
about? For example, during an 18-hour winter night at zero
degrees, following a 6-hour typical sunny clear winter day,
the night-time heat that migrates through double glass windows
is about half of the daytime solar gain. That leaves only
the other half to supply the night-time heat loss through
the roof, the other three walls, the other windows, doors,
and the cold air infiltration.
In order to get a better performance balance between house
heat loss and solar gain, window coverings are a pretty good
investment. By deploying night insulation on those same south
windows, the heat lost on long winter nights can be cut in
half, thus greatly increasing the comfort factor for the occupants.
Drafts circulating from cool air moving down the windows and
into the room are reduced; warmer surface areas are provided
for your body to feel and radiate to.
The selection of window coverings is governed by many factors
such as appearance, type, color, opacity, cost, convenience,
and insulating effectiveness. In talking with Leslie Roark,
a local installer of window coverings, he told me customers
have generally been interested first in privacy, then comfort,
then R-value, in that order. However, with heating costs rising
at future unpredictable rates, this order may soon be reversed.
Leslie’s company, Window Expressions, is one of three
locally that advertise in the Crestone Eagle. The other two
are Rock Ridgeway (It’s Curtains for Civilization) and
Kai Beetch (Kai Beetch Design). The prices for custom-made
window insulation coverings appear to range from $8-$16 per
square foot installed.
There are many types and styles of window coverings that
are effective for night insulation. One of the earliest to
be developed was Beadwall™, which is styrofoam beads
blown in at night between two panes of glass (3-4 inches apart)
to give a very effective R10 or more. Window Quilt™
is another type, with reflective mylar embedded inside fiber
batting between two decorative cloth coverings which roll
up into a valence. They have edge seals provided by plastic
tracks adhered to the window frame or trim. Warm Window™,
a Roman shade folding style with magnetic edge seals, is another
type that is available. These two might be best for old houses
with leaky windows because of the positive edge seals. Another
popular, attractive, versatile, and effective style is the
Duette™, a honeycomb shade with unique cellular construction
that provides good insulation, but no edge seal. Some of these,
including styrofoam panels placed against windows, can be
home-made to fit one’s needs, taste and budget.
The resistance to heat flow, or R-value, for these window
coverings ranges from R2.5-R4.3 for most varieties. Given
that the R-value for a plain double glass window is equal
to R1.8 at best, comfort can be improved and the nighttime
heat loss in winter cut nearly in half by installing quality
insulating window coverings. The bottom seal is most important
because it stops cold air falling down the cold glass and
into the room.
Some improvements for homes that save fossil fuel energy
may qualify for Federal renewable energy tax credits. A tax
credit, for example, means 30% of the money spent on solar
and conservation improvements for your home can be subtracted
from your income tax bill. Hidden subsidies for oil and gas
have traditionally made these fuels artificially cheap to
buy. A solar/conservation tax credit is designed to level
the playing field for saving home energy. To learn more about
passive solar home design, the new tax credits, and other
conservation options visit http://www.crestonesolarschool.com
and view the informative articles posted there.
to the Eagle!