WindowsEnergy
Savings Opportunities - Investment
Add Storm Windows
Many older homes have single glaze
windows. Older windows are notorious for air leaks and drafts around
their not-so-well sealed sashes. Cold air flows in as heat transfers
out. Although storm windows may be somewhat less desirable,
aesthetically, than window
replacements, they will provide good energy performance for a much
lower cost.
Storm windows are basically a second
window that is installed on the outside of your existing windows. They can
be raised and lowered to allow air passage and usually have screens
that slide into place. By adding a second layer of glass, an
insulating air pocket is formed between the storm window and the
original window. The seals around the storm window will eliminate heat
loss from drafting.
Storm windows are available with
tints and low E coatings, which provide an overall thermal performance
that is reasonably close to the newest double glazed windows.
The cost of a storm window naturally
varies by size and performance level. Contractors
that you contact should be able to provide documentation on the
performance of the windows they propose to install, as well as the
cost of the installation.
Window performance will
probably be specified as a U value. U value is the inverse of the R
value used for insulation. The higher the R value the better, so the
lower the U value the better.
If you do not have the funds
available to purchase and install storm windows, you can always
purchase and install plastic sheeting on the inside of each window
during the heating season. Sold as kits in hardware stores and
home improvement centers, a thin, clear, plastic sheet is installed
around the interior side of the window frame using a hairdryer to
shrink the sheet into a tight surface. If installed properly, the
created air pocket will slow heat transfer and should all but
eliminate air infiltration.
Replace your Existing Windows
In general, replacing your existing
windows with newer, higher performance models will not be cost
justified on energy savings alone, unless your windows are in poor
condition and/or you live in a very cold part of the country. As a
homeowner, an investor in your own property, you would like to obtain
as quick a payback as possible on the money you spend for this
improvement. Replacing all the windows in your house will likely run
into thousands of dollars, while your annual savings are not likely to
exceed a couple of hundred dollars.
If you have decided to replace your
windows for reasons other than future savings potential, you will
likely be justified in spending more for higher performance windows.
By contacting three contractors for quotes, you can establish a fair
price for several performance grades of windows and their installation
(See Selecting a Contractor). Each contractor should be able to
provide performance data on the windows he proposes to install. Some
will be able to perform calculations on the expected energy savings.
Dividing the difference in cost
between any two windows grades by their respective difference in
annual energy savings will yield the number of years to obtain a
payback on your investment.
(Cost of higher performance window -
Cost of lower performance window) / (Annual energy savings for higher
performance window - Annual energy savings for lower performance
window) = No. of years to recover investment dollars (Payback)
The table below lists the equivalent
return on investment for each number of payback years:
Payback
Years
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
Rate
of Return*
|
100%
|
41.4%
|
26.0%
|
18.9%
|
14.9%
|
12.2%
|
10.4%
|
9.1%
|
Compare the expected rate of return
with your other investment options (savings account, money market,
stocks, bonds). If the window payback is four years, the return is
18.9%. Money market is yielding 1.5%, stocks zero, bonds 6%.
Therefore, buying the higher performance window is a good investment,
provided you are planning to live in your house more than four years.
If you are planning to move within four years, you may be able to
recoup your money by getting a higher sale price on your house with
the window improvement. The National Board of Realtors has a website
page that lists the expected impact of various home improvements on
home value.
One final consideration: if you
borrow money to pay for the windows, subject the annual interest costs
from the energy savings costs when you compute the payback. Financing
will diminish the rate of return, but at today's rates, not by much.
To get maximum energy savings for
your investment, shop for windows with the lowest U value and air
leakage. If you live in a part of the US where heating expense exceeds
cooling expense, look for lower Solar Heat Gain Coefficients.
Elsewhere, look for higher SHGC.
Shopping Checklist:
- U value less than or equal to
0.38
- SHGC < 0.39 for heating dominant
climate, > 0.71 for cooling dominant climate
- Visible Light Transmittance (VT)
greater than 0.5
- Air Leakage < 0.3 CFM/Ft
Install Thermal Drapes/Blinds
When drawn during summer afternoons
and winter nights, insulated thermal drapes shield the inside
of the house will minimize both heat transfer and drafts, if any air
leakage is present.
The photo below shows the double air
pocket configuration of insulating blinds. Each pocket of air
adds resistance to heat flow in either direction.
Enclose the Open Area Above the
Drapes
Regardless of the R value of the
drapes, if room air can flow freely between drape and the window from
the bottom of the drape to the top, then a convection current will be
present. In the winter, as the air between the drape and window cools,
it "falls". The falling action draws in warm room air from
above and the cycle repeats. If the area around the top of the drapes
is enclosed to prevent the passage of air, the convection cycle is
broken.
Install Awnings
For windows facing south or west,
window awnings will act as a screen from direct sunlight, resulting in
less solar heat gain inside the house. Window awnings were quite
popular before residential air conditioning became commonplace.
Add Landscaping
By planting trees in strategic
locations, much of the sun's radiant heat, both direct and reflected,
can be reduced significantly. Trees also block and diffuse strong
winds that would otherwise cause a greater heat loss off of the
windows. As a bonus, trees also shade the walls and roof of the house,
providing relief to the air conditioner during the summer.
For new construction:
In heating predominant climates,
position more window area on the south side of the house and use
windows that have the following characteristics:
U value of 0.35 or less
SHGC of 0.60
or more
VT of .70 for glass only or 0.50 for whole window
Air Leakage
of 0.30 or less
Plant trees or install berms on the
north and west sides of the house to block the prevailing winds. To
counteract the effect of the high SHGC during the cooling season, use
overhangs to block the sun's rays on the south facing side of the
house and broadleaf shade trees on the east and west sides.
In cooling predominant climates,
position more window area on the north and east sides of the house.
Use windows that have the following charactistics:
U value of 0.6 or less
SHGC of 0.40
or less
VT of .70 for glass only or 0.50 for whole window
Air Leakage
of 0.30 or less
Use overhangs and/or shade trees to
minimize the windows exposure to the sun year round.
In split heating/cooling climates,
position more window area on the south and east sides of the house.
Use windows that have the following charactistics:
U value of 0.4 or less
SHGC of 0.40
or less if A/C costs are significant, 0.55 or less if A/C costs are
low
VT of .70 for glass only or 0.50 for whole window
Air Leakage of
0.30 or less
In addition to selecting the high
performance windows, further energy savings are possible with the
design of the house. Roof overhangs can be designed into new
construction to minimize sun exposure during the cooling season. In
the summertime, the sun is high in the southern sky in the middle of
the day. With a sufficiently designed overhand on the southern side of
the house, the sun's ray will be blocked from the south facing windows
throughout the day, which vastly reduces the solar heating load.
By planting broadleaf trees on the
east and west sides of the house, the sun's radiation during the
morning and afternoon can be substantially blocked during the summer
months. During the winter months, with the sun low in the southern
sky, the direct sunlight on the south facing windows will help to heat
the house. With the leaves off of the broadleaf trees during the
heating season, the sun has a clear shot at the east and west facing
windows in the morning and afternoon.
Next:
Further
Reference
Copyright © 2003 HEM Technologies, LLC. All rights reserved.
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