There have been many recent improvements and developments when it comes to windows,
making it challenging to keep up with all the newest terms and technology. There
are not only multi-pane windows, but also windows with coatings in between. Depending
on climate, consumers can now choose windows that allow more or less heat or light
waves to enter. With all these options, it can be difficult to make the best choice.
It is important to choose windows carefully and keep open-able windows to a minimum.
Even when closed, open-able windows are more likely to have air leaks. Use
only as many open-able windows as you really need for ventilation and comfort.
Types of Windows
Energy Efficient Window Design
Air Infiltration. (leakage) between the sash and the frame, depends
on the type of window and the quality of construction. Air leakage should be less
then 0.30 cfm/ft². It is important to seal between the window and rough opening.
It would be counterproductive to have an expensive leak-free window installed
that leaks around the outside. Use caulk or foam before installing window trim.
Size of Windows and Installation. Windows generally lose more heat
around the edges. In other words, everywhere the glass is fitted against a material
(a sash), there is a potential for leaking. Therefore, the objective is to reduce
the number of edges. This is most easily done by installing one large window instead
of a number of small windows. If you want the divided light look, your best bet
is to purchase windows with an applied grill. (More on window size in the chapter
Solar Heat Gain Coefficient (SHGC) is an important figure for windows
in a solar home. It measures how well a window blocks heat from sunlight. It is
the fraction of the heat from the sun that enters through a window, expressed
as a number between 0 and 1. The lower a window's SHGC, the less solar heat it
Blocking Ultraviolet Radiation indicates how well the window protects
carpets, drapes, and furniture (although visible light also 'fades fabric). Most
windows reduce UV penetration by 75%.
Inert Gases (argon or krypton) between the layers of glass in multi-paned
windows conduct heat less than air, making the window more efficient. All multi-pane
windows leak, but usually not more than 10-15% over 25 years.
Low-e coating. (low -emissivity) glass has coatings of clear silver
or tin oxide on the glass. It allows light to pass through a window but blocks
infrared radiation (heat). However, it reduces the transmission of sunlight and,
therefore, decreases solar gain. There are 2 types of coatings: Soft Coating
and Hard Coating
The soft coating is layers of silver oxide and anti-reflective
coatings applied to the inside surface of one of the panes of glass.
In hot climates, low-e coatings should be applied to the inner surface of the
outer pane, blocking the warm outside air. In cold climates, low-e coatings should
be applied to the interior surface of the inner pane, to keep the heat inside
a home. Emissivity of a window can increases the R-value nearly 60 percent.
The hard-coating is a thin layer of tin oxide incorporated
into the surface of a pane of glass during manufacturing, which makes these
coatings more durable than soft coats. However, they are not as effective as soft
coats. There are frequently new types of coating available.
Polyester Films. treated with a low-e coating, thin transparent
polyester films applied between layers of glass or just applied to the surface.
Window Specifications. Low-e windows are a must for passive solar
homes in all climates; however, specifications vary, not only according to the
climate, but also the location of the window regarding north, south, east, and
west area of the house. This will be described in detail in the sections "Window
Location" and "Glass to Mass."
Warm Edge, the non-conductive spacer between the 2 window panes,
is an important part in a window that can prevent a lot of damage. Insulating
the inside and outside frame prevents damaging condensation. It also improves
the efficiency of a window by 10 %. Condensation is a main culprit in damage to
a home. Moisture can do tremendous harm to a home (mold, rotting of material,
etc.) as described in all chapters.
Energy-Efficient Window Sashes and Frames.
Weather Stripping results in more airtight and thus more energy-efficient
windows. Hinged windows are tighter than sliding-sash windows. However, you may
find that double-hung windows from some manufacturers are tighter than casement
windows from others. It is best to check and compare.
Materials Used to Build Window Sashes.
Wood windows are (still) the number-one sellers in the U S. Wood is a renewable
resource, and it has a "warm feel." It's a much poorer conductor of heat than
metal and, therefore, fairly energy efficient. To prevent weather damage to
wood windows, metal, vinyl, or plastic is applied to the outside of the window
Metal sashes, being a good conductor, conduct the heat (or cold) out of
a house; unless inserted with "warm edges" metal, window frames are a poor choice.
Fiberglass windows are extremely strong and have a thermal expansion
coefficient close to glass. Fiberglass may become the window frame of
the future. Fiberglass windows are not as widely available and are more
expensive ; they must also be painted..
u- Value: Heat Loss is the opposite of R-value (U-value = 1/R).
U-value is the heat transmission through a material.
U-values of smaller than 0.3 are required for passively conditioned homes.
Additional Notes about Windows:
-Even high-quality multi-pane windows lose tremendous amounts of heat at night.
Cover south-facing (and other) windows at night, especially in houses that have
huge glass "walls," which are nearly impossible to cover with window shades. Cover
(especially southfacing) windows with insulated drapes or shades at night. Good
window coverings can easily add another R-3 to R-4 to windows. Anything that helps
block the flow of cold air from windows will be well worth the expense. Since
shutters require being closed from the outside every night, more convenient are
the (inside) rigid foam thermo-shutters, made from plywood and rigid foam covered
-For clearstory windows, use the most energy-efficient windows. Consider motor-driven
window shades to insulated clearstories at night.
-Payback for windows is usually between two and ten years. But also consider also
smaller, less expensive heating and cooling systems.
-High-quality windows may last longer and require less maintenance, saving time,
money, and hassle.