The selection process for your windows is just as important as selecting your carpet.  Years ago there was only clear glass or an option of a gray or bronze tinting (which really did nothing but add shading to the window). The first option we will look at is the glass.  Next we will ask what you want the frame of the window to be. 

Let's look at our options now!

Glazings: 
In the past, a single pane of glass was considered sufficient to keep the elements out of the home.   Now this is not energy efficient nor smart to have in your home.  Condensation builds up in the winter and summer on the inside causing damage to paint and wood.  The “R”, “U” and “SHGC” ratings are not acceptable.  There are some homes still being built with single glazing in aluminum and wood windows just to save money up front. 

Clear dual paned insulated glass created a dead air space between the two pieces of glass that was supposed to be the answer to the non efficient single glazing.   It works better than a single piece of glass, but still does not stop the heat gain nor hold the cold outside.

Tinting windows with a reflective film helps a little.  But, this is not the most economical route to go.

Gray or Bronze tinted glass will shade the sunlight coming through the window, but will not stop the heat or cold.

Low Emissivity Coatings are the science of today.  Low-E glass has a small initial investment, but pays itself back with reduced energy costs.

Texas is adopting new energy codes to reduce emissions from the power plants by having builders install energy efficient windows in new homes.  The idea is to reduce the Solar Heat Gain Co-efficient factor through the glass by applying Low-E (Low-emittance (Low-E) coating.  Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of low-E coating is transparent to the solar spectrum (visible light and short-wave infrared radiation) and reflective of long-wave infrared radiation. 

The level of SHGC is to be at .40 or less.   The center of glass on a Soft Coat Low-E glazing is at .41 .  The less the number the better. 

Summer Thermal Comfort
In summer, strong direct sunlight strikes people and interior surfaces, creating overheating and discomfort. Windows with low solar heat gain coefficients will reduce the solar radiation coming through the glass and associated discomfort. Low solar heat gain low-E glass (spectrally selective) reduces heat gain while still providing sufficient light and view.

Note:  In Texas, the addition of Argon gas does not affect the thermal efficiency by more than a .01 factor.

Go to www.efficientwindows.org/factsheets/texas.pdf for Texas energy cost comparisons.

Frames:     

After adding the Low-E the frame comes into the equation. 

A standard aluminum, non thermally broken, window with clear insulated glass has a SHGC of .68 .   With a factor of .68, this does not make an efficient window at all.  To make this a better window, add Low-E coating to the glazing, this will bring the SHGC to .38 .  To get better SHGC factors and more energy savings, you now go to more efficient frames.  Refer to the chart below for comparisons.  (Chart is from www.efficientwindows.org)

                                                    Whole Window Properties - Double-Glazed with Low-Solar-Gain Low-E Glass

Frame	Aluminum	Aluminum with Thermal Break	Wood	Wood Clad
U-Factor	0.60	0.48	0.32	0.32
SHGC	0.38	0.34	0.30	0.30
VT	0.57	0.53	0.50	0.50
Note: The thermal performance properties of specific glazings and frames can vary depending on product design and materials. The results presented here are averages. Consult specific manufacturers for NFRC rated U-factors and SHGCs for products of interest.

Aluminum frame:    Used mainly in the south.
Non-Thermally broken frames have been the most common.  The problem with this frame is that heat and cold are transferred to the inside of the home.  This frame is rated at the bottom of the energy efficiency list.  This is considered the most economical up front, but not long term option.

Thermally Broken aluminum frames are a better choice for keeping heat and cold outside where it belongs.    The most common solution to the heat conduction problem of aluminum frames is to provide a "thermal break" by splitting the frame components into interior and exterior pieces and use a less conductive material to join them.  This is rated second best in the energy efficiency list.  The cost factor is greater, but payback in energy savings counters that cost.

Vinyl frames:
Vinyl has come a long way in its technological development over the years.  Vinyl Plastics are relative newcomers as window frame materials in North America.   Vinyl, also known as polyvinyl chloride (PVC), is a very versatile plastic with good insulating value. Vinyl window frames do not require painting and have good moisture resistance.  Because the color goes all the way through, there is no finish coat that can be damaged or deteriorate over time -- the surface is therefore maintenance-free. Recent advances have improved dimensional stability and resistance to degradation from sunlight and temperature extremes.  Molding into custom shapes is an option easily obtainable today.  With the rigity and stability of vinyl today, the frames can take on a wood appearance.

 
In terms of thermal performance, vinyl frames are comparable with wood, while there are minor differences, depending on the frame construction. Small hollow chambers within the frame reduces convection exchange.  In terms of price, vinyl is about the same as thermally broken aluminum and less than wood.

Wood Frames: 
For many years the only  window frame material was wood, because of its availability and ease of milling into the complex shapes required to make windows.  Wood is favored in many residential applications because of its appearance and traditional place in house design. From a thermal point of view, wood-framed windows perform well with frame U-factors in the range of 0.3 to 0.5 Btu/hr-sq. ft-°F. Wood is not the most durable window frame material, because of its susceptibility to rot, but a well-built and well-maintained wood window can have a very long life. Paint protects the exterior surface and allows an easy change in color schemes.

Some wood windows have an option of being aluminum clad on the exterior to prolong the longevity of the window.

For more information on dealers with energy efficient windows, please write chris@remodelingshow.net.