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   CHOOSING AN ENERGY EFFICIENT WINDOW
 
 
 
2011 - Volume 1 Issue 2
Casas Bonitas
New Home Construction
 
Article: Bob Skolnick
 
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You can directly influence the windows by type and quantity to be ordered for your home by taking a few considerations in the design phase. The south and west walls of the house receive the most intense sunlight and the amount of window square footage should be reduced on that side of the house. Planning your roofline with an overhang that provides shade for the windows during part of the day, particularly on the south side, will reduce direct sun radiant energy. Awnings, whether fixed or retractable, also help to reduce the sun’s energy on your windows.

    "For energy efficiency,
the entire envelope must
be considered – a cheap
window may defeat the
purpose of a well-insulated wall."


Jeffery Huff,
The Design Alliance
   
  There are several types of window frames, each with its own characteristics:

Wood Frame windows are traditional and very attractive when stained to accentuate their wood grains. They perform well with U-factors ranging from .3 to .5. They do require annual staining or painting to keep them weather protected. There are also new thermally improved wood frames that have thicker frame designs and low conductance spacers.

Wood Clad Frame windows are a variation of wood framed windows with an exterior face of vinyl or aluminum. This creates a permanent weather-resistant exterior surface while retaining the attractive wood finish on the window interior. This exterior cladding eliminates regular maintenance. They are now available in a selection of exterior colors.

Vinyl Frame windows can have a variety of colors and a wood grained texture. They are very durable and maintenance free. They are resistant to degradation by sunlight. Their U-value is similar to wood frame windows.

Fiberglass Frame windows are dimensionally stable and have cavities similar to vinyl windows. Because fiberglass is stronger than vinyl, these frames can have smaller cross-section cavities and are very good for holding heavier triple-glazing. Unlike vinyl, which is colored throughout the vinyl material, fiberglass can be painted with virtually any color. The have an excellent U-factor.

Aluminum Frame windows are light, strong and durable. There are two types of aluminum, rolled and extruded. The extruded style is a much better product. They are available in anodized or factory baked enamel finishes in several colors. They are however, very heat conducive and have a high U-factor. This can be offset by ordering your aluminum windows with a thermal break, which acts as a thermal barrier. The thermal break splits the frame components into interior and exterior pieces and uses a less conductive material to join them.
   
  Resources

Artistic Entryways
& Millwork Co.

1165 McNutt Rd.
Sunland Park, NM
575-589-0281
artisticentryways.com

Rawson Builders Supply
12277 Rojas Dr.
El Paso, TX
915-791-0100
----------------
2355 Nevada Ave.
Las Cruces, NM
575-524-3568
rawson-inc.com

Subcon
770 West Palms
Las Cruces, NM
575-525-0737
subconsupply.com

Window Concepts
7355 Remcon Circle
El Paso, TX
915-584-0266
windowconcepts.com
Most communities have building energy codes that mandate minimum performance for windows. We are in the South Central Zone 3. The climate zone determines the U-Factor and Solar Heat Gain Coefficient (SHGC) to meet minimum codes. This criteria is set to qualify your windows for an Energy Star rating. Meeting or exceeding Energy Star ratings for your windows will improve your overall house energy efficiency, which we defined in our preceding Energy Efficiency article.

The walls of your home are insulated and achieve an R-value through insulation to repel and block heat and cold from penetrating into the interior of your home. Windows by nature let light into the interior and have a solar heat gain, which allows the sun’s energy to heat the interior air and surfaces of your home. Next to your roof system, windows can contribute to up to fifty percent of the heat gain in a home. The heat gain is measured in a Solar Heat Gain Coefficient (SHGC), which is a measure of how well a window absorbs or reflects heat from the sun. In this climate zone, the SHGC requirement is equal to or less than a 0.30 rating.

Windows are assigned a U-factor. This rating measures the ability of glass as it allows BTUs to pass through the glass. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value. In our South Central Climate Zone 3, windows should have a U-factor rating of less than 0.35.

The glass in your windows is equally important and today double panes of glass are standard. The space in between the two panes of glass acts as a thermal barrier and how the glass is treated determines the emissivity. Typically there is a microscopic breather tube placed through the frame into the air barrier to allow for the differences in barometric pressure from the factory elevation to the elevation of the home where the windows are to be installed.

In this region in order to meet energy tax credit requirements and receive an Energy Star certification, you must use low-E glazed glass. Low-E glass has a coating that is designed to reduce the amount of radiant heat that can be transferred through the glass. Low-E coatings or films have the ability to re-radiate a high percentage of heat back toward its source. In summer, low-E windows can be effective in reducing the amount of solar gain in the home. In winter they can reduce the amount of furnace-generated heat that can be lost to the outdoors. There are several levels of low-E coatings and which side of which pane of glass they are applied to affects the outcome. low-E glass can have a rating ranging from R-3 to R-10 depending on its properties. low-E2 glasses, also called solar low-E, or spectrally selective glass, are crucial in hot climates. They block unwanted solar gains. In hot, or even in temperate climates, tinted glasses are an added option. They also provide lower solar heat gains. However, they aren't as efficient as low-E2 glasses and yield a decreased outdoor visibility, particularly at night. All treatments to your glass must be made at the factory to maintain your warranty.

The thickness and strength of the glass must be considered. Most windows come with one-eighth inch glass and can be upgraded to a thicker glass. This is often done when the window surface area is large and subject to high winds. You can also order your windows with tempered or safety glass. This provides for more durability and if broken crumbles rather than shatters. Finally, there is also a surface where a spot free film can be applied at the glass factory to allow dirt and water to come off the glass whether rinsed or through rain. There will be no spotting. This truly makes for a low maintenance window.

The installation of the window has a material impact on the window’s performance. Your windows should be installed by a factory-trained representative from the local wholesaler representing the window manufacturer. Proper installation will eliminate water leakage, which may appear in the walls. Installed correctly, windows will be an integral part of the homes energy envelope preventing air intrusion or escape. Your heating and cooling system was sized and designed on the premise that when windows are closed they are airtight. Properly installed windows must be level and also square. Squaring not only should be measured at the window frame corners, but also diagonally from top to bottom corners. Failure to have a squared installation can affect the functioning of the window when opening and closing. Finally, windows must be sealed to the exterior wall system with a barrier (flashing, tape, caulking or foam) to make sure water and air do not enter through the edge of the window frame. You can test the tightness of your windows by using a candle around the edges and openings on a windy day and see if the flame is moved by air intrusion.

Windows have many technical elements, consult a window professional before making a decision on types of frames, glass and glass coatings. ///
 
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