Window Standards For Energy Savings

Window and window standards are an important factor when designing a building. Using windows with natural light can reduce the lighting need and reduce the amount of energy needed to light a building. Natural light can also be aesthetically pleasing for workers rather than using nasty fluorescent lighting. The other factor that can be considered for the placement of windows is for heating and cooling factors. The heating and cooling factors can work for and against the lighting factors. In hot areas, the windows should let natural light in during the day, but should block out heat from entering as well. In cooler climates both the heat and the light should be let in.

In South Korea, is estimated that 24% of the total national energy consumption (1.62 quads) goes into the building sector. It is then estimated that around 4% of the total energy consumption goes to dealing with the regulation of heat transfer from residential building windows.[1] This is not an insignificant amount of energy that is required to deal with the direct effects from sunlight penetration into both residential and commercial buildings. In the United States, there are standards for windows put in place by the International Energy Conservation Code (IECC). Generally, jurisdictions from around the country will adopt these standards to use within the local area for both residential and commercial building types. These model codes are especially helpful to set standards in different climate regions to give baselines for people to install their windows.[2]

There are three sets of standards which the codes are set within different climate zones. Prior to 2006, there were 19 climate zones which made it unwieldy for manufacturers to follow. With the 2006 IECC guideline release, the U.S. has been cut down to 8 zones. The climate zone is based on humidity and longitude with those closer to the equator having a lower number. Here in Texas we vary from zone 4 in the north, zone 3 in the middle and western parts of the state and zone 2 with the eastern and southern parts of the state.

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Between the climate areas, there are varying degrees of standards for the windows which dictate different types of windows which should be used in places with types of humidity and average sun intensity. The three factors that are used are the: fenestration U-factor, Skylight U-factor, and Glazed fenestration Solar Heat Gain Coefficient (SHGC). These factors are on different scales which indicate where on the continuum a window should fall within in a particular zone. As each window used in a specific zone must conform to these sets of standards, there are different iterations of the windows that are manufactured.[4]

As mentioned before, Texas has three window zones. The two following charts and explanations from the IECC illustrate the scale of the factors for Texas.

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The following is definitions for the different factors[6]:

Fenestration: is a vertical opening of a building like a window or door

Skylight: is a roof window that gives light or ventilation

U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. Generally given on a scale from 0.2 to 1.20. It is expressed in units of Btu/hr-sq ft-°F (US) or W/sq m-°K (European metric). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value

Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. y. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability

In Travis county, which is in zone 2, we have a high U-factor for the horizontal and vertical openings. The vertical is higher than the horizontal due to the sun being directly over vertical openings for a longer time period than horizontal openings. The SGHC is also highest of the zone due to the need for shading ability of both the horizontal and vertical building openings. I could not find cost data, but I imagine the marginal cost of the windows in the extreme climates, both hot and cold are more expensive than those in the more moderate climate zones.

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