It is important to include the glass doors of homes when calculating the exposed window area of exterior walls.

Estimate the % of glass that covers each exterior wall. Include sliding glass doors and even entry doors that have all or partial glass. If you have more than one windows type see

Multiple Window Systems

It's important to include the overhang depth of (roof, patio covers, etc) when modeling for energy efficiency to get an estimate of the solar gain of those windows/doors. For example if a west wall has a 6-foot patio cover that shades most of the windows on that wall, the solar gain will be reduced, however, if there is little overhang on the west wall (and little shade from trees) it may be a great source of solar gain which the homeowner should consider a remedy.

After entering the percent of wall to window area on the input screen, the modeling will calculate the total sq ft of glass for each wall.

If you are improving all of the windows in an orientation, the base and the improved areas will be the same. If you are only improving some of the windows, you may put a smaller number in the improved side that corresponds with the area of windows you're actually replacing on that orientation. If you are actually adding new windows to an orientation where there were none previously, the improved sqft will show a larger number than on the base side.

In the following example, 82 sqft of windows will be replaced on the North side, and 38 sqft of windows will remain the same. On the East side, all windows will remain the same. For the South side, all 120 sqft of windows are being replaced and an additional 60 sqft of windows will be added to the home. On the West side, all windows will be replaced.

Additional Notes

The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-oF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.

Solar heat gain coefficient (SHGC)
is the fraction of solar radiation admitted through a window, door, or skylight -- either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation, and external shading will determine the optimal SHGC for a particular window, door, or skylight.

The rate of air movement around a window, door, or skylight in the presence of a specific pressure difference across it. It's expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.

Sliding glass doors have a looser seal than swinging doors that allows air to leak around the weather stripping of the door. The weather stripping deteriorates as the door ages, although some models of sliding glass doors allow you to replace the weather stripping. Replacement preference should be given to hinged or swinging doors. This type of door is far more energy efficient than gliding or sliding doors.

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