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As part of the building envelope, energy efficient windows play a key role in designing a passive home. Cheap and inefficient windows will create a large thermal bridge that will negate all efforts to insulate the rest of the home; therefore, it is important to carefully choose the right windows.

Windows are rated by three main factors: U-factor, visual transmittance (VT), and solar heat gain coefficient (SHGC). The U-factor is the rate of heat flow through the window. Lower U values correspond to better insulation. VT measures the fraction of visible light that passes though the window. A high VT maximizes daylight. SHGC is the fraction of incident radiation transmitted though the glazing/window. In a hot climate where cooling is more important than heating, a low SHGC is desired. For Washington, D.C., the DOE's EERE recommends: U-factor < 0.35, SHGC < 0.35, and a high VT.

I found many companies producing highly efficient windows that implement double and triple panes and are insulated with inert, transparent gases such as argon and krypton. The most energy efficient windows use a triple pane design insulated with krypton gas. Finding accurate cost estimates is not simple as companies give prices by quoting individual designs. A high-end 48"x48" Anderson window will roughly cost $500.

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