Windows have the lowest insulation value of any element of the building envelope. They also permit radiant energy to enter a space, as opposed to the conduction that occurs through the walls and roof. Thermal radiation accounts for a large portion of the energy in natural sunlight. Closing curtains or blinds reduces the radiant contribution of the light and also reduces the conduction of thermal energy through the windows. An air space is created between the blinds and the windows that, since air is an outstanding insulator, increases the resistance to heat flow. Double- pane windows take advantage of this same premise by trapping an air pocket between the panes. This enhances thermal performance. In the heating season, the opposite applies. Radiant energy entering through the windows augments the space-heating system.
Double-paned windows with blinds or shutters are ideal for all seasons. The insulation is needed to retard the heat flow out in the winter, and vice versa in the summer. The blinds can be used to either block or enhance the flow of solar radiation for use as daylighting.
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Excerpt from Energy Conservation Projects.