Doors and Windows – There are two parts to dealing with doors and windows. The first part is that windows and doors often have an insulation value. The second part is that doors and windows that face the sun can let heat into the building. In the summer, letting sun into the building can cause the building to overheat and require cooling, unless the window has the correct shading. In the winter, allowing the sun to enter the building can reduce the heating loads.
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