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The fundamental purpose of a door is to provide access into or out of a building. Doors also serve for providing access into or out of various compartments of a building. Doors also have the following functions. They provide security, weather resistance, fire-resistance, thermal insulation, sound insulation. As to exterior doors, they must be well-designed and properly constructed. They must not leak air, wind, heat and water.
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The fundamental purpose of a door is …
to provide sound insulation
to provide thermal insulation
to leak air, wind, heat and water
to provide access into or out of a building