Steel doors also cost the least: Prices start at about $150 for a 3-ft.-wide 5 6-ft., 8-in.-tall paneled door without hardware or glazing. A steel-door system with sidelights and premium hardware can nearly equal the cost of a wood-door system, however.

All steel doors have an inner frame made of wood or, for greater strength, steel. The cavities within the frame are filled with high-density foam insulation. Premium doors typically have a 24-ga. skin and a steel frame, though some offer heavier-gauge steel (represented by a lower number). The surface usually is smooth or has an embossed wood-grain pattern.

Most steel doors are coated with a baked-on polyester finish that requires periodic repainting. Premium versions get a vinyl coating similar to the one on vinyl-clad windows for greater weather resistance. Some even have a stainable wood-fiber coating or, on really high-end versions, a laminated-wood veneer.

Steel doors usually are part of a prehung system. But if you're simply lifting the old door off its hinges and hanging a new one, remember that steel doors come with hinges attached or holes for the hinges predrilled. The hinge area on the door must match the hinge area on the existing door frame. Some doors come with an extra predrilled hole for the hinges, which allows minor adjustments to be made when hanging the door.

Also, if you choose an embossed wood grain, make sure it runs horizontally on the rails and vertically on the stiles. Finally, check the warranty. Some manufacturers will void it if you install an aluminum storm door with the steel door. The reason: Heat buildup between the doors might cause the finish to peel.
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