Many stock wood doors are a sandwich of wood-veneer skins over an engineered-wood core. This configuration minimizes the expansion and contraction that cause warping. At about $200 or so to start, they're a low-cost alternative to solid-wood doors. Look for tough, furniture-grade veneers at least 1/16 in. thick; anything thinner damages too easily.

Companies such as Lamson-Taylor, Pella and Simpson discourage bowing and warping by laminating two pieces of wood to create the stiles and rails. Split construction is also used for door panels, but they have an insulation core. The result is a wood door with an insulation value of about R-5 compared with R-2 for conventional versions. These doors cost about $300 to $500.

Solid-wood doors cost the most. A 3-ft.-wide 5 6-ft., 8-in.-high, six-panel pine door runs at least $600, while hardwood doors are even more expensive. Figure on about $2,000 to $4,000 for a complete system that includes a prehung door in its frame, hinges, locksets, sidelights and weatherstripping.

When shopping for prefinished wood doors, look for durable stains and clear finishes, such as polyurethane. High-gloss sheens offer the best protection for painted doors. Whichever finish you choose, apply it to the top and bottom edges. This helps prevent a wood door from absorbing moisture and swelling.

Also look for careful detailing. As a rule, the more intricate the carvings and moldings, and the thicker and wider the stiles and rails, the better the door. The same goes for panel thickness. For example, the high-end doors from Nord have 13/8-in. panels compared with the 9/16- and 3/4-in. panels on low-end models.