You can't judge a door by its cover, unless you know what to look for. Generally speaking, we expect traditional doors to be made of wood. But even when they are, what's inside may be another thing altogether. To save costs and improve performance, manufacturers now use fiberglass, pressed wood composites and steel to turn out doors with traditional details.

If you plan to paint your doors, you may not really care what's inside. Stain-grade wood doors come in solid wood, or more commonly, have a good quality veneer. Door height is typically 80 in. and most come in standard widths including 24, 30, 32 and 36-in.

If you're after something different or a unique size, special orders for doors are an option, and take a week or more to fill. If you're trying to closely match existing doors in your house, expect to spend time shopping-and possibly waiting.

Interior Doors:

When it's time to pick out new interior doors, you should be asking, "What's inside?" You'll find two basic types of doors: solid core and hollow core.

The solid core doors have either dense particleboard or solid wood throughout. This mass makes the door sturdier, feel better to the hand and significantly reduces sound transfer between rooms. Hollow core doors were developed to produce cost savings. These doors have a rigid core, and are commonly smooth-faced flush doors, but also may look like traditional frame and panel doors.

• Frame and Panel Doors

  These traditional doors have from 1 up to 9 vertical panels, divided by rails. The panels are typically either flat plywood or raised solid wood. Stain-grade doors cost more than paint-grade, which may have filled imperfections and mismatched grain colors.

  Shopping for lower-budget doors of this kind will yield a paint-grade, one-panel door, with a flat, composite panel. They'll probably come with a standard issue handle set. In the medium price range are four-panel, stain-grade fir doors with flat panels and better, plated lever handles. Higher budget doors can be of hardwood, stain-grade with six raised panels and a solid brass handle set.

   

• Flush doors

  These are flat, smooth doors, with wood or composite veneers over a grid core of particleboard or cardboard. The low-cost versions are meant to be painted. Better flush doors feature stain-grade maple, birch, oak or imported hardwoods.

  If you go low budget, you'll find paint-grade veneers and low-end handle sets. Medium-range budgets afford stain-grade birch with better lever handle sets. And with a high budget, you'll be able to find stain grade oak veneer doors with solid brass handle sets.

   

• Bi-fold doors

  These are a great choice when you want full access to a closet, but don't have room for a traditional door to swing open. They provide good cover for a wet bar, built-in storage and laundry closets. Two configurations are most common, louvers and panels. Louvers should be spray-finished for best results.

  When you shop at the low-end for bi-fold doors, you find 3/4-in thick, paint- grade louvered doors. In the middle price range, thickness jumps to 1 1/8-in. thick stain-grade louvered doors. And a top-of-the-line budget can afford 1 1/8-in thick stain-grade pine panel doors.

   

• Pocket doors

    Now you see it, now you don't. Pocket doors disappear into the adjacent wall cavity, requiring no clear floor space for a door swing. Typical doors need up to 10 sq. feet to swing open.

    With the right hardware, almost any door style can be hung in a pocket, not just inexpensive, hollow-core doors. The best fittings are adjustable three-wheeled hangers running in a C-shaped, overhead track. To install a pocket door, you must open the adjacent wall, a little further to one side than the doorway is wide. Plan the location carefully to avoid to rerouting wires and pipes, and make sure installation doesn't interfere with load-bearing walls.

  The lowest priced option for a pocket door will be a hollow-core, flush door on a lightweight track. Extra spending will get you a paint-grade panel door on a sturdy track. And if you make a high-budget investment, expect a stain-grade panel door on a sturdy track.

Exterior Doors:

Any door that opens to the outside must be weather resistant and solid enough to provide security. Be sure the one you choose has a solid core and measures at least 1 3/4-in. thick.

In order to resist weathering, get a clad door, covered with painted steel, fiberglass or aluminum on the outside. Pre-hung doors are fitted with excellent weather-stripping that should seal out drafts even during high winds. An awning or overhanging roof with a gutter should shield the door from direct rain.

If you want to let in a little bit of light or enjoy the view, consider an exterior door with glass panels. These also let you easily see who's outside the door. Building codes require shatterproof, tempered glass. And you should probably consider energy-efficient double-glazing as well.

• Sliding doors

  Sliding glass doors encourage you to go outside, or at least to take a look. They provide ideal access to patios and same-level decks. Sliders have come a long way from the heat-robbing, bulky beasts of decades past. Solid wood or fiberglass composite frames, while heavier than all-aluminum, conduct less heat.

  You can shop for sliders in any price range. At the low end of the spectrum you'll find aluminum-framed doors with tempered glass. Mid-range options include anodized aluminum frames with double-paned glass. And the best among sliders offer exterior-clad wood frames, divided lights and double-paned glass.

   

• French doors

  Elegant French doors are near the top of many remodelers' wish lists. Typically installed in pairs, these may swing in or out, depending on traffic and surrounding furniture and floor levels. Their multi-pane construction often adds an interesting architectural element. Some, with removable grills, can cost far less than true divided lights, and they pop out quickly for easy glass cleaning.

  If there is a step down to a deck or patio, you should install the doors to swing in, to prevent falls. If the exterior and interior are at the same level, you can open the doors out to save interior space. Keep in mind, however that this exposes the hinges - so be sure to specify non-removable pin hinges.

  Lower-priced French doors will have a steel or fiberglass frame. The mid-priced doors have wood frames and removable window grilles. Top-of-the-line doors boast exterior clad wood frames and true divided lights.