Here's a truism that applies to homes as well as people: Good looks aren't enough.

It's especially true for siding, which has to pair aesthetic appeal with such mundane qualities as durability, low maintenance and reasonable cost.

Siding - and the trim and detail work that accompany it - is a big-ticket item that most homeowners don't need to purchase more than once or twice in a lifetime. But it can be critical to creating a home's beauty and personality.

If a beating from time and nature is forcing siding replacement, or if you simply want to upgrade, you need to know what you're buying.

With the exception of nomadic cultures and those who live in extreme climates, humans have always adopted three basic materials - wood, earth and rocks - for their structures.

The development of surface materials that differ from the structure itself is a relatively modern phenomenon, but we haven't strayed that far from our roots. Sawn lumber, brick and stone, or the appearance of them, still constitute the dominant visual themes we use to wrap our homes.

In some cases, the look is literal, created by the actual materials. Increasingly, though, a manufactured or hybrid material is being recruited as a substitute.

Over the last few decades, metal (steel or aluminum) and vinyl cornered a big share of the siding market, largely because of low-maintenance requirements and lower installation costs.

These horizontal-lap systems feature matching trim pieces, and on existing homes are often applied directly over old wood siding. Both had mixed reputations early on, but newer-generation products feature more durable materials and contoured rigid-foam backing to add insulating value.

Vinyl reigns as the low-cost solution, with installed costs for economy-grade goods at a few thousand dollars for a typical three-bedroom home. Expect to spend two to four times that for premium natural or engineered materials.

Detractors still argue that neither metal nor vinyl offers the deep textures or detailing of traditional materials, and there are other drawbacks. Metal siding can dent or lose its factory finish, and vinyl colors are limited and can't be painted. Also, their hollow lap configuration makes both susceptible to peeling off in extremely high winds.

Despite the drawbacks of some manufactured siding materials, there are plenty of reasons these new players have entered the marketplace. Cost, maintenance requirements and environmental factors rank highest.

The best siding lumber, for example, comes from old-growth redwood and Western red cedar trees, which are increasingly scarce and sometimes off-limits because of their environmentally sensitive habitat.

These trees yield heartwood that is stable and rot-resistant, and many are milled into horizontal lap siding or shingle siding, but premium grades are pricey. For best results, these materials need to be sealed or "back-primed" before installation. Some manufacturers offer their goods preprimed or prestained, a real time-saver.

When it comes to siding, today's smaller and younger trees aren't sawn as much as shredded, creating a fiber stew that is mixed with binder resins. When formed in a hot press, this composite is shaped into engineered wood "boards" that mimic milled lumber, even down to a realistic textured surface. The outer layer is sealed with a factory-applied primer, and the uniform quality is considered a big advantage over solid lumber.

Unfortunately, even high-quality sawn or engineered wood siding is susceptible to moisture-related seasonal movement or water penetration, causing the boards to shrink and/or swell, depending on conditions.

Poor surface sealing, especially on board ends, accelerates this moisture cycling and often causes premature paint failure or literal disintegration of the composite. Solid-color and semitransparent latex stains stay flexible enough to move with the material, and thus fare better than most paints. Carefully installed and sealed, engineered wood composites are affordable and durable.

Relatively recently, another engineered hybrid has earned high points for its performance. Fiber-cement siding, made from wood fiber mixed with portland cement and other additives, is a solid-bodied material that comes in the form of lap planks, plywood-size panels, and shingles. It is cost-competitive (especially the lap siding), resistant to fire, water and insect damage, and dimensionally stable.

Offered preprimed and sometimes prefinished, fiber cement holds paints and stains well because the material doesn't move much in response to moisture or temperature. In addition to posing as a wood surface, fiber-cement siding comes in variations that mimic stucco, brick and other masonry surfaces.

Fiber-cement and engineered-wood materials typically require a clear sheathing substrate, so for renovation work the old siding must be removed. This is a good idea anyway, allowing you to insulate exterior walls, reinforce or replace sheathing, and install a new water/wind barrier.

Whatever siding material you choose, treat this investment seriously. Contractors installing cheap materials might charge thousands less than competitors with top-notch goods, but the quality difference will be painfully obvious and detract from your home's value.

Few other materials are as crucial to both protecting your home and adding curb appeal.