In the last decade, a range of new trim has overtaken the exteriors market. Trim stock is now available in plywood, OSB, MDO, hardboard, low-density fiber-cement, cellular PVC, and polyurethane. The variety is staggering, and, as you might suspect, so is the range of performance.

Three Categories Products commonly bundled under the term “composite” trim can be grouped into three general types: fiber-resin, fiber-cement, and plastics. Typically, the fiber-resin wood and fiber-cement products are slightly less expensive than clear-grade softwoods, while plastic runs a bit more.

Durability is the chief reason to choose any of these materials over wood. All are designed to resist cupping, warping and splitting, and to hold paint longer than dimensional stock. But the performance largely depends on how it's used, and not all types are well suited to custom work.

Fiber-resin materials include some combination of wood fiber and glue. Although resin chemistry has changed dramatically since the OSB and hardboard siding failures of the 1980s, all fiber-resin materials will swell if they get wet. Materials made with a PVC polymer, such as CertainTeed's Weatherboards, or with an MDO core, such as Pacific Wood Laminate's ClearLam, will absorb much less moisture than products made with phenolic resin and wood flour. Nevertheless, installers should be careful to flash edges and keep the surface intact. Anytime the surface is broken — either by countersinking fasteners or ripping an edge — fiber-resin products are at risk of swelling. These products generally perform best if fasteners are not countersunk, and if the material is installed in stock sizes, not ripped.

Fiber-cement trim is a different material than the siding. Typically referred to as “low-density” fiber-cement, the trim material is significantly lighter than the siding, though it is still heavier than most trim stock of comparable size. While water won't cause the material to deteriorate, Jim Blahut, a general contractor in Beach Haven, N.J., reports swelling at cut ends and edges, and at scarf joints. “I was meticulous in the installation, back-priming every cut and careful not to overdrive nails. I wanted to know how the material performed, and to know the installation would not be the issue. So I am confident saying that any water that got into this material caused it to swell. It might work better if I wasn't ripping it, trying to make it work on an elaborate Victorian exterior. A production builder might have a very different experience.”

Plastics come in a wide range of formulations. The most common are polyurethane and cellular PVC. Polyurethane is a closed-cell material that won't absorb water and is molded, rather than extruded, which allows it to be formed with ornate details. But it tends to be the most expensive option, costing as much as $12 per linear foot for an elaborate piece of crown with dentils.

The most versatile trim seems to be cellular PVC. Priced in the range of clear cedar, cellular PVC is completely water resistant and has performed well in extremely wet conditions for almost 20 years. Like any plastic trim, it has a high coefficient of thermal expansion, which causes it to expand and contract with temperature changes. However, installers report a variety of methods to stabilize joints, principally by solidly fusing joints with PVC cement (Gorilla PVC gets high marks in this department).

PVC trim stock is mixed with a foaming agent that injects tiny bubbles into the material. A majority of products are made with a “celuka” foaming process that distributes smaller, tightly packed gas bubbles near the edges of an extrusion and larger bubbles towards the center. This increases the density near the surface, but voids are common when the material is ripped.

Azek is an exception. Made with a different foaming process, Azek has a dense, uniform consistency throughout that can be worked just like wood. According to Anthony Saduk of Woodbine, N.J., who specializes in ornate exteriors on high-end seaside homes, voids are rare. “We have a great detail that puts a bead on the edges of cornerboards. It's tough to get a perfect bead on an 18-foot board because the material is floppy and has to be continuously supported. But there's no other material, except wood, that could hold this detail.” Saduk doesn't typically paint Azek, which is treated for UV resistance, and he says it doesn't look at all like plastic. “What I'm selling is a truly maintenance-free exterior,” explains Saduk. “But it shouldn't look like it came out of a box.”