Interior trim best practices:
This article describes current choices of materials used for interior woodwork and trim in buildings and gives advice on ordering and using solid wood trim, finger jointed moldings, MDF medium density fiberboard composit trim, urethane moldings and trim, and flexible moldings.
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This article series discusses and provides a best construction practices guide to the selection and installation of building interior surface materials, carpeting, doors, drywall, trim, flooring, lighting, plaster, materials, finishes, and sound control materials.
Once the domain of premium softwoods, such as clear pine, poplar, and other easily machined woods, interior trim is just as likely now to contain a mix of finger-jointed stock, medium density fiberboard (MDF) molded urethane for decorative trim, and flexible polyester moldings that must bend around curved surfaces.
Our photo (left) shows the most basic and widely-used budget-trim installed in homes in the U.S. beginning in the 1960's: clamshell trim. At WINDOW / DOOR AIR LEAK SEALING HOW TO we (D Friedman) describe a project we undertook to remove this clamshell trim, seal gaps around windows and doors, and install custom-cut but elegantly-simple rectangular profile mahogany trim in this home.
Wood moldings and other finish lumber are graded for visual properties only. In general, the higher the grade, the more uniform the grain and color will be, and the fewer the defects, such as small knots, pitch pockets, and other natural markings.
In some species, there is also a marked color difference between heartwood and sapwood. Some customers might like the natural variation found in lower grades; others find it objectionable.
Most lumberyards stock only a few molding profiles in pine and even fewer in hardwoods. Specialty molding suppliers, however, offer a far wider variety of stock profiles in both softwoods and common hardwoods.
Molding suppliers also stock a variety of architectural ornaments, such as rosettes and plinth blocks, that can dress up a job or match a traditional style without the cost of custom millwork.
[Click to enlarge any image]
Most wide, flat moldings are recessed or “backed out” a little to reduce the tendency to cup. Cutting kerfs in the back of flat board stock will accomplish the same effect (see Figure 5-18).
While some lumberyards stock small quantities of milled hardwood boards and a few molding profiles, most larger jobs require the purchase of rough stock from a hardwood supplier or millwork shop. Hardwood trim characteristics are shown in Table 5-10.
If a job requires all clear stock that is “color matched” with minimal color variation from board to board, you will probably need to purchase the highest grade available, often FAS (firsts and seconds), and may still need to cull some pieces. For jobs where more grain variation is acceptable, No. 1 Common or No. 2 and 3 Common may suffice.
FAS is at least 80% clear stock with minimum boards 6 inches wide by 8 to 16 feet long. No. 1 is at least 65% clear with narrower boards, and No. 2 and No. 3 are 50% and 33% clear, respectively.
Providing the shop with a specific cut list of finished pieces is the best way to guarantee that they deliver the pieces needed for the job. For a premium, you can obtain all-heartwood, all-sapwood, or color-matched boards for uniform color in glue-up work and throughout the job.
Also, the millwork shop can plane the stock on one or both sides, joint one or both edges, and sand one or both faces as needed. Generally, the millwork shop can dress the boards far more economically than a contractor can in the field or in a small shop.
A job with hardwood trim may also require profiled moldings, such as baseboard, chair rail, or crown. Custom hardwood moldings require a substantial lead time and a setup fee to make the cutter knives. Many shops keep cutters on hand for standard profiles, as well as custom profiles from prior jobs. Using an existing cutter can significantly cut costs and lead time.
Finger-jointed stock is widely used for paint-grade door and window jambs, as well as profiled moldings. Finger-jointed stock generally performs well, but in some cases, joints between the individual pieces will “telegraph” through the painted finish due to minute differences in the swelling and shrinking of the individual pieces of wood (see our photo of finger spliced interior trim, looking at the edge of a trim board, below left -DF).
To avoid this problem, sand any uneven joints before applying any finish. Also, back-priming the material will reduce any moisture movement after installation, minimizing problems with telegraphing. While the glued-up finger jointed board connections are quite strong, it is indeed possible to break a board at the finger joint splice, as our second photo shows, above right -DF. But handled with reasonable care, nailed in place, and properly prepped and painted, these joints are virtually impossible to see in interior trim wood.
Finger-jointed exterior trim, unlike its interior trim cousin, is exposed to weather and has proven less durable than hoped at some homes, as illustrated at TRIM, EXTERIOR CHOICES, INSTALLATION.
Medium-density fiberboard (MDF) is a fine-grained composite material made from wood particles and resin bonded under heat and pressure. The resin is generally urea-formaldehyde, a known lung irritant, but a few manufacturers offer alternative products made with the more stable phenol formaldehyde or other low-emission resins. Our photos (below) show the painted-side and back side of composite trim boards on display at a New York Home Depot® store -DF.
SierraPine Composite Solutions makes Medex, a moisture-resistant MDF product, and Medite II, an interior panel, both using a formaldehyde free resin called MDI (methylene diisocyanate).
In many markets, MDF has become the material of choice for trim and casework due to its low cost, ease of machining, and excellent appearance when painted. It is uniform in consistency and dimensionally stable. MDF trim is available preprimed in a number of standard molding profiles, and 4x8 MDF panels are easy to cut to size and can be routed or shaped to a clean, crisp profile.
However, a 3/4-inch 4x8 panel weighs 95 pounds versus 75 pounds for birch plywood, making MDF sheets a challenge to maneuver.
While MDF offers many benefits, it is not problem free. Cutting and milling creates a super fine dust, which requires workers to wear tight-fitting respirators. Shops should have a good dust-extraction system as well.
The urea-formaldehyde makes the dust more irritating to eyes and lungs and off-gasses to some extent after installation, making the product unacceptable to some (see Chapter 7, “Formaldehyde,” page 287).
Because of hardness, MDF moldings must be installed with pneumatic nailers, which tend to pucker the material around the nail. These “mushrooms” must be chiseled off prior to filling the nail holes.And although it holds paint well, cut and routed edges of MDF will absorb water-based primer and swell.
To avoid these problems, edges of MDF moldings and trim should be sealed with a shellac-based or oil-based primer or painted with special finishes formulated for use with MDF. Due to its potential for absorption at edges, MDF is not a good choice for wet areas. Edge nailing is also not recommended, so MDF is not well suited to applications such as jamb extensions.
See important details at Definition & Characteristics of MDF Medium-Density Fiberboard
Although pricey, polyurethane foam moldings (also called polymer moldings) are popular for ornate decorative work. The leading manufacturer, Fypon, makes a wide range of large crown and cornice moldings, as well as architectural ornaments for mantles, decorative ceilings, and other decorative elements. Our photos (below) show the face side and back side of primed polyurethane trim sold at a lumber supply store in New York -DF.
Urethane foam moldings are sold preprimed, and they can be cut, planed, and sanded like wood—only more easily because of their lighter weight.
Urethane trim and moldings are installed with proprietary caulk or adhesive rather than nails, although a few finish nails are often used to hold them in place while the glue dries. Butt joints and miters are bonded with the same adhesive. Larger moldings are limited in length to 10 to 12 feet, requiring multiple joints on long runs.
Flexible moldings made from dense polyester resin have been available since the late 1960s, but they have improved a lot in recent years.
Newer formulations are easier to nail, more resistant to cracking, and come in a wide variety of profiles, in both paint and stain grades as well as in pre-finished wood-grain patterns (2nd photo from page top -DF, and (Figure 5-19 below).
Most manufacturers offer thinner profiles and softer formulations for tighter curves, as well as fire-retardant formulations. Less expensive rigid versions are also available for straight runs. While originally developed for interior use, many of these products are suitable for exterior applications as well.
The stain-grade material has an embossed grain, but must be stained after installation due to the stretching of the surface and requires a heavy pigmented oil-based or gel-type stain with a clear topcoat.
Most flexible moldings are made to order and can perfectly match typical finger-jointed or MDF profiles if specified correctly when ordered—manufacturers have thousands of molds matched to various manufacturers’ stock moldings.
Simple curves such as baseboard or chair rails generally do not need pre forming, but crowns, arch top casings, and most small-radius curves must be preformed by the manufacturer for the specific radius needed. Manufacturers can accommodate ovals, ellipses, and other irregular curves if provided with accurate design specs.
The material cuts easily with standard woodworking tools, but it needs to be held in a jig or sandwiched between wood blocks for difficult cuts. Most manufacturers recommend installation with construction adhesive, panel adhesive, or gel-type super glue, with a few finish nails to hold the molding in place while the glue dries.
Pneumatic pin nailers work well when installing flexible moldings and trim. But ...
Watch out: However, nailing too close to the edge may distort or crack the rubber material. Large moldings such as crown need wood backing or triangular blocks to prevent the molding from bowing in. (see Buy Interior Finish Product Resources for a list of suppliers.)
-- Adapted with permission from Best Practices Guide to Residential Construction.
Polymer (Urethane), MDF, and Vinyl Trim Producers & Sources
Flexible Trim Manufacturers & Sources
-- Adapted with permission from Best Practices Guide to Residential Construction.
This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons. Our page top photo shows chestnut trim in a Poughkeepsie, NY home constructed in 1900 and restored by DJF.
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