Best Practices Guide to Installing Interior Trim
Interior trim stock, profiles, materials, MDF trim, urethane trim
TRIM, INTERIOR INSTALLATION - CONTENTS: Interior trim types, installation procedures and recommendations. Wide Range of Trim Stock Profiles. Table of Properties of Hardwoods Used for Interior Trim. Grading Standards for Interior Trim: FAS "Firsts and Seconds" . Ordering Procedures for Interior Trim.
Custom Profiles Available for Interior Trim. Finger-Jointed Moldings Used as Interior Trim. MDF: Medium Density Fiberboard Composite Trim: Guide to Selection & Use. Guide to using Urethane Moldings and Interior Trim. Guide to Flexible Moldings as Interior Trim
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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.
Guide to Selecting & Installing Interior Trim: Best Practices
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
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.
Wide Range of Trim Stock Profiles
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
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).
Table of Properties of Hardwoods Used for Interior Trim
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.
Grading Standards for Interior Trim: FAS "Firsts and Seconds"
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.
Ordering Procedures for Interior Trim
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
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.
Custom Profiles Available for Interior Trim
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 or Finger Spliced Wood Moldings Used as Interior Trim
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.
MDF: Medium Density Fiberboard Composite Trim: Guide to Selection & Use
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.
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.
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
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.
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 as Interior Trim
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’
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.
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.)
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com
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