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Figure 5-18: (C) J Wiley, S Bliss Best Practices Guide to Installing Interior Trim
Interior trim stock, profiles, materials, MDF trim, urethane trim

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

Guide to Selecting & Installing Interior Trim: Best Practices

Flexible molding trim (C) Daniel FriedmanThis 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.

Article Contents

As described in the book, Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) Chapter 5, Interior Finish:

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.

Wide Range of Trim Stock Profiles

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.

Figure 5-18: (C) J Wiley, S Bliss

[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.

Table 5-10: Common Domestic Hardwood Characteristics (C) J Wiley, S Bliss

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 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.

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).

Finger jointed interior trim photo (C) Daniel Friedman Finger jointed interior trim photo (C) Daniel Friedman

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.

Finger jointed interior trim photo (C) Daniel Friedman Finger jointed interior trim photo (C) Daniel Friedman

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

Urethane Moldings and Interior Trim - Polyurethane trim;

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.

Finger jointed interior trim photo (C) Daniel Friedman Finger jointed interior trim photo (C) Daniel Friedman

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).

Figure 5-19: (C) J Wiley, S Bliss

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 (Steve Bliss, J Wiley & Sons) .

Where to Buy Interior Trim Products

Polymer (Urethane / Polyurethane), MDF, and Vinyl Trim Producers & Sources

Flexible Trim Manufacturers & Sources

-- Adapted with permission from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) .

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction (Steve Bliss, J Wiley & Sons) , 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.

Trim Installation Guides

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

MDF Crown Molding "Alexandria" pattern for sale at Home Depot stores, Medium Density Fiberboard trim, primed Discussed at InspectApedia.comOn 2019-08-05 by (mod) - what's the difference between MDF and Fypon trim boards?

Jimmy,

Since Fypon produces a wide range of trim type products I infer from your question you're asking about the difference between Fypon™ trim and MDF trim boards.

MDF includes wood fibres or cellulose plus waxes and adhesives in its makeup.

Fypon (trim) is a polyurethane (resin) foam product.

Quoting from our definition of

MDF Medium-Density Fiberboard Definition & Characteristics [above on this page]

MDF (Medium Density Fiberboard) products combine a mixture of wood fibers, a bonding resin, and usually waxes that are combined under high temperature and pressure to produce a medium-density fiberboard product very widely used in the production of trim, cabinets, shelving, and other products, including some products wrapped with an exterior skin of vinyl or plastic to permit outdoor exposure or other uses.

Fypon polyurethane mouldings & decorative trim products discussed & cited at InspectApedia.com (C) 2020 Fypon™ - is a trade or business name for a specific urethane (resin) product formed into decorative trim (or other building product) produced by Fypon, LLC 1750 Indian Wood Circle, Maumee, OH 43537 USA Website: http://www.fypon.com Tel: (800) 446-3040

Fypon produces a wide range of products such as balustrades, lightweight decorative "faux-wood" beams, shutters, mouldings, decorative trim, window & door trim such as crossheads, pediments, pilasters, vinyl column wraps.

Fypon Trim:

Here is how the company describes their moulding products:

Material Type: Polyurethane

Product Type: Crown, dentil, decorative, cornice, smooth flat trim, brick, trim, casing, apron, chair rail, baseboard trim, bed, moulding accessories and timber flat trim

Basic Product Information: Moulding to fit any interior style, no unique tools needed for installation, low maintenance with the ability to stain or paint - retrieved 2019/08/05 original source: http://www.fypon.com/products/mouldings.aspx

Definition of polyurethane

Polyurethane is a synthetic resin in which the polymer units are linked by urethane groups, used chiefly as constituents of paints, varnishes, adhesives, and foams. A foam product that's sufficiently hard and rigid, covered with a vinyl or polyurethane or other plastic or resin "skin" can be formed into "trim" boards that are light weight and that (in some formulations) can be used indoors or outside.

Beware that where exposed to high temperatures we sometimes see plastic and urethane foam trim products that are severely deformed, such as around a window or door where the polyurethane foam trim has been painted a dark colour and is exposed to heat and sunlight.

Opinion: having worked with both MDF trim and polyurethane trim both indoors and outdoors I'd say that each has its advantages and its risk points.

Properties of MDF trim vs Polyurethane Trim
Feature MDF Polyurethane
Adhesive Recommended, PVA adhesive  
Cutting Triple-chip carbide finishing blade; Fine toothed handsaw; can be routed. Seal end cuts. Dulls blades quickly. 80-tooth carbide blade; Cut long, spring into place;
Cost $1.70 / ft.

$1. - $2.50 / ft.

Flexibility More rigid Easier to fit onto wide radius corves, may pucker
Interior/Exterior

Some products only for interior use.

See TRIM, EXTERIOR CHOICES, INSTALLATION		  
Nailing

Hand nail or nail gun;

Pre-drilling needed if hand-nailing.

nailer reduces chipping, size depends on application

Puckering may occur at nail heads.

Glue, Nail, or both; add nails to avoid buckling.

Some always require adhesive / glue.

Pre-sand surfaces to be glued or glue joints may fail.

No pre-drilling needed

Hand nail or finish nailer

Polyurethane Construction Adhesive
Painting Yes, pre-primed. Don't use traditional wood stains.

Yes, pre-primed. Use a glaze or gel stain or use Acrylic Latex or Oil based paint

Resin-based flexible moulding does not accept wood stains. Gel type stains from Minwax, WoodKote, Jel'd Stain, or Zaro will work.

Shrinkage Minimize by gluing, end gluing, proper nailing or stapling Uncommon
Temperature   Stiffens below 60°F, remains nailable.
Variety of Form Predominantly panels & trim; More extensive, e.g. medallionis
Moisture & Weather Resistance

Absorbs moisture, protect from weather, acclimate to jobsite.

Industry standard: 0.3% expansion/contraction in response to RH variation beween 40 RH & 80 RH.

Low moisture uptake, May warp in high heat.
Weight

Heavier than polyurethane.

3/4" thick MDF weighs 3.2 lbs per sq.ft.

3/4" thick ultralight MDF (Trupan) weighs 2.5 lbs. per sq.ft.

Very lightweight.

3 1/2" Crown molding, 1.25 oz. / ft.

Half or less the weight of MDF.

 

Notes to the table above:

Trim Board Weight Details

Trim Board Cost Comparisons using Crown Moulding as an example

These cost comparisons are just approximate; per foot costs vary by moulding width and profile, type of priming, & other features.

References for Installation of MDF vs Polyurethane Trim Mouldings

On 2019-08-05 by Jimmy - what's Different between Fypon & MDF material ?

Different between Fypon MDF material


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