Explore the World of Wood in Historic Preservation
Dr. Michael O.Hunt

Yellow Poplar Problem
-or-
Yellow Poplar Opportunity
November 10, 2008
Michael O. Hunt and Daniel L. Cassens

THE PROBLEM

The across grain, vertical crack and the knife blade inserted in the drip edge of the third lap of siding from the bottom in the left hand picture above indicate decay in new-growth yellow poplar siding after about six years (1999) of exposure.  The adjacent right-hand picture shows the back side of this lap of siding after removal (2005).  The drip edge is oriented to the top of the picture.  Extensive decay is evidenced along much of the length of drip edge of the piece.
The siding shown is on a 1993 garage addition to a 160 year old historic house clad in old-growth yellow poplar clapboard/bevel siding that was restored at the same time.  Very few strips of siding had to be replaced on the restored house.  It was observed that much of the original siding on the house was heartwood.
It is recognized that sapwood of yellow poplar has no decay resistance.  Partially, for this reason both sides of new siding strips had been brush coated with Woodlife Classic® water repellent preservative before installation.  However, decay after such a brief exposure in a supposed dry application was unexpected.  During removal of the decayed lap of siding and a section of decayed band board it was discovered that there was considerable evidence of water staining behind the siding.  The garage addition had been constructed using standard practice at the time.  House wrap (Tyvek®) was fastened to the sheathing.  Next the yellow poplar siding had been fastened to the studs and in direct contact with the house wrap.  Rain driven water was trapped on the impervious house wrap in contact with the back side of the siding.  For a fuller discussion of this problem and its ramifications click on Decay of Siding @ 602 N. 5th St., Lafayette, IN. (Click here for an annotated Power Point version.)

BACKGROUND 
Today’s yellow poplar is a tremendous timber species specifically in Indiana and in general in eastern United States.  It grows fast, tall, straight, and in dense stands.  It is easy to harvest, saw, dry, and machine.  Large old growth trees composed mostly of heartwood were used for numerous functional uses such as flooring, furniture, and interior and exterior millwork including siding and windows.  Its performance was excellent and both authors of this report own structures in excess of 125 years in age where yellow poplar windows and siding are still in serviceable condition.  Unfortunately, today’s “millwork grade” yellow poplar is mostly sapwood and many contractors have continued to use it just as the old growth supply but very often with the resulting problem of early decay.
Yellow poplar constitutes 13% of the timber volume in Indiana.  In terms of volume it is surpassed only by the oaks.  It is a medium to low valued species.  Recognizing a growing national and international trend that affects markets, all Indiana state forest lands and all Indiana classified forests are certified as being sustainable.  But landowners are disappointed because these beautiful timber trees do not have a high value and the hardwood lumber industry complains that it is difficult to turn a profit when processing this species.  There is just not enough demand for the volume available.
As mentioned above, yellow poplar was used widely from the eastern seaboard to the Mississippi River valley for siding, trim and ornamentation in the construction of historic buildings well into the 20th century.  If new-growth yellow poplar is to be used as a “replacement in kind” material in historic restoration and rehabilitation projects its decay resistance property must be improved. 

RESEARCH
The purpose of this research is to develop preservative-treated yellow poplar that economically and effectively can be used in above ground contact construction applications.  The use of preservative treatments to make new growth yellow poplar comparable in performance to its old growth counter-part and even comparable to expensive western red cedar, a naturally durable wood preferred for exterior trim, has not been investigated - not even on a preliminary basis.  More recently fiber cement products are being used in historic restoration projects.  The successful preservative treatment of new growth yellow poplar would mean that it would be directly competitive with the very expensive western red cedar and fiber cement products and that it could be promoted for above ground uses, such as, windows, doors, casings, siding, soffits, fascia, and corbels.  It might be sold as lumber to be machined on site or as stock profiles, typical of large volume applications.  The species would become a sought after asset, thereby encouraging landowners to better manage timber, providing existing wood manufacturers with increased demand and probably creating demand for treating facilities and marketing opportunities.
The Purdue Wood Research Laboratory (PWRL) is involved in current proof-of-concept research to develop a niche market for preservative treated yellow poplar for above ground use.  A number of cooperators and interested parties are involved.
Arch Wood Protection, Inc. of Conley, Georgia, is providing  pressure treatment of yellow poplar specimens with their commercial preservative Wolman AG® preservative with and without water repellent.  Wolman AG® is a “green” organic preservative in that it contains no heavy metals, e.g., copper, chromium and arsenic.  Yellow poplar specimens supplied by PWRL have been treated by Arch and returned to the Purdue University campus for evaluation.  A status report of this evaluation will be reported below.
SEEMAC Inc. of Carmel, Indiana, will provide in-kind market research once the proof-of-concept research has been shown to be successful.  SEEMAC will research the following potential uses of preservative treated yellow poplar:

Representatives of pertinent historic preservation and restoration agencies encouraging the successful outcome of this research are Antonio Aguilar, architect and Acting Chief, Technical Preservation Services Branch, National Park Service, and David B. Duvall, Historical architect, Indiana Division of Historic Preservation & Archaeology.

Interestingly, treated southern pine sapwood and heartwood mimic that shown for yellow poplar.  Preservative treated southern pine dominates the huge treated wood products marketplace in eastern, southern and central states of the United States.  Arch markets similarly treated southern yellow pine wood products as Wolmanized® L3 Outdoor® Wood.

Evaluation of treated specimens-                
Three separate but related efforts of evaluation are underway:             

Paint adhesion  From a technical standpoint for new growth yellow poplar to be successful in the construction marketplace it must have adequate decay resistance for above ground use and it must continue to demonstrate research-documented outstanding paint performance.  The previously mentioned Arch Wood Protection analysis of pressure preservative treatment data and the developing research of the comparative decay resistance of heartwood of yellow poplar and southern pine address the adequacy of decay resistance issue.
Favorable weathering resistance of painted yellow poplar has been documented by research at the U.S.D.A. Forest Products Laboratory (W. Feist 1987 Forest Products Jour. 37(3): 15-22).  Painted yellow poplar weathered similarly to ponderosa pine, fir, hemlock and spruce.  In more recent Purdue University field exposure research (Hunt et al. 2003 FPJ) painted yellow poplar was equivalent in weathering performance to western red cedar, the gold standard for painted wood performance.  Research has been performed to determine if the treatment with Wolman® AG preservative with and without water repellent affected the surface adhesion of paint.  Long term, paint fence type of weathering is not feasible at this time.  A quicker evaluation to give a preliminary go/no go outcome was necessary.  Therefore, it was hypothesized that the surface adhesion of a coat of primer paint to the surface of treated wood would be a valid predictor of the long term weathering performance.  ASTM D 3359-08 Standard Test Methods for Measuring Adhesion by Tape Test (ASTM) was selected for this purpose.  Prior to preservative treatment in selecting specimens for treatment matched specimens were retained for controls.  Two groups of controls were used: one group was matched untreated yellow poplar specimens with one coat of primer paint and the second group was matched untreated yellow poplar specimens with one coat of Woodlife Classic® water repellent preservative plus one coat of primer.   These groups represent the two types of yellow poplar specimens used in the multi-year field weathering research (Hunt et al. 2003).  The specimens in the long-term weathering exposure had two top coats of paint in addition to the primer.  However, the logic in the adhesion by tape test is that long term field exposure performance would be directly correlated to the presence or absence of good surface adhesion of the primer.  For a detailed report click on Yellow Poplar Paint Adhesion Test report.