Investigating CLT’s Ability to Fight Fungus

The growing reputation of cross-laminated timber (CLT) as a sustainable, cost-effective, and innovative building material has prompted researchers at the Forest Products Laboratory (FPL) to build upon past research and investigate the material’s ability to fight against fungus.

Intact cross-laminated timber panel section (left); 4-in. cube cut from panel section for scaled-up decay testing.

(A) Intact cross-laminated timber panel section; (B) 4-in. cube cut from panel section for scaled-up decay testing.

Praised for its many benefits, including speed of construction, cost, sustainability, excellent thermal and sound insulation, and fire restriction qualities, the pre-fabricated building material has made a name for itself in the construction and worldwide mass timber market. CLT has already made an appearance in a variety of high-rise apartment buildings in the Pacific Northwest and Southeast United States, urging scientists in the Lab’s Durability and Wood Protection Unit to further examine how the timber fairs against a rainy, humid climate.

The study builds upon past conclusions that untreated CLT is susceptible to mold and a variety of fungi. While decay can be reduced with preservatives such as boron, researchers are using more methods to investigate resistance treatments.

Scientists have implemented soil block assay tests on numerous random samples of CLT, and also plan to conduct mass loss and x-ray density profiling to assess decay in CLT.It is hoped that this exploration will help researchers develop more targeted fungal reduction methods for CLT.

The project will conclude in early 2017.  For more information on CLT and fungal resistance, read the full Research in Progress report.

Blog post by Francesca Yracheta

Termites of the Frozen Tundra: How Do They Survive?

One look at our snowy landscape in Wisconsin and it would be easy to assume termites couldn’t survive here. These wood-destroying critters are a common concern for people living in warmer climates, but amazingly, certain species of subterranean termites have been introduced and become established in the northern United States and Canada, as well.

map for termite distribution

Simplified map showing distribution of eastern subterranean termites along its northern range. (Only selected populations are shown to highlight major zones of activity.)

Termites’ ability to adapt to a colder climate was studied in the 1970s and 1980s by Forest Products Laboratory (FPL) researcher Glenn Esenther. Today, FPL entomologist Rachel Arango is still fascinated by the phenomenon.

“Insects are poikilothermic, which basically means that body temperature is determined by the external environment,” explains Arango. “So, temperature is a major factor in species distribution.”

Figuring out how certain populations of subterranean termites can tolerate lower temperatures is important for predicting the potential spread and economic impact of termites, as they are already responsible for billions of dollars in damage to structures annually in North America.

Arango and her termite team will directly compare northern and southern populations of eastern subterranean termites in hopes of better understanding how some are able to survive in colder regions. Concurrent tests between Wisconsin and Mississippi populations will include studying basic differences in cold tolerance, behavioral investigations, and research into genetic differences.

“We hope to identify some of the physiological changes that might increase cold tolerance,” says Arango.

Arango expects the research will be beneficial not only for understanding termites, but for broader research into invasive species, as well, particularly as global climate conditions change.

Results are expected by May 2019. For more details on the study, see this Research in Progress report.

Patience is a Virtue: Impressive Decades-Long Research Results

It takes patience to be a scientist. Research can be time consuming, especially when you’re working in the Forest Products Laboratory’s (FPL) Durability and Wood Protection research work unit. Part of the unit’s mission is to treat wood with preservatives and track how long the treated wood can fend off decay. Success can mean decades-long studies.

FPL's Valley View field test site.

FPL’s Valley View field test site.

Recently, several members of the unit made the trek to FPL’s Valley View test site, an unassuming field west of Madison, Wisconsin, where wood in various forms is left to endure the elements, often for years. Their mission: to inspect treated wood stakes that have been buried in the ground for 40 years.

Ground-level view of intact 2 x 4 field stake showing no visible deterioration after more than 40 years of soil contact.

Ground-level view of intact 2 x 4 field stake showing no visible deterioration after more than 40 years of soil contact.

The stakes, both solid wood and plywood, are made of southern pine or western wood species, such as Engelmann spruce and Douglas fir. They were treated with either chromated copper arsenate (CCA) or ammoniacal copper arsenate (ACA).

The results are astounding. There were stakes, both solid wood and plywood, that achieved a rating of 10 (sound) after 40 years of soil contact.  For comparison, untreated southern pine stakes in contact with soil typically last one-and-a-half to two years before failure (rating of zero) due to decay fungi.

Field tests are critical for establishing the durability of treated wood products that are used for construction of decks, bridges, and utility poles. The test site at Valley View has been maintained by FPL since the early 1950’s. Matched sets of many of the studies at Valley View site are also installed in the Harrison Experimental Forest in southern Mississippi, which represents a more severe decay environment due to increased average temperature and rainfall.

Examples of sound (rating of 10) 2 x 4 (top) and plywood field stakes after 40 years of soil contact in southern Wisconsin.

Examples of sound (rating of 10) 2 x 4 (top) and plywood field stakes after 40 years of soil contact in southern Wisconsin.

Blog contributed by Grant Kirker, Amy Bishell, and Stan Lebow.

Cottage Conundrum: FPL Fields Cleaning Question

All of the research and expertise of the Forest Products Laboratory (FPL) is in vain if we can not use it to better the American people. Our lab receives many questions for our researchers, and recently, we received a question regarding T1-11 cedar siding.

Dear FPL,

I have a problem with the T1-11 Cedar Siding (plywood) that was installed on my cottage back in 1986. There is a black fungus on all sides of the cottage, even where the front is exposed to the direct sunlight most of the day. I would like to have a vinyl siding installed over the T1-11, but am not sure if I should have that done without washing the siding first. I have washed the siding in the past and the fungus has returned! I would appreciate any advice you can provide to me.

Cottage Conundrum

Luckily, Mark Knaebe, a Natural Resources Specialist with the Forest Products Marketing Unit, had the answer!

Dear Cottage Conundrum,

The most important question here is what do you really have?  If you used what is commonly called a “wood bleach” which is oxalic acid, and it brightened up, then you have iron stain. Iron stain is not a fungus.  


Although it leaves wood black, iron stain is not a fungus. The above example was caused by corroded fasteners.

If you used a chlorine or oxygen bleach and it brightened it up, then you have mildew, which is a form of fungus, and it may not be a huge problem except it that it can keep wood wetter for longer periods which can promote decay fungus (rot) in very wet areas.

I would not give up on the T1-11 just yet.

If you are set on putting vinyl over it, and it is iron stain, you can ignore the black. If you have mildew, you should find out why first, and if bleach cleans it up, you could just bleach and put up the new siding paying special attention to proper flashing. Traditional penetrating coatings contained oils which are food for mildew so as soon as sunlight destroys the mildewcide, mildew can quickly return, so that might be what you’re experiencing.

The bottom line is find out what it is first, and take the next steps from there. Thanks for your question!



Temperature Down, Danger Up : Wood Heating Caveats From FPL

Most residential buildings in the United States employ wood as a primary construction material, and increasingly, commercial buildings are following suit. Although researchers at the Forest Products Laboratory (FPL) have spent the better part of a century formulating new treatments and methods for improving the fire durability of wood, fire safety remains a serious consideration, particularly during the winter.

Fire retardant test at FPL, 1940s.

A fire retardant test at FPL during the 1940s. FPL research has helped improve building codes, wood treatments, and testing standards.

This danger can be compounded depending on your method of heating. As the temperature goes down, if you choose to heat your home with wood, fire danger goes up. Proper precautions should be taken to ensure that the fire stays contained in the stove or fireplace, lest it spread to the surrounding structure.

According to The Wood Handbook: Wood as an Engineering Material, one of the most important problems associated with home fires is the smoke produced. The term smoke is frequently used in an all-inclusive sense to mean the mixture of pyrolysis products and air that is near the fire site. In this context, smoke contains gasses, solid particles, and droplets of liquid — but why is smoke so dangerous?

Smoke presents a potential hazard because it interacts with light to obscure vision, but the toxicity of combustion products is the primary concern. Fire victims are often not touched by flames but die a s a result of exposure to smoke, toxic gasses, or oxygen depletion. These life-threatening conditions can result from burning contents, such as furnishings as well as from structural materials involved.

The toxicity resulting from the thermal decomposition of wood and cellulosic substances is complex because of the wide variety of types of wood smoke. Composition and the concentration of individual constituents depend on such factors as the fire exposure, oxygen and moisture present, species of wood, any treatment of finishes that may have been applied, and other considerations.

The vast majority of fires that attain flashover (a fire’s sudden spread when an area is heated to its flashpoint) do generate dangerous levels of carbon monoxide, independent of what is burning. Carbon monoxide is a particularly insidious toxic gas and is generated in significant amounts in wood fires.

Even small amounts of carbon monoxide can be toxic because the hemoglobin in the blood is much more likely to combine with carbon monoxide than with oxygen, even with plenty of breathable oxygen present. Generally, two approaches are used to help deal with the smoke problem: limit smoke production and control the smoke that has been produced. The control of smoke flow is most often a factor in the design and construction of buildings.


Wall test conducted in the large vertical furnace at FPL showing a wall panel at the point of fire burn through. Information of this nature is used in building designs to help ensure time for people to exit a burning structure and help contain the fire and smoke.

Draftstops are one useful control measure construction engineers implement. Draftstops are barriers intended to restrict the movement of air within concealed areas of a building. The are typically used to restrict horizontal dispersion of hot gases and smoke in larger concealed spaces such as those found within wood joist floor assemblies with suspended dropped ceilings or within an attic space with pitched chord trusses.

Doors can also be critical in preventing the spread of smoke and fire, even if they are made out of wood. Doors left open or doors with little fire resistance can easily defeat the purpose of a properly fire-rated wall or partition. Listings of fire-rated doors, frames and accessories are provided by various fire testing agencies. When a fire-rated door is selected, details about what which type of door, mounting, hardware, and closing mechanism must be considered.

Finally, keep in mind that smoke rises, and that when evacuating a burning building, clean air can usually be found closer to the ground. For more information on home fires, and tips to keep you and your family safe, visit

For more information about the fire resistance of wood, please see Chapter 18 of The Wood Handbook: Wood as an Engineering Material.