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Thermally Modified Wood

Materials and Bioeconomy

Thermally modified wood used in a boardwalk at Sax Zim Bog.

Thermally Modified Wood Program at NRRI

Program Contact:  Matt Aro,, (218) 788-2700

About the Program

Thermal modification of wood is a chemical-free treatment for increasing wood durability, enhancing resistance to rot and decay, and improving dimensional stability. This emerging, chemical-free technology has the potential to create and expand forest products markets, particularly for traditionally underutilized and low-value species. Thermally Modified wood produces sustainable value-added wood products with extended service life and reduced environmental impacts.

NRRI has invested in installation of a Thermal Modification Pilot Plant, one of the few pilot-scale thermal modification kilns of its kind in North America. Several projects have been completed or are underway; summaries of selected projects are described below. We have also completed several product development projects with our industrial collaborators.

Research Projects and Applications

Value-added Products from Small Diameter, Underutilized Tree Species

A project in partnership with the U.S. Forest Service Forest Products Laboratory and Michigan Technological University is looking at using thermal modification technology to add value to small-diameter logs from underutilized tree species. More information can be found here: PDF iconThermal Modification for Underutilized Species

Publications from this project:

Particle size analysis of airborne wood dust produced from sawing thermally modified wood, Aro, M.D., Geerts, S.M., French, S. et al. Eur. J. Wood Prod. (2019) 77: 211. 

Presentations from this project:

Biological Durability of Hygrothermally Modified Hardwoods and Softwoods.  Aro, M., Richter, D., Larkin, G., French, S., Keranen, E., and Laks, P.  2017.  American Wood Protection Association (AWPA) Annual Meeting.  Las Vegas, NV, April 9-11.

Project progress report: 

    1. NRRI Technical Report NRRI/TR-2019/18
    2. NRRI Technical Report NRRI/TR-2020/12

Development of a Strategic Marketing Plan for Thermally Modified Timber

This is a collaboration with the University of Minnesota Twin Cities campus to evaluate consumer perceptions and generate strategies for marketing of thermally modified wood. It was funded by the U.S. Department of Agriculture, Agricultural Marketing Service.

Final summary report for this project:

PDF iconProfessional Consumer Perceptions of Thermally-Modified Wood

Marketing strategy recommendations: 

PDF iconMarketing Strategy Recommendations for the U.S. Thermally Modified Wood Industry

Professional consumer perceptions of thermally modified wood:

PDF iconProfessional Consumer Perceptions of Thermally-Modified Wood

Thermally Modified Engineered Wood Materials

This National Science Foundation Partnerships for Innovation project focused on thermal modification technology to improve dimensional stability and resistance to biodegradation and weathering to a variety of engineered wood materials. PDF iconDownload the final summary report

Publications from this project:

  • "Mechanical and Physical Properties of Thermally-Modified Plywood and Oriented Strand Board Panels," Forest Products Journal, v.64, 2014.
  • "Tensile strength of thermally modified laminated strand lumber and laminated veneer lumber," Wood Material Science & Engineering, 2016.
  • "Durability of Thermally Modified Engineered Wood Products," Proceedings of the International Research Group on Wood Protection, 47th Annual Meeting.
  • "Destructive and Nondestructive Evaluation of Thermally Modified Laminated Strand Lumber and Laminated Veneer Lumber," Proceedings of the Forest Products Society, 69th International Convention.

Evaluating Thermally Modified Wood for Timber Bridge Applications

The NRRI and the USDA Forest Service, Forest Products Laboratory (FPL), with financial support from the USDA Forest Service, are conducting research to provide much-needed baseline data that would lead to increased use of thermally modified wood, particularly in severe biodeterioration hazard regions. The scope of this research includes:

  1. Thermally modifying select wood species at a range of treatment intensities
  2. Conducting leaching tests on the thermally modified wood and chemically analyzing the leachates
  3. Exploring potential co-treatments to improve the durability of thermally modified wood
  4. Constructing a demonstration boardwalk from thermally modified wood

Ultimately, this research has potential to grow the thermally modified wood market and provide economic returns to regions that harvest wood, as well as to manufacturers and distributors of thermally modified wood.

Publications from this project:

PDF iconImproving Exterior Performance of Thermally Modified Timber (TMT).  Kirker, G., Aro, M., and Donahue, P.  U.S. Forest Service, Forest Products Laboratory; Research in Progress FPL-RIP-4723-038.

Manufacture of Cross-Laminated Strand-Veneer Lumber

PFI-RP: Manufacture of Durable and Stable Cross-Laminated Strand-Veneer Lumber for Mass Timber Construction

Thermal modification allows for efficient utilization of lower-value and underutilized trees (such as small-diameter trees), which can accelerate forest restoration efforts, restore ecological balance in the forest, reduce the amount of hazardous fuels on public and private lands, and provide favorable economic returns from low-value timber.  Because of their small size and volume limitations, small-diameter stems are difficult to convert into traditional products (i.e., structural lumber).  However, this raw material source could be reconstituted into a value-added wood composite product.  More efficient use of our wood resources, particularly small-diameter material, can enhance the value proposition of mass timber products, such as cross-laminated timber (CLT). 

The NRRI, Washington State University (WSU), and public and private cooperators, with funding from the National Science Foundation Partnerships for Innovation program, are conducting research focused on utilization of small-diameter timber using thin-strand veneer and laminated strand veneer technology developed at WSU, coupled with thermal modification technology advanced at the NRRI. This research could provide a potential solution to address the issues of dimensional stability and degradation due to moisture infiltration and accumulation of mass timber materials such as CLT.

Demonstration of a Thermally Modified Eastern Larch Boardwalk to Catalyze Markets for Low-value Timber

With financial support from the U.S. Forest Service Wood Innovations grant program, the NRRI, Friends of Sax-Zim Bog, and industry partners constructed a wooden boardwalk at the Sax-Zim Bog near Toivola, MN. The boardwalk was manufactured from thermally modified Eastern larch timber, a low-value tree species threatened by the Eastern larch beetle.