ThermoWood Mass Loss CalculatorThermal Modification Mass Loss and Class
Calculate the mass loss that occurs during thermal modification (ThermoWood) and estimate the likely durability class, equilibrium moisture reduction and dimensional stability improvement.
Why this calculation matters
Mass loss indicates both the durability gained and the strength given up. It is what tells you whether the board belongs in cladding or must not carry load.
ThermoWood Mass Loss Calculator
Thermal Modification Mass Loss and Class
Base species before modification.
Commercial ThermoWood sits in a narrow band, and it is SPECIES-SPECIFIC: Thermo-S is 190 ±3 °C for all species; Thermo-D is 212 ±3 °C for Pine and Spruce, 230 ±3 for Radiata Pine, 225 ±3 for Hemlock, 208 ±3 for Ayous, 190 ±3 for Iroko and 212 °C (−2/+5) for Ash and Frake. Temperatures outside those bands are laboratory or experimental heat treatment, not commercial ThermoWood — the estimate still runs, but do not read the result as a ThermoWood property.
Peak temperature hold time.
Oven-dry mass before treatment.
| Property | Before | After Treatment | Change |
|---|
How this is worked out. Mass loss is not something you can read off a published table for an arbitrary temperature and time: it depends on the species, the kiln, and the schedule. The figure here comes from an in-house model fitted to the published relationship between treatment temperature and mass loss, not from ITWA or VTT data for a specific product. Use it to compare schedules and to sanity-check a supplier's claim. For a real number, ask the producer for their measured mass loss on the actual batch.
About ThermoWood Mass Loss Calculator
Thermal modification (the ThermoWood process) heats wood to 160-240°C in a low-oxygen environment, permanently altering its chemistry. The wood loses mass as hemicelluloses break down - and this mass loss is the single best predictor of the improvements gained: lower equilibrium moisture, better dimensional stability, and higher decay resistance. This tool estimates mass loss and the resulting performance class. It links thermal modification temperature to expected mass loss — a quick window into how the thermal modification process changes the wood.
Where Is This Used?
Class temperature is species-specific. The ITWA handbook sets the Thermo-D temperature per species, not once for the class: Pine and Spruce 212 ±3 °C, Radiata Pine 230 ±3 °C, Hemlock 225 ±3 °C, Ayous 208 ±3 °C, Iroko 190 ±3 °C, Ash and Frake 212 °C (−2/+5). Thermo-S is 190 ±3 °C for all species. Enter the temperature your supplier actually used — a class name alone does not fix it.
Formulas Used
Mass loss % ~ f(temperature, duration, wood type)ML = base(T) x duration_factor x type_factorThermo-S class: ~180-200°C, mass loss 3-7%Thermo-D class: ~210-230°C, mass loss 7-15%EMC reduction ~ proportional to mass loss (up to ~50% lower)Durability improves from class 4-5 toward class 1-2 (EN 350)How the calculation works
Mass loss is the accepted proxy for how far the treatment has gone, because the hemicelluloses that break down are both the food source for decay fungi and the part of the wood that holds moisture. More loss means more durability and less movement, and also less bending strength.
Show the formula
Mass loss is interpolated across the published treatment bands: 160-180 C rising to 3%, 180-200 C to 6%, 200-220 C to 11%, and above 220 C beyond that, then adjusted by hold duration and by 1.15 for hardwoods. Mass after treatment = mass x (1 - mass loss/100). EMC reduction and the indicative durability class follow from the mass loss.Mass loss is interpolated between the published ITWA treatment bands, not measured for a specific producer. The durability result is capped at Class 2, which is the published ceiling: the International ThermoWood Association classifies Thermo-D as durable under EN 113, and no standard ThermoWood class reaches Class 1. Above 215 C the tool says so, because ITWA gives that as the maximum and the class descriptions do not extend past it.
References and what each one provides
- ThermoWood HandbookInternational ThermoWood AssociationITWA treatment classes, the 215 C maximum, and the published durability rating for each class.
- EN 350CENThe durability class scale the result is expressed on.
Unverified values are marked as such rather than presented as sourced.
Limitations of this calculation
Indicates treatment intensity from mass loss. It does not account for process variation between producers, timber thickness effects, uneven treatment through a charge, or the actual strength reduction in a specific board.
Do not use this for Process validation, treatment certification, or structural design of modified timber.
Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.
- Finnish ThermoWood classes: Thermo-S ~190 °C, Thermo-D ~212 °C
- Mass loss rises with temperature and duration; EMC falls as mass is lost
Formula checked against the sources above by an automated regression test (tests/verify-phase3a.js) that derives each expected value independently of this page. Last reviewed .
How to use this tool
- Enter your project details below, then press Calculate.
- Pick your class target and wood type from the dropdown.
- Fill in treatment temperature, hold duration, initial mass.
- Press Calculate.
Worked example
Common mistakes to avoid
- Confusing mass loss with density change. Mass loss is measured oven-dry to oven-dry — moisture is excluded.
- Expecting higher mass loss to always be better. More treatment means more rot resistance but also more brittleness.
- Comparing mass loss between different species at the same temperature. Species react differently to the same schedule.
Frequently Asked Questions
Wood species data
Density, hardness and movement for 60 timbers
Full listing in the ThermoWood species reference.