ThermX vs. LBNL THERM An honest feature comparison for 2D heat transfer analysis

THERM is the free, Windows-only reference tool from Lawrence Berkeley National Laboratory. ThermX is a modern, cross-platform alternative built around CEN/EN ISO workflows — it reads and writes THERM's THMX files, runs on Windows, macOS, Linux and in the browser, and costs €10 once. Here is how they actually compare.

🔥 ThermX product page 🔁 Switching from THERM guide 🎓 Tutorials

The Short Version

Choose ThermX if…

…you work on macOS or Linux, want to run analyses in a browser, or your deliverables are CEN/EN ISO based — ψ-values per EN ISO 10211 and fRsi condensation checks per EN ISO 13788 are built in, with no manual post-processing.

Stay with THERM if…

…you submit NFRC certification simulations (NFRC procedures specify THERM/WINDOW), or your workflow is deeply tied to LBNL WINDOW glazing-system integration. THERM is free, mature, and remains the North American reference tool. You can also use both — ThermX reads and writes THMX files.

Feature-by-Feature Comparison

Feature	ThermX 1.x	LBNL THERM 8
Platforms	Windows, macOS, Linux, Browser	Windows only
Runs in a web browser	✓ thermx.modeltok.com	—
Price	€10 one-time	Free
Installer size	~4–7 MB	~138 MB
THMX files (read & write)	✓	✓ native format
ψ-value per EN ISO 10211 (built-in)	✓ Component Editor	Manual post-processing
fRsi per EN ISO 13788 (built-in)	✓ automatic, critical point marked	Manual, from surface temperatures
U-factor (ISO 15099)	✓	✓
CEN & NFRC boundary condition presets	✓	✓
EN ISO 10211 Annex A validation cases built in	✓ Case A.1 & A.2	—
Material database	37+ materials (EN ISO 10456) + custom	✓ THERM library
Automatic meshing	Delaunay + Ruppert refinement, quality sliders	✓ quadtree mesher
GPU-accelerated solver	✓ WebGPU / wgpu	—
DXF geometry import & export	✓	Underlay (tracing) only
DWG import & export	✓	—
Drawing sheets & PDF reports	✓ Sheets tab, vector PDF	✓ print report
CMA spacer/divider files	✓ THMX import	✓ with WINDOW integration
Technology	Tauri 2 (Rust) + TypeScript	.NET WinForms

Comparison reflects ThermX 1.7 and publicly documented LBNL THERM 8 capabilities. Spotted something out of date? Tell us and we will fix it.

Platforms: Windows-Only vs. Everywhere

THERM officially runs on Windows only. On macOS or Linux you need a virtual machine or Wine — workable, but slow to set up and unsupported. ThermX ships native installers for Windows (.msi), macOS (.dmg, universal binary) and Linux (.deb), each only a few megabytes.

There is also a full browser version: the same geometry editor, mesher and FEM solver running on WebAssembly and WebGPU. Nothing to install — useful for quick checks, teaching, and locked-down office machines.

EN ISO 10211 & EN ISO 13788 Workflows

THERM was built around NFRC window rating; European thermal-bridge deliverables need extra hand work. ThermX implements the CEN/EN ISO methods directly:

ψ-value (EN ISO 10211) — define the flanking components (1D U-values and lengths) in the Component Editor and ThermX computes Ψ = L_2D − Σ(U_j · l_j) automatically. In THERM you read off heat flows and finish the calculation in a spreadsheet.
fRsi (EN ISO 13788) — the temperature factor fRsi = (T_si,min − T_e) / (T_i − T_e) is computed automatically and the critical (coldest) surface point is marked on the canvas for condensation risk assessment.
EN ISO 10456 / 6946 — the material library uses EN ISO 10456 tabulated conductivities, and boundary conditions apply Rsi/Rse surface resistances per EN ISO 6946.
Validation — EN ISO 10211 Annex A reference cases (A.1, A.2) are built into the Validation panel, so you can verify solver accuracy yourself.

Step-by-step guides: how to calculate a ψ-value, fRsi condensation risk check, and modelling a balcony thermal bridge.

THMX Compatibility: Keep Your THERM Files

Switching cost is the biggest worry, so ThermX treats THERM's THMX format as a first-class citizen: File → Import THMX reads materials, boundary conditions, polygons, BC assignments and mesh settings; File → Export THMX writes files that open in THERM 7.x and 8.x. CMA (Component Modeling Approach) spacer and divider files import too.

That means you can adopt ThermX gradually — or use it alongside THERM on projects where a colleague stays on Windows. See the Switching from THERM guide for a concept-by-concept mapping and an import walkthrough.

Where THERM Is Still the Better Choice

An honest comparison cuts both ways. THERM is free, has nearly three decades of use and validation behind it, and a large community of users and tutorials. If you produce NFRC certification simulations, NFRC procedures specify THERM and WINDOW — ThermX is not a substitute for certified rating work. THERM's tight integration with LBNL WINDOW for glazing systems is also more complete than ThermX's CMA file import.

ThermX's case is different: native EN ISO ψ/fRsi outputs, three desktop platforms plus the browser, a modern editor, and DXF/DWG geometry exchange — for the price of a lunch.

Frequently Asked Questions

Can ThermX open my existing THERM files?

Yes. ThermX reads and writes THMX files, including CMA spacer/divider files. Geometry, materials, boundary conditions and BC assignments are imported; you re-mesh and re-solve in ThermX.

Is there a THERM for Mac or Linux?

LBNL does not ship THERM for macOS or Linux. ThermX runs natively on both (and in any modern browser), which is exactly the gap it was built to fill.

Is ThermX free?

ThermX costs €10 as a one-time purchase with free updates within the major version — no subscription. THERM is free; you are paying for cross-platform support, built-in EN ISO methods and active development.