Lawrence Berkeley National Laboratory (LBNL) offers two free, Windows‑based tools for analysing the energy performance of windows and building components: WINDOW and THERM. Both are widely used by manufacturers, test labs, and building compliance specialists in the UK and around the world. This guide explains how to download the latest versions, what each tool does, where to find official manuals, and what you need to know about compatibility.
How to Download WINDOW and THERM Software from LBNL
WINDOW Software
Latest version: 7.8. Free download from LBNL. Used for whole‑window thermal analysis.
THERM Software
Latest version: 7.8. Free download from LBNL. Used for detailed 2D heat transfer analysis.
User Manuals
Official PDF guides available for both WINDOW and THERM – linked directly in this article.
Compatibility
Reported to work on Windows 7, 8, 10, and 11. See the clarity section below for details.
- Both WINDOW and THERM are free, publicly funded tools developed by LBNL.
- WINDOW 7.8 includes advanced glazing system analysis, gas‑fill optimisation, and frame‑spacer modelling.
- THERM 7.8 complements WINDOW by providing finite‑element analysis for edge‑of‑glass and frame heat transfer.
- The official download page at windows.lbl.gov is the only trusted source – avoid third‑party mirrors.
- UK users can use these tools to comply with Part L building regulations and generate BFRC energy ratings.
| Software | Latest Version | Developer | Primary Use | Price | OS Support |
|---|---|---|---|---|---|
| WINDOW | 7.8 | LBNL (USA) | Whole‑window U‑value, SHGC, and VT calculation | Free | Windows 7/8/10/11 |
| THERM | 7.8 | LBNL (USA) | 2D heat transfer analysis for frames and glazing edges | Free | Windows 7/8/10/11 |
To download WINDOW 7.8, visit the official WINDOW download page. The page offers a full installer for first‑time users and a smaller package for those who already have a previous version. For THERM 7.8, go to the official THERM download page. LBNL notes that WINDOW 7 and THERM 7 can be installed alongside version 8 on the same computer.
What Are the Main Features of WINDOW 7.8?
WINDOW 7.8 is built for thermal and optical analysis of windows, glazing systems, and other fenestration products. It calculates total window performance indices such as U‑value (thermal transmittance), solar heat gain coefficient (SHGC), and visible transmittance (VT).
Under Part L of the Building Regulations, UK manufacturers often need U‑values and SHGC data for compliance. WINDOW provides the industry‑standard output used by BFRC and other certification bodies.
Glazing and gas‑fill modelling
The 7.8 release includes updated gas mixtures and an expanded glazing database. Users can model double and triple glazing with varying gas fills, low‑e coatings, and spacer materials.
Frame and spacer analysis
WINDOW handles the whole window as a system, including frame and edge effects. For more detailed frame analysis, LBNL recommends pairing it with THERM.
NFRC compliance
Version 7.8.71 was approved by the National Fenestration Rating Council (NFRC) in 2023 for simulation and certification work. Later builds (e.g., 7.8.80) are available for testing and development.
What Is THERM Software and How Is It Used?
THERM is a finite‑element analysis tool that models two‑dimensional heat transfer through building components such as window frames, doors, curtain walls, and glazing edges. It is frequently used alongside WINDOW to refine the thermal performance of frame and spacer details.
Where THERM fits in the workflow
WINDOW provides the whole‑window result, but the detailed geometry of frames and edge seals can significantly affect the final U‑value. THERM allows you to draw or import the cross‑section, assign materials, and run a 2D heat‑flow simulation. The result can then be fed back into WINDOW for an accurate overall rating.
Use WINDOW for the basic glazing system, then export the geometry to THERM for detailed frame modelling. The updated U‑value from THERM can be re‑imported into WINDOW to produce a certified whole‑window rating.
ISO 15099 implementation
LBNL states that WINDOW 7 and THERM 7 implement the ISO 15099 algorithms, ensuring consistency with international standards. The documentation page links technical notes and supporting papers that describe the added capabilities in the 7.x generation.
Availability and licensing
THERM is free to download, as confirmed by LBNL’s public hosting and by demonstration material. There is no UK‑specific restriction; it is a globally downloadable Windows application.
Where Can I Find the LBNL Window Manual and Installation Help?
LBNL maintains a documentation page that provides user manuals for both tools. The THERM 7 User’s Manual is still being updated section by section rather than as a single static document. For WINDOW 7, a complete PDF manual is available. Additionally, LBNL offers an NFRC WINDOW / THERM Simulation Manual that covers the workflow for certification.
Only download the installers and manuals from windows.lbl.gov. Third‑party mirrors may contain outdated or altered files. LBNL does not host its software on any other site.
Installation instructions are included in the download packages. LBNL advises that a full installation is required for first‑time users, and that later versions create new directories so multiple major versions (e.g., 7 and 8) can coexist. Some THERM 7.8 installer packages are explicitly not compatible with THERM 7.4, 7.6, or 7.7 – treat 7.8 as a separate installation line.
What Is the Version History of WINDOW and THERM?
- – WINDOW 7.6 added support for dynamic windows and electrochromic glass.
- – THERM 7.6 brought enhanced CAD import and better mesh generation.
- – WINDOW 7.7 improved frame modelling and updated NFRC compliance features.
- – Release of WINDOW 7.8 and THERM 7.8, including updated gas mixtures, new glazing databases, and bug fixes. NFRC approved version 7.8.71 for certification use from October 2023.
This timeline is based on LBNL release notes available on windows.lbl.gov.
What Is Certain and Uncertain About LBNL Software Compatibility?
| Established information | Information that remains unclear |
|---|---|
| Both WINDOW and THERM are free to download and use. | Exact system requirements for Windows 7 – you may need a specific .NET Framework version; check the LBNL site. |
| The official download site is windows.lbl.gov. | Whether all features are available “for free” – in practice, all features are free, but some advanced databases may require registration. |
| Version 7.8 is the latest as of 2025. | Whether LBNL formally guarantees Windows 7 support – the visible pages do not explicitly name Windows 7. |
| WINDOW and THERM can be installed alongside version 8. | There is no official Mac version, but the tools can be run via virtualisation software such as Parallels or Wine. |
Why Does WINDOW Software Matter for UK Energy Compliance?
The UK government’s Part L building regulations require accurate thermal performance data for windows. WINDOW software (together with THERM) is the industry‑standard tool used by test labs and manufacturers to generate BFRC energy ratings. The tools are free, developed by a world‑class US lab, and widely accepted by certification bodies.
UK users can also consult the UK Government Buying Standards for Windows for further context on performance requirements.
For other UK‑focused product guides, you may find the Instax Mini 99 – Complete UK Guide to Price Modes and Film and the Instax Mini – 2025 UK Buying Guide & Comparison useful.
What Do the Official Sources Say About WINDOW and THERM?
“WINDOW is a publicly funded computer program used for calculating total window thermal performance indices.”
Windows & Daylighting, LBNL
“THERM is a state‑of‑the‑art computer program developed at LBNL to model the two‑dimensional heat transfer through building products.”
THERM User Manual, LBNL
What Should You Know About LBNL WINDOW and THERM?
WINDOW and THERM are free, powerful tools for anyone involved in window energy performance analysis. They are the de‑facto standard in the UK fenestration industry and are essential for generating compliant BFRC ratings. Always download from windows.lbl.gov, check the latest version numbers, and use the official manuals for guidance.
Frequently Asked Questions
What is B&M Windows?
B&M Windows is a UK double‑glazing company. It is unrelated to the LBNL software. Ignore it if you are searching for WINDOW/THERM tools.
Can I use these tools on a Mac?
WINDOW and THERM are designed for Windows only. No official Mac version exists. You can run them in a virtual machine (e.g., Parallels) or use Wine.
Are there online alternatives?
LBNL does not offer browser‑based versions. The desktop software is required for full functionality.
What is the IGDB?
The International Glazing Database (IGDB) is a collection of optical and thermal data for glazing products. It is used within WINDOW to model real glass types.
Do I need to register to download?
No registration is required for the basic downloads. Some advanced database updates may request contact information.
Is THERM 7.8 compatible with Windows 7?
LBNL does not explicitly list Windows 7 support on the current download pages. The software is described as Windows‑based, but you should check the installer’s system requirements before installing.
Which file should I download on the WINDOW page?
First‑time users should choose the “Full” installer. The smaller installer is for those who already have a previous version of WINDOW 7 installed.
Can I use WINDOW and THERM for UK BFRC certification?
Yes. UK test labs and manufacturers use WINDOW and THERM to produce the U‑value and SHGC data required for BFRC ratings under Part L.
Where can I find the WINDOW User Manual PDF?
LBNL provides the WINDOW 7 User Manual as a PDF on its documentation page. You can also access it via the direct PDF link.
How do I know which version of THERM to install?
The latest stable version is 7.8.80. If you need NFRC‑certified simulation, use version 7.8.71, which is the approved release for certification purposes.
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