Window U-Factors & Thermal Bridges with LBNL WINDOW 8

Windows are the weakest thermal link in most facades, and they are not a single material — the centre of the glass, the edge of the glass, the frame and the way the window meets the wall all behave differently. LBNL's WINDOW program (version 8) is the tool that pulls those pieces into one whole-window performance figure, and it works hand-in-hand with THERM to handle the thermal bridges inside and around the window.

What WINDOW 8 calculates

WINDOW computes the whole-product U-factor and Solar Heat Gain Coefficient (SHGC) of a window, along with visible transmittance and condensation resistance, following NFRC and ISO methods. It does this by area-weighting three zones that each perform differently:

• Centre-of-glass — built from a glazing system (panes, coatings, gas fills) defined in WINDOW's glazing and Optics libraries.
• Edge-of-glass — the zone near the spacer, where the frame and spacer bar create a thermal bridge that raises heat loss.
• Frame — whose thermal performance is modelled in THERM and imported into WINDOW.

The THERM + WINDOW workflow

The two programs are designed to be used together:

1. Build the glazing system in WINDOW (or import optical data via Optics/IGDB).
2. Draw the frame cross-section in THERM, import that glazing system into the sightline, and let THERM solve the 2D heat transfer through frame, spacer and edge-of-glass.
3. Send THERM's frame and edge-of-glass U-factors back into WINDOW.
4. WINDOW area-weights all the zones into the total window U-factor and SHGC.

This is why frame and spacer choices matter so much: they are thermal bridges that THERM quantifies and WINDOW rolls into the number a compliance model or facade spec actually uses.

The install psi-value: the bridge people forget

Even a superb window loses performance if it is badly installed. The junction where the window meets the wall — the reveal, sill and head — is itself a linear thermal bridge with its own installation psi-value. That is modelled in THERM as a junction (window plus surrounding construction), and it is easy to overlook: a low-U window set into a poorly detailed opening can underperform its rating in practice. Treating the install line as a thermal bridge in its own right closes that gap.

Why it matters for compliance and comfort

Whole-window U-factors feed directly into energy models and code compliance, while the edge and install details drive the coldest interior surface temperatures — and therefore condensation risk — around glazing. Modelling windows properly in WINDOW and THERM gives both a defensible performance number and a comfortable, condensation-free result. For UK junction psi-values more broadly, teams often pair this with AutoPSI.

This article is general guidance and reflects information available at the time of writing. WINDOW and THERM are developed by LBNL; always follow the rating method (NFRC/ISO) required for your project.