✅ For Best Experience: This article covers advanced technical stuff. To get the most out of this guide, we recommend starting with our introduction to the topic: Boundary Types Guide.
Adiabatic Boundary
When modeling a junction around an opening, you're calculating the heat loss through the wall or roof structure around the frame, not through the window or door itself (as its performance is calculated separately).
To do this, you must apply an adiabatic boundary (a zero heat flow line) where the frame meets the wall or roof. Think of this as a thermal "stop sign" 🛑 in the model, telling the software that heat does not cross this specific line. This isolates the junction's performance for an accurate calculation.
Specific Cases:
Lintel, Jamb, and Sill: For windows and doors, these junctions are modeled with the internal (red) and external (blue) boundaries on the wall surfaces and an adiabatic (green) boundary at the frame.
Metal Lintels: If a lintel has a sloped face, it should be modeled using a series of steps to approximate the slope if your software doesn't support diagonal lines.
Rooflights: The model includes an adiabatic boundary where the rooflight frame (or its upstand) meets the roof structure.
New to this guide? If you're just getting started, we suggest reading our Boundary Type Guide first for reference.
How to Model Lintel
Modeling Sloped Surfaces (Metal Lintels). Some modeling software works best with simple rectangular shapes. If you need to model a component with a sloped face, like certain metal lintels, the standard method is to approximate the slope using a series of steps.
Modeling Perforated Metal Plates. Modeling every single hole in a perforated metal plate within a large building model is often impractical and computationally intensive. Instead, the standard approach is to calculate an "equivalent thermal conductivity" for the plate and use that value in your main model.
How to Model Jamb
How to Model Sill
Any internal (or external sill) should be included in any model of the sill junction and, in the case of a sill that is integral with the frame, the adiabatic boundary at the frame includes the vertical ‘connection’ of the frame with any integral sill.
How to Model Rooflight
When you model the junction where the rooflight meets the roof structure to find its thermal bridge value (Ψ-value), you should use an adiabatic boundary (zero heat flow) at the point where the frame or upstand connects to the roof. This ensures you are only calculating the heat loss around the opening, not through the rooflight unit itself.
For regulatory purposes, the value you typically need to check against building code requirements is the U-value of the rooflight unit itself (glazing and frame), not the overall U-value of the entire opening.
