The U-value of a building element represents its overall thermal resistance without bridging. In a 2D thermal model, the U-value arrows tell the software which parts of the model represent the main building elements (like walls or floors) and what their U-values are. The correct placement of these arrows is critical for an accurate Psi-value calculation.
The Psi-value is calculated by subtracting the heat loss through the plane elements (as defined by the U-value arrows) from the total heat loss through the entire 2D model. If the U-value arrows are placed incorrectly, the calculation will be inaccurate.
In AutoPSI, the U-value arrows are automatically placed for you in Quick Model and Projects. In Free Model, you have full control over their placement, allowing you to model complex or non-standard junctions.