Note: Boundary Types are predefined on all of our Quick Model Templates. For more flexibility, you can draw and define Junction Boundaries using our Free Draw Template.
Boundary conditions define the environment on either side of the construction. In thermal modelling, we typically use three types of boundary conditions:
Boundary Condition | Description | Application in AutoPSI |
Internal | Represents the heated internal environment. It has a specific temperature and surface resistance. | Applied to all surfaces of the model that are exposed to the inside of the building. |
External | Represents the unheated external environment. It also has a specific temperature and surface resistance. | Applied to all surfaces of the model that are exposed to the outside air or unheated spaces like ventilated attics. |
Adiabatic | A line of symmetry across which no heat is allowed to flow. | Used to represent a perfect continuation of the building element, such as the centreline of a party wall or an intermediate floor. |
Getting the boundary conditions right is fundamental to accurate thermal analysis. AutoPSI simplifies this process by pre-setting the boundary conditions in Quick Model and Projects. In Free Model, you can draw the boundary conditions directly onto your model, giving you the flexibility to model any situation. Remember ventilated airspaces aren't included in the calculation and adjustments to boundary conditions are needed*.
Heat Flow Direction | Boundary Type | Resistance |
Downwards (Floor Slab) | Internal | 0.17 |
Downwards (Suspended Floor) | Internal | 0.10 |
Horizontal (Wall) | Internal | 0.13 |
Upwards (Roof) | Internal | 0.10 |
External Exposed | External | 0.04 |
Underfloor space downwards heat flow | External | 0.10 |
Underfloor space Horizontal heat flow | External | 0.13 |
*Horizontal (Wall) Ventilated Cavity | External | 0.13 |
*Upwards (Roof) Ventilated Cavity | External | 0.10 |
*Low E layer next to ventilated cavity | External | Calculate Resistance |
Other Colors: Different colors may be used to represent other specific environments, such as a ventilated attic or a crawl space under a floor, which will have temperatures different from the standard internal or external conditions.