✅ 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.
Roofs Insulated at the Ceiling
When insulation is at the ceiling level, the attic space is typically ventilated and colder than the living space.
Boundary Setup:
The attic space temperature (TL) is modeled as a separate boundary, usually set to 1°C when the inside is 20°C and the outside is 0°C.
The surface of the ceiling insulation facing the attic is also set to this 1°C boundary condition.
The sloping roof structure itself is not included in the model unless it contains metal rafters, their high thermal conductivity requires that the sloping part of the roof must be included in the model to accurately capture the thermal bridge.
Examples:
Roof Eaves: The model includes the wall and the ceiling. The attic space is treated as a 1°C environment.
Roof Gable: Similar to the eaves, the attic space is set to 1°C.
Party Wall: The junction with a neighboring property's roof is also modeled with the 1°C attic temperature.
Roof eaves (insulated at ceiling)
For a roof insulated at the ceiling level, it's standard practice to ventilate the roof space (loft). In this scenario, the temperature of the roof space, TL, will be very close to the external air temperature, Te.
Note for Non-Ventilated Roofs: If a roof space is intentionally sealed and non-ventilated, you must calculate TL using a heat balance between the U-value of the ceiling (UC′) and the U-value of the roof (UR′)
Roof gable (insulated at ceiling)
Roof Party-wall (insulated at ceiling)
New to this guide? If you're just getting started, we suggest reading our Boundary Type Guide first for reference.
Roofs Insulated on the Slope
When insulation follows the slope of the roof, the attic space is part of the heated interior.
Boundary Setup:
The model includes the insulated roof and the wall.
There is no separate attic temperature; the internal boundary (red line) follows the underside of the sloped insulation.
Examples:
Roof Eaves: The internal boundary extends up the sloped ceiling.
Roof Gable: The model shows the wall meeting the sloped, insulated roof.
Roof eaves (insulated on slope)
Roof gable (insulated on slope)
Roof Party-wall (insulated on slope)
New to this guide? If you're just getting started, we suggest reading our Boundary Type Guide first for reference.