Add Subfloor

A subfloor is the space beneath your floor system, often raised above the ground and ventilated. While it might seem like just a structural gap, it plays a major role in how your building handles heat loss and moisture. That’s why subfloors are included in Total R-value calculations, following guidance from CIBSE Guide A.

Floor

To begin setting up a subfloor, click 'Add Subfloor' from the main area of the Floor Designer. Start by filling out the floor conditions.

Soil Type

Start by selecting the Soil Type beneath the floor. Options include:

• Clay (1.5 W/m·K)

• Sand (2.0 or 3.0 W/m·K)

• Rock (3.5 W/m·K)

This value sets the soil’s thermal conductivity, which affects how heat transfers from the floor to the ground.

Perimeter of Exposed Floor

Set the Perimeter of Exposed Floor to account for ground heat loss calculations.

Choose your floor structure, such as slab-on-ground or suspended.

Ventilated Cavity

Next, define the air space beneath the floor.

Floor Height Above Ground

Set in millimetres. This determines whether the space counts as ventilated or not.

Subfloor Ventilation Area

Enter the area of ventilation openings per metre of perimeter, in m²/m.

Wind Speed

Provide the average wind speed at 10 m above ground level, which helps determine ventilation rates.

Wind Shielding Factor

Choose from:

• Sheltered – limited wind exposure

• Average – typical suburban conditions (default)

• Exposed – open terrain with strong winds

These values influence how air moves through the cavity and how effectively it removes heat and moisture.

Wall

Finally, set the properties of the surrounding walls enclosing the ventilated space.

Wall Width

Total perimeter of walls enclosing the cavity.

Wall U-value

Thermal transmittance of the wall material.

Wall Depth into Ground

How far the wall extends below the ground surface.

These inputs affect the way heat interacts with the ground and the stability of the thermal environment under the building.