Cavity

Cavity Ventilation Rate

Cavity ventilation rate refers to the rate at which air moves into and out of a cavity within a building structure, such as a cavity wall or a ventilated rainscreen. Ventilation can help remove moisture from a cavity, reducing the risk of condensation and related problems. In hygrothermal modelling, the cavity ventilation rate is a critical factor that can affect the moisture balance in building elements.

Fixed Ventilation Rate

Configure a specified fixed air change rate representing the constant rate of air change per hour.

ASHRAE Fundamentals 2009 (Fixed)

ASHRAE Fundamentals 2009 Fixed air change rate due to wind and thermal forces, based on provided inputs.

Wind Speed m/s

Configure the speed of the outdoor air flow driving wind-induced ventilation through the opening.

Opening Effectiveness

Configure a dimensionless factor (typically 0.5–0.6 for perpendicular winds, 0.25–0.35 for diagonal) accounting for how effectively wind direction drives air through the opening.

Opening Inlet Area

Configure the free (unobstructed) cross-sectional area of the opening through which air enters.

Discharge Coefficient

Configure a dimensionless factor (typically ~0.6–0.65) accounting for flow contraction and friction losses as air passes through the opening.

Height Difference m

Configure the vertical distance between the inlet and outlet openings, which drives buoyancy (stack-effect) airflow.

Cavity Temperature °C

Configure the air temperature inside the cavity or ventilated space, used to determine the buoyancy driving force.

Outside Temperature °C

Configure the ambient outdoor air temperature, compared against the cavity temperature to calculate the stack-effect pressure difference.

ASHRAE Fundamentals 2009 (Dynamic)

ASHRAE Fundamentals 2009 Dynamic air change rate due to wind and thermal forces, based on provided inputs. The dynamic air change rate uses a an EPW file, and therefore does not require direct inputs for Wind Speed, Cavity Temperature and Outside Temperature.

Emissivity

Configure the Emissivity of the bounding surfaces of all cavities. Cavities default to 0.9.