ASHRAE Fundamentals Cavity Ventilation

Cavity ventilation in wall systems can provide an important mechanics to increase the drying potential in wall and roof systems. Academically, there are no agreed approaches to cavity ventilation from first principles, equations, CFD and physical testing.

The following equations are based on the Bernoulli equation, as per ASHRAE Fundamentals – Natural Ventilation 16.12. They have been adopted as the minimum cavity ventilation inputs required to generate a theoretical air flow rate (Q) at the openings. Q is then converted to air changes per hour (ACH) based on the proposed cavity dimensions.

Where an air flow rate (Q) is know or a more advanced calculation method is nominated, it can be directly specified as a fixed air change rate.

Flow Caused by Wind Only

The equation quantifies airflow rate (Q) driven exclusively by wind through ventilation inlet openings, considering the effectiveness of openings (Cv), the area of openings (A), and wind speed (U) while excluding thermal influences. It accounts for various aspects of wind, including average speed, prevailing direction, seasonal and daily variations, and local obstructions. This equation serves as a tool for determining optimal opening sizes to achieve desired airflow rates based on wind conditions.

Q_{\text{wind}} = C_v \cdot A \cdot U

Qwind = airflow rate, m³/s

Cv = effectiveness of openings (Cv is assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.35 for diagonal winds)

A = free area of inlet openings, m²

U = wind speed, m/s

Flow Caused by Thermal Forces Only

The equation calculates airflow rate (Q) due to thermal forces (stack effect) based on factors like opening area, temperature difference, and height from lower opening to neutral pressure level (NPL), considering indoor and outdoor temperatures. It incorporates a discharge coefficient to account for viscous effects and adjusts for temperature differences based on indoor and outdoor temperatures.

Q_{\text{thermal}} = C_D \cdot A \cdot \sqrt{\frac{2g \Delta H_{\text{NPL}} \left( T_i - T_o \right)}{T_i}}

Qthermal = airflow rate, m³/s

CD = discharge coefficient for opening

ΔHNPL = height from midpoint of lower opening to NPL, m

Ti = cavity temperature, K

To = outdoor temperature, K

The above applies when Ti > To. If Ti < To, replace Ti in the denominator with To, and replace (Ti − To) in the numerator with (To − Ti ). An average temperature should be used for Ti if there is thermal stratification. If the building has more than one opening, the outlet and inlet areas are considered equal. The discharge coefficient CD accounts for all viscous effects such as surface drag and interfacial mixing.