> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/user-guide/workflows/moisture/cavity.md).

# Cavity

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## 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.


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