# Determine "Effective thickness of cavity behind exterior baffle"

I appreciate your help in advance.
I would like to know how to determine the field named "*Effective thickness of cavity behind exterior baffle*" which appears in the class "*SurfaceProperty:ExteriorNaturalVentedCavity*" from EnergyPlus.
I have found the following definition:

"*This field is used to enter a nominal gap thickness (m) for the collector. If the baffle is corrugated, use the average depth. This distance value is only used when the collector is near horizontal to determine a length scale in the vertical direction for buoyancy calculations. For example, if the collector is mounted on a flat roof, its tilt-adjusted height is zero and the program will use this gap thickness as a length scale rather than the height from the previous field.*"

Does this mean that if I am using "*SurfaceProperty:ExteriorNaturalVentedCavity*" to simulate a vertical ventilated façace, this field is not used by EnergyPlus? This way, I do not find any other field to indicate the thickness of the cavity to EnergyPlus.

I have also asked ChatGPT. I know it can make mistakes, but I have tried. This has been its answer:

----------Given the specific details of your **vertical, ventilated cavity**:
- **Cavity thickness**: 0.3 meters (300 mm)
- **Emissivity**: 0.85 (relatively high)
- **Orientation**: Vertical
- **Ventilated**: Airflow within the cavity
Let's ensure that the **effective thickness** estimate is accurate, taking into account the vertical orientation and ventilation.

### Step-by-Step Process:

**Vertical Cavity and Convection**:- In a
**vertical cavity**, convective currents are stronger due to the natural buoyancy of warm air rising. This creates more**air movement**within the cavity, which increases**convective heat transfer**and decreases the**thermal resistance**of the cavity. - Since the cavity is also
**ventilated**, external air is entering the cavity, which further reduces its thermal resistance by enhancing convection.

- In a
**Emissivity Impact**:- The
**high emissivity**(0.85) indicates significant**radiative heat transfer**between the surfaces of the cavity, which further reduces its thermal insulation performance. Radiative heat transfer between surfaces becomes a more dominant factor when emissivity is high, causing more heat to be transferred.

- The
**R-Value for a Vertical Ventilated Cavity**:- For a
**ventilated vertical air cavity**with**high emissivity**and a thickness of**0.3 m**, the**ASHRAE Handbook**and similar sources suggest an**R-value**(thermal resistance) in the range of**0.10 m²·K/W to 0.12 m²·K/W**. - This lower R-value reflects the fact that vertical ventilation significantly increases heat transfer through convection, reducing the overall insulation provided by the cavity.

- For a
**Calculate Effective Thickness**: To find the**effective thickness**in EnergyPlus, we convert the estimated R-value into an equivalent**thickness of an insulating material**with known thermal properties (e.g., a material with**thermal conductivity λ = 0.04 W/m·K**).

Using the formula:

Effective Thickness = Rvalue / Thermal conductivity

For an **R-value of 0.10 m²·K/W** (a reasonable estimate for your case):

Effective Thickness = 0.1/0.04 = 2.5 cm = 25 mm

For an **R-value of 0.12 m²·K/W** (upper estimate):

Effective Thickness = 0.12/0 ...