> 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/model.md).
# Model
## Orientation
Orientation refers to the geographical direction a building or a part of the building (like a wall or window) faces. Orientation can significantly impact the amount and intensity of solar radiation a building receives, which in turn affects the thermal and moisture dynamics within the building.
## 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 rain screen. 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.
## Floor Area
Floor area refers to the total area of the floor of a space, measured in square meters. In hygrothermal modelling, floor area directly correlates to internal moisture loading, with larger floor areas typically having lower moisture loading than smaller areas.
## Floor to Ceiling Height
This is the vertical distance from the floor to the ceiling of a room. It can influence the volume of air in a room and thus impact internal moisture loading.
## Surface Resistances
In the context of hygrothermal modelling, surface resistances represent the resistance to heat and moisture transfer at the internal and external surfaces of a building element (like a wall or roof).
### Custom Resistances
{% hint style="info" %}
This group of inputs describe concepts applied to heat transfer surfaces that are of an advanced nature. Careful consideration must be given before using these. [More](https://bigladdersoftware.com/epx/docs/8-0/input-output-reference/page-012.html).
{% endhint %}
#### Exterior Convection Coefficient
The rate of convective heat transfer between the exterior surface of a building component and the surrounding air. It represents the effectiveness of heat exchange at the exterior boundary.
#### Cavity Convection Coefficient
The convective heat transfer coefficient within a cavity space, such as within an insulated wall or roof assembly. It characterizes the rate of heat transfer through convection within the cavity.
#### Interior Convection Coefficient
The convective heat transfer coefficient between the interior surface of a building component and the indoor air. It reflects the efficiency of heat exchange at the interior boundary.
#### Exterior Vapour Coefficient
The rate of moisture transfer between the exterior surface of a building component and the surrounding air. It indicates the effectiveness of moisture exchange at the exterior boundary.
| Paint Type | Thickness (m) | Sd Value (m) | Vapour Transmission Coefficient (kg/(m²⋅s⋅Pa)) |
| -------------------------- | ------------- | ------------ | ---------------------------------------------- |
| Speckel Default | 0.0002 | 0.032 | 0.0000000065 |
| Water-based Emulsion Paint | 0.0002 | 0.04 - 0.1 | 0.000000005 - 0.000000002 |
| Acrylic Paint | 0.0002 | 0.2 - 0.4 | 0.000000001 - 0.0000000005 |
| Latex Paint | 0.0002 | 0.08 - 0.16 | 0.0000000025 - 0.00000000125 |
| Silicate Paint | 0.0002 | 0.006 - 0.02 | 0.0000000333 - 0.00000001 |
| Oil-based Paint | 0.0002 | 0.4 - 1.0 | 0.0000000005 - 0.0000000002 |
| Vapour-Barrier Paint | 0.0002 | 2.0 - 4.0 | 0.0000000001 - 0.00000000005 |
#### Cavity Vapour Coefficient
The moisture transfer coefficient within a cavity space, such as within an insulated wall or roof assembly. It characterizes the rate of moisture transfer through the cavity.
#### Interior Vapour Coefficient
The moisture transfer coefficient between the interior surface of a building component and the indoor air. It reflects the efficiency of moisture exchange at the interior boundary.
---
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