> 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

## 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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