Internal Conditions

In hygrothermal modelling, internal conditions of a building play a critical role in influencing the building's thermal and moisture performance.

Specify Temperature

The indoor temperature of a space impacts the heat flow through building components. High internal temperatures can increase the rate of heat transfer from inside to outside in colder weather and vice versa in warmer weather. This has a corresponding impact on moisture transfer.

The default temperature, as per ASHRAE 160, is 21 °C. This represents a continuously heated space. While this is unlikely to represent most buildings, it typically assumes a worst-case scenario and should remain at 21 °C in most cases.

Specify Relative Humidity

Internal relative humidity levels can affect the moisture transfer within building components. High internal relative humidity can lead to condensation within walls or roofs if these surfaces reach the dew point temperature. This can potentially lead to issues like mould growth and structural damage.

The relative humidity is typically uncontrolled in most buildings and is a function of external and internal conditions.

Air Exchange Rate

Air exchange, also known as air change or ventilation rate, plays a crucial role in hygrothermal simulations. It represents the amount of air that is added to (infiltration) or removed from (exfiltration) a building or a specific room within a given period.

Air exchange rates can vary significantly depending on the type of building, its construction, the effectiveness of its ventilation system, and how airtight the building is. These factors should be considered when defining the air exchange rate for hygrothermal modelling purposes.

Other standards, such as ASHRAE 62.1 for commercial and institutional buildings or ASHRAE 62.2 for residential buildings, would typically be referenced for specific air exchange rates.

Relative Humidity Mode

Two modes are available to account for internal relative humidity, the simplified and intermediate methods. The simplified method determines the design of indoor humidity based on the average daily outdoor temperature. In the Intermediate method, the indoor design humidity is ascertained from hourly weather data and the specific type of HVAC equipment used. Only the Intermidaiate method is suitable for building code compliance.

Moisture Generation Mode

Moisture generation mode refers to the methods or conditions through which moisture is produced or introduced into the indoor environment of a building. It's an essential aspect of simulations because moisture production can significantly affect indoor humidity levels and hence the hygrothermal performance of the building.

Three moisture generation modes are available, number of people, number of bedrooms, and custom.

People - For design purposes, there will be a minimum of two occupants in a space, with an additional person for every bedroom beyond the master bedroom. The specific rates for design moisture generation can be found in ASHRAE 160/DA07.

Bedrooms - For design purposes, the number of bedrooms can also determine moisture generation rates. The following specific rates for design moisture generation can be found in ASHRAE 160/DA07.

• For a 1-bedroom home with two occupants, the moisture generation rate is 7 litres/day or 0.8 x 10-4 kg/s.

• For a 2-bedroom home with three occupants, the rate is 9 litres/day or 1.0 x 10-4 kg/s.

• For a 3-bedroom home with four occupants, the rate is 10 litres/day or 1.2 x 10-4 kg/s.

• For a 4-bedroom home with five occupants, the rate is 11 litres/day or 1.3 x 10-4 kg/s.

• Add one occupant for each additional bedroom beyond four and increase the moisture generation rate by 1 litre/day or 0.1 x 10-4 kg/s.

Custom - For buildings with different occupancy types, the design values for moisture generation should be specified in kilograms per second (kg/s) and obtained from a mechanical services engineer.