# What Transient Simulations Are?

Nowadays (2010) the building components and assemblies are evaluated in most cases by calculation methods with the stationary (constant) boundary conditions. This means that the calculations for heat transport (energy evaluation, heat loss, temperature factor, thermal bridges, etc.) and moisture transport (annual balance of evaporated and condensate water vapour) are carried out at constant external temperature and relative humidity. Some energy calculations are calculated at least monthly average outdoor air temperature. However, all these calculations ignore the following stationary outdoor climatic phenomena:

- sunlight (shortwave radiation),
- longwave radiation,
- rain,
- wind.

Current computational methods of heat and moisture transport did not take into account the certain physical phenomena that occur in everyday situations and can not be described by stationary methods. These physical phenomena are:

- heat storage,
- moisture storage,
- capillary condensation,
- transport of liquid moisture,
- effect of moisture on thermal conductivity.

**Transient simulations of coupled heat and moisture transport**
(eg Software WUFI®
) are numerical calculations using the transient boundary conditions
- often with a time step within a few minutes. Such a short time step
can include in the calculation of the short but significant climatic
phenomena (shortwave and longwave radiations, rain, wind, etc.)

**Transient simulations** can better describe physical phenomena
that occur widely in building materials and components, and that are
not able to be described by steady state methods.

For these reasons,** transient simulations result in significant improvement
in the accuracy and reproducibility of hygrothermal simulations**
and they are **significantly closer to reality**.

*Page created: 29/11/2010; last update: 30/11/2010.*