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Advantages and Disadvantages of Transient Simulations

The use of transient simulations of coupled heat and moisture transport in building materials and components, of course, brings benefits but also some technical disadvantages. It is important that the benefits outweigh.

Disadvantages of Transient Simulations

Higher demands on the knowledge of materials - if you want to get better, more accurate results, it is logical that it should be used more accurate input of material properties than the commonly used in hygrothermal calculations. Usually there are two physical parameters; the thermal conductivity, λ [W/mK] and the water vapour diffusion resistance factor (µ-value), µ [-].

When using transient simulation tool - software WUFI® it is good to known the other physical properties of the material: bulk density, porosity, specific heat capacity, moisture storage function (sorption isotherms), the dependence of thermal conductivity on moisture and temperature, water absorption coefficient or moisture-dependent µ-value. The advantage is that it is not necessary to obtain all physical parameters for all in-build materials and most of the physical parameters can be determined very accurately by simple measurements. Another advantage is that the WUFI® software has an extensive database of materials for which physical parameters were already measured.

Higher demands on the knowledge of boundary conditions - for accurate and realistic assessment of coupled heat and moisture transport with the help of numerical simulations, it is necessary to know the exact outdoor climate conditions of a building location and initial conditions of in-built materials.

Outdoor climate conditions mean at least the hourly values of the following quantities: exterior air temperature, outdoor relative humidity, rainfall, wind direction and velocity, and direct and diffuse solar radiation and long wave radiation. The advantage is that the WUFI contents many climatic files from around the world. The disadvantage is that the climatic files about the Czech and Slovak Republic are still missing (2010) and it is necessary buy them.

The initial material condition is meant to determine the amount of moisture in building materials and components. The amount of moisture is necessary to determine mainly for reconstructions that have problems with higher level of in-build moisture.

Higher demands on the calculation - the transient calculations using computer simulations take more time than the commonly used steady-state calculation methods, but the difference is not significant.

The calculation for one-dimensional numerical simulations of coupled heat and moisture transport lasts on common office computer in seconds. Advanced calculations are for a two-dimensional field, which depends on the number of elements, in which a detail is divided and the time period of the calculation. Calculation of the 2D field lasts in the tens of minutes. Numerical simulations of coupled heat and moisture transport for the whole building are the most challenging. They can last several hours.

Advantages of Transient Simulations

Results close to reality - A great advantage of transient numerical simulations of coupled heat and moisture transport in building materials is a significant approach to reality, that has been validated in many in-situ measurements mainly in the Fraunhofer Institute of Building Physics, Germany for last 15 years. Moreover the WUFI® also meets the requirements of standard EN 15026 for numerical software.

The advantage of transient numerical simulation is that we can determine with a high degree of accuracy how will be heat and moisture transport over the time at your preferred location in the building components, which will help to optimize the newly designed assemblies (components) and also to select suitable materials for the reconstruction of a original building.

Savings in operating and capital costs - Precise determination of heat loss of the building leads to later on the optimal design of the building envelope, as well as the optimal dimensioning of the building heating system. For these reasons, transient numerical simulations save the operating cost of the building.

Possibility to simulate events that otherwise can not simulate - Using transient numerical simulation tool - the WUFI® software allows to simulate physical phenomena, which would not be possible or it would be difficult to measured in reality (for example, use of climate conditions of a completely different location or to simulate moisture transport without capillary transport).

It can be useful for testing of new building systems in a new climate environment or for using new materials (elements) in the building exterior wall/roof assemblies. The use of these simulations led to the discovery of "The smart vapor retarder" that releases moisture depending on the environment in which it is located.

Graphical outputs - The advantage of transient numerical simulations using software WUFI® are graphical outputs. That means: graphs (mathematical data) and videos (graphs over time), which show temperature and moisture in the building component in time. Such outputs can be used for illustrative presentations of products and buildings systems, but also they can be used for education purposes.

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