Modelling air cavities – overview
The equivalent conductivity model
To describe the thermal properties of air cavities in thermal simulations for building science applications a so-called equivalent conductivity model is usually applied. This model assumes that the cavity is “filled” with a “solid air material” having a specific thermal conductivity. In reality however, the heat transport in such cavities consists of complex radiative and convective mechanisms. These are dependent on numerous factors such as geometry, size, surface temperatures, temperature gradients, surface emissivities and heat flux direction.
To simplify the calculation equivalent conductivity models have been developed. In these models, the parameters mentioned above are being used to calculate a specific value (equivalent thermal conductivity) which considers radiative and convective effects in a simplified manner. The model is therefore able to cover certain effects (dependency on size of the cavity, emissivity of the surface, direction of heat-flux, temperature levels and gradients) using a simplified arithmetic formula that has been worked out based on empiric studies. For building science applications these formulas are laid out in the standards ISO 6946 and ISO 10077-2.
Simulating air cavities with HTflux
The type of thermal air cavity model used for the simulation can be selected in the Materials dialog using the combo box in the section thermal conductivity (1). If necessary the specific paramters can be viewed and changed with a mouse-click on the paramters input field next to it (2). In general however, it is recommended to pick the according material of the public data materials database, as all other properties will then be defined correctly (heat capacity, density, vapor diffusion,…)
Using HTflux you have basically four options allowing you to thermally simulate air cavities:
ISO 6946 – automatic
For most applications, this option might be the best choice. HTflux will automatically detect the geometry, dimensions of the air cavity and its surface temperatures and calculate the equivalent conductivity accordingly. It is necessary to make sure you have provided the correct orientation of the model in the boundary dialog, so that the direction of the heat flux versus gravity can be taken into account correctly. Apart from that, this “automatic air material” works fully automatic. It will automatically detect or split cavities and calculate specific conductivities for each of them.
⇒ find more on automatic ISO 6946 air cavities here.
ISO 10077-2 – automatic
This material is defined according to the standard ISO 10077-2. It is very similar to the automatic ISO 6946 material and it will also detect the cavity parameters automatically. The ISO 10077-2 has been designed to define the calculation processes of window frames. For this reason, the main difference between this definition and the ISO 6946 material mentioned above is that this model will always assume a horizontal direction of the heat flux. Gravity effects on the convection, resulting in a relatively increased upward and decreased downwards heat-flux are not considered in this model, the model assumes a horizontal heat-flux direction.
⇒ find more on automatic ISO 10077-2 air cavities here.
ISO 6946 – air layer (manual)
The ISO 6946 air layer model allows you to “manually” specify relevant parameters for the air layer. It is therefore less flexible and specific, but allows you have full control of the calculation process. It is best used for extended, even air layers, such as void floors or air gaps in double wall constructions, especially if you need to stay in line with pre-calculated conductivities used for U-values or alike.
⇒ find more on ISO 6946 air layers here.
Well-ventilated layers / cavities
In the case of well-ventilated layer (e.g. rain-screen claddings, vented roof,…) the cavity models above are not appropriate. The ventilation of such layers also implies heat-exchange with the environment. Therefore the temperatures within such layers are usually close to the external-temperatures. It is imporant to note that according to ISO 6946 an air layer is seen as well-ventilated very quickly. The following conditions are used to determine the minium size of any openings (to the external environment):
- for vertical air layers: >1500 mm² per meter of length (in horizontal direction)
- for horizontal air layers: >1500 mm² per square-meter of surface
The appropriate method for such cases is to use a boundary condition, usually with the external temperature, sometimes with a weighted temperature value. However unlike the standard external boundary condition, you will have to use an increased heat transfer resistance value (see heat transfer resistances).
Constant value – “unmoved” air
This option will only be suitable for very special applications. If you wish to apply other air cavity models or if you have empirical data available you might also define the conductivity of the air cavity by setting a specific constant value manually. For this method, you can import the “AIR” material of the material database and overwrite constant value for the thermal conductivity. Be aware that the predefined default value of 0.025 W/mK is valid only for unmoved, stationary air cavities. This exceptionally low value reflects very special scenarios, where the air is effectively trapped in very small cavities with dimensions less than 1mm. Most insulation materials are based on this principle of “trapped air”, however as you will probably not simulated these effects on such a microscopic scale you will hardly ever use this approach.