2D:Dialog EditSurfaceCoefficients

Dialog: Edit Surface Coefficients

Use this dialog to specify the surface coefficients to be assigned to a surface segment of the component.

"Adiabatic / System Border": If your component and the boundary conditions are symmetric with respect to some symmetry axis, then no heat or moisture transport will occur across the symmetry axis, even if the axis does not coincide with some impermeable construction element. You can save on computation time by modelling only one half of the construction and treating the symmetry axis as an impermeable boundary.
Checking this option will disable the edit boxes for the surface transfer coefficients and disallow any heat or moisture flow across this surface segment.

"Treat as Inner Surface": WUFI automatically computes the vapor transfer coefficients from the heat transfer coefficients. The formula used for this distinguishes between exterior and interior surfaces. So if the surface segment under consideration is on the indoor side of the component, check this option; if the surface is on the outdoor side, leave the option unchecked.

"Sd value [m]",
"Vapor Transfer Coefficient [kg/m²sPa]": There are two possible contributions to the water vapor transfer resistance at the surface:

• the natural resistance due to the boundary air layer close to the

surface. This resistance is usually computed by WUFI from the heat transfer coefficient specified for that surface and automatically taken into account (see below for how to override this if needed).

• a possible additional resistance due to a surface coating, if such

a coating shall be accounted for by only having its sd-value specified instead of being explicitly modelled in the component assembly (see the topic "Surface Coatings" for details).

This dialog offers two ways to specify the desired diffusion resistance:

• Sd-value [m]:
  

Use this option to specify an additional resistance, if any. WUFI will automatically compute and add the natural component. For example, if you wish to model a layer of tiles with sd = 2 m in this way, enter "2" as the sd-value.

• Note for the experts: if you enter a negative number here, WUFI will

use it (with positive sign) as the total  transfer resistance (i.e. it will add nothing). This may be useful if you need to specify a given total resistance in terms of an sd-value.

"Vapor Transfer Coefficient [kg/m²sPa]":
Use this option to specify the total resistance, expressed in terms

of the vapor transfer coefficient.
In most cases you will have no need to model any additional surface layer, and you'll want to leave it to WUFI to deal with the natural component of the transfer resistance. Then simply enter sd-value = 0.

"Heat Transfer Coefficient [W/m²K]": The heat transfer coefficient governs the convective and (long-wave) radiative heat exchange between the component and the surroundings.

"Short-Wave Radiation Absorptivity [-]": The short-wave radiation absorptivity determines the fraction of total incident solar radiation that is absorbed by the component.

"Long-Wave Radiation Emissivity [-]" This coefficient controls the emission and absorption of thermal radiation. It is usually ignored by WUFI and can safely be set to zero.
If a weather file in Test Reference Year format ( *.TRY or *.DAT) is used, WUFI will use the emissivity to estimate nighttime radiative cooling. However, we generally recommend to switch off the cooling effect by setting the emissivity to zero unless you have adequate counterradiation data. For a discussion of these issues please see the topic Long-wave Radiation Emissivity.

"Rain Water Absorption Factor [-]": The rain water absorption factor takes into account that some of the rain water hitting the component surface splashes off on impact and is not available for capillary absorption. For ordinary walls, a value of 0.7 appears adequate for most cases. Set it to zero if the facade is protected from rain and no rain absorption shall take place at all. Set it to 1 if all rain water shall be available for absorption (as on a flat roof).

"Surface Type": This option is not used in the current version of WUFI-2D.

"Surface Group": This option is not used in the current version of WUFI-2D.

"Color in Model": Select the color with which the surface segment whose properties you are editing shall be displayed in the component diagram.