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Surface properties tab on MaterialsRelated TopicsRelated Topics
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Surface properties are only used in simulations when the material is located on an inner or outer surface of the construction.
Note: The absorptance values in this object impart surface properties to the construction and should be applied to the thermally significant inner and outer layers in the overall assembly. Attempting to trick the program by modelling thin “paint” layers to apply surface properties is not a good idea as the thermal models were not intended to support such strategies.
The thermal absorptance represents the fraction of incident long wavelength radiation that is absorbed by the material. This parameter is used when calculating the long wavelength radiant exchange between various surfaces and affects the surface heat balances (both inside and outside as appropriate). Values for this field must be between 0.0 and 1.0 (with 1.0 representing “black body” conditions).
The solar absorptance field in the Material input syntax represents the fraction of incident solar radiation that is absorbed by the material. Solar radiation includes the visible spectrum as well as infrared and ultraviolet wavelengths. This parameter is used when calculating the amount of incident solar radiation absorbed by various surfaces and affects the surface heat balances (both inside and outside as appropriate). Values for this field must be between 0.0 and 1.0.
The visible absorptance field in the Material input syntax represents the fraction of incident visible wavelength radiation that is absorbed by the material. Visible wavelength radiation is slightly different than solar radiation in that the visible band of wavelengths is much more narrow while solar radiation includes the visible spectrum as well as infrared and ultraviolet wavelengths.
This parameter is used when calculating the amount of incident visible radiation absorbed by various surfaces and affects the surface heat balances (both inside and outside as appropriate) as well as the daylighting calculations.
It is very important to check the surface reflectance values used in EnergyPlus daylighting calculations because any library values used will be generic and the actual surface reflectance will typically depend more on any surface finish such as paint than on the material itself.
Tip: You can calculate Visible absorptance from a surface Light Reflectance Value (LRV) by dividing the LRV by 100 and subtracting from 1.
Values for this field must be between 0.0 and 1.0.
This field is a character string that defines the relative roughness of a particular material layer. This parameter only influences the the calculated exterior convection coefficient. A special keyword is expected in this field with the options being “VeryRough”, “MediumRough”, “Rough”, “Smooth”, “MediumSmooth”, and “VerySmooth” in order of roughest to smoothest options.
The colour data is used for display purposes when the texture is not available for any reason. It is not used in any of the calculations.
The texture is used to represent this material when it is specified as either the inside or outside material of a construction which is selected in the building model. It is also used to represent the material in diagrams of any construction using this material.
The following data items are used to define the surface properties of the material when it is used in a Radiance simulation. They allow materials to be defined as transparent or semi-transparent.
Note: settings in this section do not affect EnergyPlus or any other simulations.
The material class defines how the material is defined in Radiance daylighting simulations. The options are:
To account for non-zero component block transmissivity, the material class should be set to 3-Translucent or 4-Translucent (simple).
The fraction of light that is absorbed within the material. This option is only available when the material class is set to 3-Translucent.
Reflectance + Transmissivity + Absorptance = 1
Reflectance is the proportion of incident light that is reflected away from the surface.
Enter a specularity value for the material surface to define the way that light reflects from it. The value entered must be between 0 and 1. A value of 0 provides purely diffuse reflection, while a value of 1 gives no diffusion, representing mirror-like behaviour. Specularity fractions greater than 0.1 are generally not very realistic for typical building materials. The specularity of metals is usually 0.9 or greater.
Note: the Visible absorptance must also be adjusted to complete the surface property behaviour for Radiance calculations.
Specular reflection is the mirror-like reflection of light from a surface where each incident ray is reflected, with the reflected ray having the same angle to the surface normal as the incident ray. Some devices used to enhance daylighting performance such as light shelves and blinds with directional reflection properties require a non-zero value of specularity.
Roughness is specified as the rmf slope of surface facets. A value of 0 corresponds to a perfectly smooth surface, and a value of one would be a very rough surface. Roughness values greater than 0.2 are not very realistic.
The transmissivity is the fraction of penetrating light that travels all the way through the material at normal incidence, i.e. the fraction that is not absorbed in one traversal of the material.
A transmissivity value of 0 means fully opaque and 1 means fully clear.
Transmittance, the value usually measured, is the total light transmitted through the pane, including multiple reflections. To compute Transmissivity (tn) from Transmittance (Tn) use:
This option is only visible when one of the 3-Translucent or 4-Translucent (simple) Material class options are selected.
Translucent objects are infinitely thin.
The fraction of light that is diffusely scattered as it is transmitted. This option is only visible when the 3-Translucent Material class option is selected.