Parameters for Transparent Layers

Parameters

Details

Name

Name of the material

Description

Details about the layer

Width

The width of the layer.

Conductivity

The thermal conductivity of the material.

Vapour Diffusion Factor

The specific moisture diffusion resistance of the material (the ratio of the permeability of air (1.92e-10 kg/Ns ) to the permeability of the material).

Solar Transmittance

The fraction of solar radiation at normal incidence which is transmitted by the layer.

External Solar Reflectance

The fraction of external solar radiation at normal incidence which is reflected by the layer.

Internal Solar Reflectance

The fraction of internal solar radiation at normal incidence which is reflected by the layer. This will differ from the External Solar Reflectance if the layer is unsymmetrical (for instance a glass pane with a metallic coating on one side).

Light Transmittance

The fraction of incident visible radiation at normal incidence which is transmitted by the layer.

External Light Reflectance

The fraction of external visible radiation at normal incidence which is reflected by the layer.

Internal Light Reflectance

The fraction of internal visible radiation at normal incidence which is reflected by the layer.

External Emissivity

The fraction of external long-wave radiation which is absorbed by the layer.

Internal Emissivity

The fraction of internal long-wave radiation which is absorbed by the layer. This will differ from the External Emissivity if the layer is unsymmetrical.

Blind?

Check the box if the layer is a blind, otherwise the box should be unchecked. This affects the treatment of convection, both in Tas and in the calculation of CIBSE and Pilkington parameters. In the calculation of Pilkington parameters, convection coefficients for gas layers adjacent to blinds are automatically increased by a factor 4/3 to allow for the effects of convection currents in the larger gas space enclosing the blind. The same procedure is adopted in Tas, where it is extended to the treatment of convection heat exchange between an internal blind and the room air. Here a convection coefficient of 4/3 the usual value is applied. (This contrasts with the treatment in CIBSE and Pilkington procedures where the blind is tacitly assumed to be at room temperature, implying an infinite convective heat transfer coefficient.)