Glazed Construction - Construction Layers
Construction Layers (Outside to Inside) grid
The construction may consist of up to 4 glazing panes separated by air gaps. The panes are listed in order from outside to inside. With the exception of air gaps, each layer is assigned a pane type, consisting of a description and a set of optical properties describing the pane. Unlike an opaque material, a pane type describes the properties of the layer (pane) rather than the substance of which it is made. The pane properties are stored in the Project Materials database, but may be edited within the layer. Any new pane type created by edits of this kind will be added to the list of Project Materials. Cavity layers (air gaps and other gas-filled cavities) can be described in three different ways: in terms of a gas, a convection coefficient or a cavity resistance.
Material: the id and description of the material composing the layer, or alternatively ‘Cavity’.
Thickness: the thickness of the layer.
Conductivity: the thermal conductivity of the material. Values for commonly used building materials are listed in Table 6 in the Apache Tables document and in CIBSE Guide A.
Angular dependence: specifies the angular dependence of the pane’s optical properties. The options are as follows:
‘Fresnel’: Calculates the angular dependence using the Fresnel equations and the specified refractive index, with adjustments to cater for special coatings.
’Explicit’: Explicit user-specified angular dependence (see ‘More data’).
‘Constant’: Optical properties are independent of incidence angle.
‘LBNL’: Uses standard profiles for angular dependence, as specified by Lawrence Berkeley National Laboratory.
Gas (optional): gas type of the cavity. If set, this is used to calculate the cavity convection coefficient.
Convection coefficient (optional): the convection coefficient for heat transfer across the cavity. If set (or calculated from Gas), this is used, with the adjacent pane surface emissivities, to calculate the cavity resistance.
Resistance (optional, air gap only): the thermal resistance of the air gap, taking account of both convection and radiation across the gap. The air gap resistance will be higher if a low-emissivity coating is applied to either of the surfaces facing into the cavity. If Cavity convection coefficient is set, it is used, with the adjacent pane surface emissivities, to calculate the cavity resistance. For guidance on setting air gap resistance and other glazing parameters, see the Hints & Tips section.
Transmittance: The transmittance of the pane for solar radiation at normal incidence.
Outside reflectance: the reflectance of the outside surface of the pane for solar radiation at normal incidence.
Inside reflectance: the reflectance of the inside surface of the pane for solar radiation at normal incidence.
Refractive index: The refractive index of the material composing the pane. Editable only if the angular dependence is set to ‘Fresnel’.
Outside emissivity: the emissivity of the outside surface of the pane. This is used to calculate the thermal resistance of the adjacent surface or cavity (unless this resistance has been specified explicitly).
Inside emissivity: the emissivity of the inside surface of the pane. This is used to calculate the thermal resistance of the adjacent surface or cavity (unless this resistance has been specified explicitly).
Visible light specified (not currently used): Indicates whether the pane data contains visible light parameters (which in due course will provide an alternative route to deriving the construction’s visible light transmittance).
Note: For an Electrochromic Layer, the values for Thickness, Conductivity and Angular Dependence must match on both the Clear and Dark sub-layers. When these values differ, the Electrochromic Layer will be highlighted red, the OK button on the construction window disabled and the Derived Parameters window hidden:
