Manufacturers’ data
The properties of glazing panes and constructions are published by manufacturers. The salient properties for an individual pane are the solar direct transmittance, reflectance and absorptance. These values should sum to one.
When creating a glass pane type (material) the direct transmittance, reflectance and absorptance may be used directly with no modification. However, for a double glazed unit you will find that the solar radiant properties are usually given for the full unit rather than the individual layers. The properties given are typically U-value, short-wave shading coefficient and total shading coefficient. Separate U-values may be specified for the mid-pane properties and the unit as a whole, including the frame.
The procedure for matching glazing properties for a complete glazing unit is as follows:
1. For any known glazing panes (for instance 6mm clear float), assign properties from the materials database.
2. Make a guess for the properties of panes for which no specific data is available.
3. Match the manufacturer’s mid-pane U-value by suitable choice of air gap resistance. The air gap resistance includes the effect of low emissivity coatings facing into the glazing cavity. Do not adjust the CIBSE Net U-value (including frame) or the emissivities of the outside or inside surface.
4. Click on Derived Parameters to display the derived properties of the glazing and compare the shading coefficients displayed with those provided by the manufacturer. Adjust the reflectance, absorptance and transmittance of the unknown panes to match these parameters. Be careful to ensure that the sum of the reflectance, absorptance and transmittance is 1. If the manufacturer’s data is in the form of transmittances rather than shading coefficients, use the displayed values for T(D) and T(R) at 0º incidence. These transmittances are related to the shading coefficients by a factor of 0.87.
5. Adjust the CIBSE Net U-value (including frame) to match the manufacturer’s value for this parameter. This operation must be done last, as the Net U-value is automatically updated to the calculated mid-pane U-value after edits to any other parameters.
Interstitial blinds and ventilated cavities
Blinds within a glazing cavity (which may be ventilated) can be modelled in one of two ways. The example given below assumes triple glazing:
Method 1
The blind may be modelled by using a fourth layer of glass but assigning the properties of the blind to this pane.
Using a fourth pane of glass to model the blind. In this case, the manufacturer's blind transmittance, absorptance, reflectance data can be used to set up a glazing material. Set the refractive index to 1.0.
The manufacturers' blind transmittance of short-wave solar radiation value can be entered directly as the transmittance of the blind material.
The absorptance is calculated by reducing the absorptance of the actual blind by a factor to account for the convection of the absorbed heat by natural ventilation in the cavity.
If the ventilation is 100% effective in removing heat absorbed by the blind, enter 0.0 as the absorptance of BLIND.
If there is no ventilation enter the manufacturers’ value directly. The actual value will lie somewhere between 0 and 100% of the manufacturer's value.
You will have to estimate the actual value to use based on the likely flow of air through the cavity and the heat removal rate from the blinds.
It is likely that the proper value to use is between 40 and 95% of the manufacturers' absorptance value.
Method 2
Alternatively, the outer cavity may be modelled as a separate room. In this case the blind can be modelled in the normal way, by attaching an internal shading device to the outer pane.
The double-glazed unit between the outer cavity and the room is now an internal window.
Air partitions
If a space is subdivided with the option Remove partition set, the partition between the spaces is automatically replaced by a hole. Holes are completely transparent to solar radiation and MacroFlo treats them as open to the passage of air. In some instances it is appropriate to adjust the MacroFlo characteristics of such an opening (as described in the MacroFlo Methods manual), and to make this possible the hole must be replaced by a window. To ensure the correct characteristics for solar and long-wave radiation exchange the window should be constructed as a single pane with the following properties:
General
Outside surface emissivity: 1.0 Inside surface emissivity: 1.0 Frame: 0.0
Construction layer
Reflectance: 0.0 Absorptance: 0.0 Transmittance: 1.0 Refractive index: 1.0
Editing materials
It is possible to edit certain material properties in two places: the Construction layers grid in the Project construction dialog (Sections 4 and 5) and the dedicated Project materials dialog (Section 7).
Material changes are not synchronised between these dialogs when both are visible so making changes in both places may result in those changes being lost.
In general, it is recommended that changes to materials are made using the Materials dialog.
