Simple Glazing Definition

If only basic manufacturers data on the glazing system is available then you may need to use the more approximate Simple glazing definition method. In this case the glazing properties for the overall glazing system are defined using the three parameters:

• U-value,
• Solar heat gain coefficient (SHGC) and
• Visible transmittance (VT) data.

You can read details of the method used behind the scenes in EnergyPlus to represent the simple glazing definitions in the report Modeling Windows in Energy Plus with Simple Performance Indices by Dariush Arasteh, Christian Kohler and Brent Griffith.

U-Value

This field describes the value for window system U-Factor, or overall heat transfer coefficient. Units are in W/m2K. This is the rated (NFRC) value for U-factor under winter heating conditions. The U-factor is assumed to be for vertically mounted products.

Note: The maximum U-value is 5.8 W/m2K because this is the most conductive glazing system possible given the above standard conditions that can realistically be achieved. If you need to model a glazing system with a very high U-value (above 5.8 W/m2K) the published rating probably contains the frame, in which case you should add a frame to obtain the required overall glazing + frame U-value.

Tip: You can find some detailed information about the U-value to use and how the NFRC and EU format U-values differ on the DesignBuilder b.i.g. website

Solar heat gain coefficient (SHGC)

Enter the value for solar heat gain coefficient for the combined glazing system, a fractional value from 0 to 1 without units. This is the rated (NFRC) value for SHGC under summer cooling conditions and represents SHGC for normal incidence and vertical orientation.

Background information on solar transmission

• g-value is the coefficient commonly used in Europe to measure the solar energy transmittance of glass - called a Solar Factor on some window literature (%) i.e. 53% = 0.53;
• Solar Heat Gain Coefficient (SHGC) is used in the United States and most commonly refers to the solar energy transmittance of a window or door as a whole, factoring in the glass, frame material (wood, aluminium, etc.), sash (if present), divided lite bars (if present) and screens (if present). SHGC may also refer to the solar energy transmittance of the glass alone (sometimes more specifically termed center-of-glass SHGC), in which case it is analogous to g-value.
• Shading Coefficient was succeeded by SHGC in the United States, but older windows and doors may still refer to their SC value. The relationship between SHGC and SC may be approximated as: SHGC = SC × 0.87;

 

g-values and SHGC values ranges from 0 to 1, a lower value representing less solar gain. Shading coefficient values are calculated using the sum of the primary solar transmittance (T-value) and the secondary transmittance. Primary transmittance is the fraction of solar radiation that directly enters a building through a window compared to the total solar insolation, the amount of radiation that the window receives. The secondary transmittance is the fraction of inwardly flowing solar energy absorbed in the window again compared to the total solar insolation.

Source Wiki Solar Gain

Visible transmittance

Enter the visible transmittance for the glazing coefficient, a fractional value from 0 to 1. It is the rated (NFRC) value for visible transmittance at normal incidence.

Frame

Simple glazing data can either include or exclude the effect of the frame. If the glazing data includes the frame then you should make sure to switch off frames in the openings model data to ensure that they are not double-counted.

Note: When using the simple glazing definition it is important to understand that there may be significant differences in performance between the simple window system and the usual more detailed model. This option should therefore only be used when layered data is not available.