Window Operation and Free Aperture

Openings tab in model data under External Glazing, Internal Glazing and Roof Glazing headers



The operation of windows can be defined by entering data under the Operation header. This data is only shown:


  1. When Natural ventilation is activated on the HVAC tab at the current level and Calculated natural ventilation is selected, or,
  2. For Internal glazing or both Calculated and Scheduled natural ventilation.

Control option (internal windows only)

The control option allows you to define how internal windows behave in natural ventilation systems and whether they operate purely based on a schedule or, when using Calculated natural ventilation, whether they are linked to the main natural ventilation controllers defined on the HVAC tab. This data is not available for external windows. The options are:


For Calculated natural ventilation:



For Scheduled natural ventilation:


Operation schedule

The schedule that specifies when window venting is available. A zero or negative schedule value means window venting is not allowed. A value greater than zero means venting can occur if other venting control conditions (specified by natural ventilation Control Mode) are satisfied.


This schedule allows you to turn off venting at certain times of the day (at night, for example), of the week (on weekends, for example), or of the year (during the winter, for example).


If used with natural ventilation Control Mode = 4-Constant, the ventilation rate is constant only when this schedule allows venting; otherwise the ventilation rate is set to zero. If Ventilation Control Mode = 1-NoVent, this schedule has no effect.


The windows are 'open' when:


Tzone_air > Tsetpoint AND Tzone_air > Toutside_air AND the schedule value = 1


The ventilation rate is calculated based on the pressure difference across the opening calculated from wind and stack pressure effects using equations described in Natural Ventilation Modelling.

Free Aperture

The size and position of window opening airflow paths can be defined by entering data under the Free Aperture header. This is data is used in both:


  1. EnergyPlus Calculated natural ventilation Airflow network simulations and
  2. CFD simulations


Note: While free aperture data is shown when the Scheduled natural ventilation model option is selected, this data is not used in EnergyPlus simulations. In this case the data is only made available to allow it to be used for CFD boundary conditions.

Opening position

The position of the opening in the window can be selected from



Windows should be rectangular to have an airflow opening in them and the position of the opening on the window can be defined by selecting from the above list. Options 3-Right and 4-Left will give identical simulation results as the EnergyPlus Airflow Network only accounts for the z-position of openings. The difference is useful for CFD simulations though.


Note: Non-rectangular windows (or rotated rectangular windows) can still be used as airflow openings. In this case for Airflow Network opening size purposes, EnergyPlus treats triangular windows as if they were rectangular with height and width based on the maximum dimensions in each direction.

% Glazing area opens

The % Glazing area opens data allows you to define the openable window "free area" as a percentage of the total window area. For example enter 20 if the window free area is 20% of the total window area. In simple terms this is the fraction of the window that can be opened, but it should also take into account the effect of any obstructions and in practice a more correct definition for the free area is the "cross-sectional (measurable, geometric) area".


The "free area" is multiplied by the discharge coefficient to give the "effective area" for the Airflow Network calculations within EnergyPlus.


Tip: A good rule of thumb to approximate of the free area for a top hung window is the bottom rectangular "opening area" plus 2 x the triangular side opening area. For closely spaced windows the side triangles should be reduced to account for the reduced effective opening area. A similar calculation can be made for side and bottom hung windows.


A couple of good reference for how to measure or otherwise obtain this data are:



Note: if modulation is switched on (it isn't by default) the glazing opening areas defined here are multiplied by a factor to account for external conditions in EnergyPlus Airflow Network simulations when using Calculated natural ventilation.

Tip: In some cases it is best to switch off the Lump similar windows on surface model option when using Calculated natural ventilation. Particular cases where this is important are a) when there are windows of the same size and type but different % Glazing area opens values on the same surface and b) where the air flow rate through individual windows is required, e.g. when preparing boundary conditions for a linked EnergyPlus CFD study.

Scheduled Natural Ventilation

Airflow through individual external windows, vents, doors and holes are not modelled explicitly with Scheduled natural ventilation because the natural ventilation airflow from outside is defined using the total natural ventilation flow rate and operation on the HVAC tab. No operation data or free aperture needs to be entered on the Openings tab for external windows in this case.


Note: Natural Ventilation can be overridden to be off on the HVAC tab for both Calculated and Scheduled model options.