Openness and Free Area - Inputs

What is Openness and Free Area?

The free area is a way to describe how effective openings in a building are.

The easiest way to imagine how free area works is to imagine the free area being equivalent what you would get if you had a completely unobstructed hole as the aperture for natural ventilation. 

Most architectural openings are not completely unobstructed. Also often what is drawn in the 3D model is not all operable glazing either.  

Why do I care?

Generally speaking the higher the free area, the more effective natural ventilation will be at generating air changes in a space. This is usually true up to a point, after which increasing free area does not make much difference.

Many codes and standards set Free Area requirements for prescriptive compliance of naturally ventilated designs. These standards usually expect the free area through the apertures to be equal to around 5% of the floor area of the space being conditioned by those windows.

How window openings work in Sefaira for Natural Ventilation

Sefaira uses glazing drawn in the 3D model as the basis for the windows that will be naturally ventilated in a model. Any window imported as glazing is considered to be operable for the purposes of natural ventilation.

In reality, the amount of glazed area that can actually be opened will depend on the types of windows used in the proposed building and how they open, as well as other factors such as stoppers to minimise opening size, whether there are fixed windows, and whether there are impediments like insect screens.

Sefaira offers 2 ways to account for these impacts. Both include the “collections” feature which means that the values can be adjusted differently for each orientation.

% Glazed Area That Opens

This first property sets the % of glazed area that can actually be opened. The default is 100%.

This means that it is assumed that all of the glazing drawn in the 3D model imported into Sefaira can be opened.

If some of the glazing in your model is in fact fixed glazing, you can reduce this percentage to reflect that. 

So for example, if you wanted to reflect a window assembly such as the window below, then the % of glazing that is openable is approximately 75%. The bottom left part of the assembly is fixed glazing and so not openable.

Alternatively you could draw the 3 glazing planes shown below in your 3D model and tag the fixed glazing as fixed. Then set the "Glazing that is Operable" percentage to 100%.


Free Area (%) of Opening

The free area is a way of measuring what the equivalent completely open area is for a glazing assembly. The value entered is the maximum free area for the window assembly type.

For example, a sash window can only ever have a maximum free area of 50% (because the windows slide on top of each other). If there is also a fly screen, it might only be 35%.

For any opening piece of glazing, there is typically a free area associated with that opening depending on the type of window it is and the other aspects that might limit its opening.

A table is provided to support the user in understanding what some typical free area limits might be depending on the window type and other constraints. This is attached below.

In the example above where the % glazed area operable is 75%, the free area might be set at the value for a top-hung window. If the window could be opened up fully, that free area value entered could be 80%. If the window had a stopper, you would enter 20%.

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