How Free Area calculations work

What does Free Area mean?

If you're designing a naturally ventilated building, the fastest way to check your opening sizes are about right is to use a free area calculation on a zone by zone basis. 

The "Free area" of an opening is a way of describing how well air can travel through for the purposes of natural ventilation. It helps your design account for how the types of windows you use and the impact of other treatments added to them, like the window opening type, insect screens and security stoppers.

Explain in simple terms how it's calculated.

Think of a room that is 20m2 with two sash windows (double-hung window).

  • Each sash window has an area of 2m2.
  • Only half of it can be opened (one half slides over the other) so the free area of each window is 50% x 2m2 = 1m2.
  • Let's assume each window has an insect screen as well which only allows for 60% free area. The net free area of each window is now 1m2 x 60% = 0.6m2

This is how the free area of each window is calculated.

In Sefaira as in many standards, we care mostly about the free area as a proportion of the floor area. In the example above:

  • Floor area = 20m2
  • Total window free area = 2 windows @0.6m2 each = 1.2m2
  • Free Area for room = 1.2m2 / 20m2 = 6%

Why do I care?

The free area of all openings in a zone expressed as a percentage of the floor area is a very common way for codes and standards to test prescriptive compliance for natural ventilation systems.

The following table summarises some of the codes and standards that use this free area ratio as a way of determining compliance without analysis. 

Code or Standard Where it mainly applies Free Area Requirement
BREEAM 2011 HEA 02 (p84-85) Mainly the UK 5%
AS-1668 Part 2 Australia 5%
Title 24 Section 120.1 California 5%

Sefaira provides you with a very quick way to see whether the free area of the glazing drawn in the 3D model, adjusted by openness factors, meets the targets above in every zone, saving a lot of time doing hand calculations.

How the Free Area output works in Sefaira

The free area output tab makes it really easy to test whether your design complies with prescriptive free area requirements. Even if you're not using these requirements to prove your design is compliant, the values help hone in on the right free area values to begin natural ventilation studies.

Step 1 - activate natural ventilation

First to do free area calculations, you need to switch on the Natural Ventilation feature. The free area card will prompt you to go to the Nat Vent input tab to do this if you haven't done it already. If Natural Ventilation is not on, then we assume all windows are fixed closed for free area calculations and all zones will fail.

Step 2 - determine your criteria

Set your pass criteria using the first card in the Free Area output section (example below). The default free area target is 5% of the floor area but it's adjustable.

Step 3 - How we calculate free area for glazing planes

The glazed area for each zone in your model is calculated. This is the total area of planes that are tagged as glazing in your 3D model and that are external glazing. (If you tag a plane as fixed glazing in your 3D model it will not be counted here.)

Use the openness card to decide the % of glazing that opens (eg a window might be half-fixed and half-openable) and the free area % of the opening itself. 

Sefaira will then work out the free area for each glazing plane in each zone as follows:

Free area of glazing plane = (area of glazing plane) x (% that opens) x (% free area)

The free area available to each zone is assumed to be the sum of the free area of all glazing planes in that zone.

The excel download for Free Area will provide you with the glazed area per zone and all the results.

Step 4 - How we check whether the zone passes and colour the zones

The free area % is determined for each zone as follows:

Free area % for zone = (sum of free areas for all glazing planes) / (zone floor area) x 100%

We then compare this value to the target set in step 2. The table below shows how we colour the zones:

Free area outcome How we colour zone Pass or fail?
More than 1% under target Red Fail
Up to 1% under target Orange Fail
Up to 1% over target Lime Green Pass
More than 1% over target Dark Green Pass

Example of Coloured Floor Plate:

Click on zone to get details about how it specifically is going:


Step 5 - Summary

In the criteria box it will then tell you how many zones failed and where. The worst zone is one of the zones with the poorest free area score.

How do I get my project to pass?

There are 3 ways you can improve the Free Area ratio of a zone:

- Add more opening windows to the zone (you can do this in your 3D model and re-upload it)

- Increase the Openness properties in the Natural Ventilation Tab (make sure the values you put are realistic for your project design)

- Reduce the zone size or change the space plan layout (the % is based on the area of your zone after all!)


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