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NCC 2019: Natural light, a debate in the making!

Updated: Feb 6, 2019

For anyone that loves natural light in buildings (who doesn’t?), they will be delighted to see an awesome introduction in the hot off the shelf NCC 2019 changes to our beloved building code bible for minimum compliance.

With advances in daylight design, glass performance and a greater adoption of climate sensitive metrics, will this new requirement help guide projects to a minimum design requirement that drives better outcomes or is this simply another design team obligation that will get packaged up in an architects or consultants compliance fees?

What’s new?

Finding itself down in Health and Amenity - FP4.1 provisions for natural lighting become mandatory for Class 2, 3 or 9 buildings, or a Class 4 part of a building by May 2019!

Let’s first examine the all-important Performance Requirement to see what the NCC 2019 is aiming to deliver.

“Sufficient openings must be provided and distributed in a building, appropriate to the function or use of that part of the building so that natural light, when available, provides an average daylight factor not less than 2%”.

Wow, a bit of a mouthful but the idea is clear enough….. windows in a building must provide natural light that are fit for purpose and guided by the average daylight factor metric and a magical value of 2% for both Verification an Deemed-to-Satisfy methods.

So, what is the average daylight factor and is it the right tool to guide a minimum level of natural light? Let’s look at the Verification method first…Morpheus, throw me the blue pill!

FV4.3 Verification of suitable provision of natural light

So, an oldie and not necessarily a goodie, average daylight factor is a design metric that sets a minimum quality of light required by the code. And it is not surprising, as it remains one of the most widely used methods of establishing compliance and rating tools performance (Green Star, BREEAM, LEED, Green Mark, etc.) globally.

Now, I have written on this before and ill save on my word count, but average daylight factor is a somewhat antiquated design metric as a simple back of envelope ratio of the illumination level within a room provided by daylight to the level of daylight outside the room during overcast (worst case) conditions.

Useful, back when we had access to pencils and not PC’s, but arguably limited in modern design as it assumes no sun and thus is insensitive to orientation and arguably to the climate of the location you are designing in! Not a great start but average daylight factor equations are highly variable, so let’s hold judgement and look under the hood!

As you can see, this equation would leave few in the dark (boom boom !!) as to how to calculate an average daylight factor.

Put simply, it uses the area of the windows (W) and walls (A) and adds window transmittance values (T) (to allow for natural light in overcast conditions) and surface reflectance values (R) (to reflect light from internal surface areas). It then aims to account for external structures that may obscure the so-called view of the sky (θ) and therefore the transmittance of natural light and applies an angle to account for such an obstruction.

Here is a nice simple graphic, extracted from the NCC 2019 Guide to Volume One. Grab a copy here as it has some nice additional guidance that helps with the interpretation of NCC 2019.

The result of this equation, generally a small value between 1 and 8%, with 2% representative of a room that can be considered daylit, but artificial lighting will still be needed to perform visual tasks. Go above 5% (in the UK anyway where average daylight factor was born), issues of glare and visual comfort become topical!

Well, while I have to say that I would rather sit through 10 recent Tom Cruise films than do average daylight factor calculations (although Magnolia is awesome) and question any adoption of a metric that is being phased out by far more useful metrics Internationally, I reluctantly feel compelled to support it as a backstop to natural light is needed in such Building Classes and equally across all States.

But what if you’re not a fan of Tom Cruise like me? Don’t fancy going down the calculation pathway for each window in “living” and “sleeping” areas with different room finishes, window sizes or external obstructions? Well, we can take the red pill of course and meet the Performance Requirements by the Deemed-to-Satisfy provisions. Is this a more effective approach to ensuring natural light for all? Let’s have a look.

F4.1 Provision of natural light

F4.1 sets the focus of the Performance Requirement very clearly, where “living” and “sleeping” areas within Class 2 (multi-residential), Class 3 (student accommodation and hotels), Class 4 (where a part of a building is a dwelling) Class 9a (hospitals and health care), Class 9b (schools and universities) and 9c (aged-care) buildings need to provide better natural light.

F4.2 Methods and extent of natural light

Within F4.2 , we can start to decipher the ways and means of avoiding average daylight factor calculations by understating the aggregate of windows and roof light areas. Here it is in a nut shell:

- Glazing that is not less than 10% of the floor area (of the room) and are open to sky, or

- Roof lights that is not less than 3% of the floor area (of the room) and are open to the sky,

- Or, a combination of the above.

As you delve into part b of F4.2, it sets some interesting minimum requirements for minimum boundary distances for Class 2, 3, 4 and 9 and minimum sill heights for Class 9b and 9c, which can only be a good thing (if you are on the ground floor that is!).

F4.3 Natural light borrowed from adjoining room

Finally, when combined with F4.3, you guessed it from the title, you can start accounting for glazing or openings from an adjoining room to meet the Performance Requirement in a Class 2, Class 4 or in a sole-occupancy unit of a Class 3 building. Again, focusing on aggregating access to natural light, F4.3 provides us with similar numbers to F4.2, namely:

- Glazing that is not less than 10% of the floor area (of both rooms) and are open to sky, or

- Roof lights that is not less than 3% of the floor area (of both rooms) and are open to the sky,

- Or, a combination of windows and roof lights required in F4.2.

Now, I am sure many of you have already spotted something very interesting. In F4.2 and F4.3, there is no requirement for using any specific window transmittance values or surface reflectance values? So, how is it possible to meet a 2% Performance Requirement target without mandated Deemed-To-Satisfy window transmittance values or surface reflectance values? There is no requirement to model the room here, so this has left me perplexed as to how the Performance Requirement is being met?

Another very interesting observation is the use of the 10% requirement, not with standing the lack of performance values for the windows and walls. Surely a 10% target would be easily achievable given our culture of overly glazing buildings? Let’s look at a couple of examples.

Sleeping areas, the focus of F4.1, are on average between 13m² and 20m² in Class 2, 3, 4 and 9 buildings, giving us a target 1.3 m² and 2 m² respectively, based on the 10% floor area. If we said such sleeping areas where square, that would give us roughly 9 m² and 11 m² façade area (based on a 2.4 m room height) and only a target of 15% and 19% window to wall ratio?

OK, so not all sleeping areas are equal, and they are certainly are not always square. Let’s do the same very rough review assuming the façade is half the width of the internal wall (a rectangle) and the same overall floor area as above. In this case, our façade areas are only 6m² and 8 m² and our target window to wall ratios are still a moderate at 21% and 26%. 30% is a good target, even on the west!

Do we need this Performance Requirement for Natural Light?

So, we have taken the blue and the red pill (they say you should never mix your pills!!!) and if you have made it this far into the post, you will realise i am perplexed by my interpretation of this minimum provision for natural light. But the question remains, is this a valuable additional to our building code legislative tome of now 748 pages?

The DTS red pill has left me top ended, chasing a Performance Requirement that fails to clarify the full methodology or is not very challenging to illustrate compliance based on no additional effort beyond paperwork. The Verification blue pill uses an outdated metric that is not even specific to location or climate to illustrate an average level of daylight but the lights are still on?

While I am on the fence and feel further clarification will be provided/discussed in the months to come, I think it is important for light quality to be prioritised at the Federal level to form at least a consistent minimum requirement across all States, particularly in hospitals, schools and aged-care facilities. Yes, we have local council score cards, rating systems and sustainable management plans, but these are often lost in the competing picture for façade performance and aesthetics, making natural light a low priority for some.

What’s your thoughts? A fair appraisal or a step in the right direction?


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