Condensation management: Which ceiling area do I measure?

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NCC condensation provisions require a calculation to determine if a roof is sufficiently ventilated to receive discharge from an exhaust fan. This video describes how that calculation is performed, and answers some common questions on this topic.


You may recall that NCC 2019, released last year, has some new provisions which aim to manage condensation. These provisions apply to both BCA volumes: Part F6 in Volume One and Part 3.8.7 in Volume Two. Not long after NCC 2019 was adopted we started to get some questions about the new provisions and this is one of them: When you talk about the respective ceiling area, which ceiling do you mean? Do you mean the ceiling of the room being ventilated? Or the whole ceiling that the roof covers?

I'm about to give you the answer. But first I need to show you the background. The new provisions require that some exhaust systems are discharged directly to outside, or into a ventilated roof space. In order to be a ventilated roof space there has to be evenly distributed openings, and to come up with the minimum aggregate area of those openings you need to apply the fraction as outlined on the screen to a ceiling area: 1/300th of the respective ceiling area if you have a roof pitch of more than 22 degrees. Or 1/150th if you have a shallow roof whose pitch is less than or equal to 22 degrees. But what is the respective ceiling area?

So here's a hypothetical floor plan, a small residence: we've got a kitchen, living and bedroom, all naturally ventilated. And in the bottom right we have a wet room which is the bathroom and laundry combined and for the purposes of this exercise it has an exhaust fan discharging into the ceiling space. So we need to measure the respective ceiling area so that we can determine the size of the openings that we need in order to make the roof a ventilated roof space. So, do we measure the ceiling of the room being ventilated? Highlighted in yellow here. Or do we measure the whole ceiling that the roof covers?

The answer is this one, the respective ceiling area is the area of the entire ceiling that is served by the roof receiving the discharge. Because, when you think about it, this is a measure of how well ventilated the roof space is, and that's a function of the area of the entire ceiling that that roof covers, it's not a function of the area of the room which is being ventilated. Something to note on this one is that we're only talking about the roof which is receiving the discharge.

For example, in buildings like this one, we have various roof spaces for the building which are not connected, like this two storey house. When you're calculating the openings required for, say, the upstairs bathrooms, you only use the ceiling area for the upstairs roof area. You don't measure the lower roof area. Because that lower roof area does not contribute to how well ventilated the roof area receiving the discharge is. Or if you have parts of the roof which are blocked off from one another, like this townhouse, which has a separating wall continuing to the underside of the roof, then you only measure the part of the roof that's receiving the discharge.

You don't measure the neighbour's roof because that has nothing to do with how well ventilated your own roof is. While we're on this topic we'll step through a worked example. The area of this ceiling is 86.4 square metres. The total amount of ventilating area required is calculated by multiplying this area by a factor. There are two different factors: one for low roofs with a pitch of 22 degrees or less. And for taller roofs where the pitch is more than 22 degrees, we have a different factor, like this one. We have more than 22 degrees, so we're going to multiply by 1/300.

That gives us a total ventilating area of 0.29 square metres. And of that total ventilating area, 30% has to be within 900 millimetres of the top of the roof. So we find 30% of 0.29 square metres and that gives us 0.09 square metres. You have a few options available, let's use some gable vents today. The vents have to be evenly distributed. So for this gable roof you would need two gable vents, one at each end, each at least 0.045 square metres.

Now that isn't much, a vent the size of an A4 piece of paper would cover that easily. You could also use whirlybirds instead, especially if you have a hipped roof. When you use a whirlybird, I'm personally happy with taking the throat area of the whirlybird as the ventilating area. You could spend some time if you want, figuring out the area between the vanes, but I figure that any air that enters the throat is going to leave that roof space.

The smallest whirlybird that you can buy from a popular hardware shop has a 250 millimetre diameter shaft, that gives an area of 0.049 square metres. So two of those would suit this roof quite nicely. 30% has to be within 900 millimetres of the top and we've done that with the gable vents or whirly birds. The remaining 70% is 0.2 square metres and that's found by using eave vents evenly distributed around the eaves. Now what happens if our roof pitch is 22 degrees or less?

The ceiling area, of course, is multiplied by 1/150. Notice how the ventilating area required doubles when the pitch is less? This is because shallow roof spaces are more prone to condensation because there is less volume for moist air to take up. Relative to this question, there is another question and that is, isn't a tiled roof without sarking already a ventilated roof? Can't I simply discharge my exhaust into that roof space and not bother with the new ventilation opening requirements that we've just talked about?

If you think the answer to this question is yes, you're probably thinking of Volume Two,, that I've shown here which used to allow a required exhaust fan to discharge into a tiled roof which wasn't sarked. The idea being that the exhaust air can escape through gaps between the tiles. What I've outlined in red was removed for 2019, in order to accommodate the new condensation provisions. So the answer to this second question is no, a tiled roof without sarking is no longer a ventilated roof space under the Deemed-to-Satisfy Provisions.

You can still use a mechanical exhaust instead of windows to ventilate a sanitary compartment, laundry or kitchen or a bathroom, in accordance with 3.8.5. You just need to discharge that exhaust in accordance with the new condensation provisions found in 3.8.7. That means it's going to be going directly outside, or to a roof which has ventilation openings like those we just talked about.

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