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NCC 2019: Measures to minimise condensation impact

The new provisions in the National Construction Code 2019 that help minimise the impacts of condensation relate to the installation of pliable building membrane (sarking) to control ingress of moisture through the external wall, the discharge of exhaust fans and ventilation of the roof space.

The new provisions contained in the National Construction Code (NCC) 2019 allow for the mitigation of condensation within Class 1, 2 and 4 parts of a building. Further measures are being considered for NCC 2022.

What is condensation?

Condensation can be attributed to changes in climatic conditions resulting in increase in humidity and seasonable temperatures. An increase in the build-up of condensation in the home can lead to mould growth that can have an effect on the health of the occupants in the building.

Many practitioners have raised concerns that with the increasing levels of energy efficiency provisions in buildings, their greater levels of insulation and the fact that they are being built to be more airtight, the design and construction of buildings is causing increased levels of condensation.

It’s important to note that the new provisions seek to minimise health impacts through the management of condensation. It does not look at eradication of condensation as it acknowledges that dealing with condensation in buildings is a complex matter and is much about how the building is used as it is about how it is built.

This is reflected in the NCC Performance Requirement, which states:

P2.4.7 Condensation and water vapour management

Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.

To satisfy this Performance Requirement there are a number of Deemed-to-Satisfy solutions:

  • If you are installing a pliable building membrane (sarking) in the external wall and you are in climate zone 6, 7 or 8 (this includes Victoria, Tasmania, ACT, Sydney West and some regional areas in SA and WA), the sarking must be a vapour permeable (breathable) membrane, and be placed on the outside of the insulation
  • In addition to installing the wall sarking, the NCC also requires that exhaust fans installed in a kitchen, bathroom, toilet or laundry must be ducted to the outside air or ducted into a ventilated roof space
  • The exhaust/ducting provisions did exist somewhat in NCC 2016 under the ventilation provisions – however, they are now much more specific
    • The exhaust fans for bathrooms and toilets must have a minimum flow rate of 25L/s and for the kitchen and laundry a flow rate of 40L/s
    • The exhaust from the bathroom, toilet and laundry must discharge directly to the outdoor or to a ventilated roof space
  • Where the exhaust is discharged into the roof space, the roof space must be provided with ventilation openings evenly distributed throughout the roof space including:
    • having 30% of the ventilation openings within 900mm of the ridge or highest point of the roof space and the rest provided by eave vents
    • the total unobstructed area of the openings will be dependent on the pitch of the roof and the respective ceiling area. (1/300 of the respective ceiling area for roof pitch of more than 22° and 1/150 for roof pitch less than 22°)
  • For Class 1 buildings (houses) the kitchen exhaust /rangehood is not required to discharge into a roof space (can use re-circulating range hoods). However, for Class 2 building (apartments) or Class 4 parts of a building (dwelling over a shop or caretakers accommodation) the kitchen exhaust must be ducted to the outside air unless a re-circulating range hoods is used and in that case it is not mandatory to duct it though advisable.

The ducting requirements will present some challenges – particularly for a bathroom, toilet or laundry located centrally in the house – so consideration of ducting and pathways within floor/ceiling spaces earlier in the build process is recommended.

Let’s look at the new requirements in more detail.

Clause 3.8.8.3 (b) of Volume Two requires exhausts from bathrooms, toilets and laundries to be discharged to the outside air either directly or via a shaft, duct or a ventilated roof space.

Exhausting directly to the outside is preferable. However, there may be cases where this is not achievable and will need to duct into the roof space.

Clause 3.8.7.4 requires the roof space be ventilated through evenly spaced openings that have:

  • a total unobstructed vented area of 1/300 of the respective ceiling area where the  roof pitch is more than 22° and 1/150 for roof pitch 22° or less, and
  • 30% of the ventilation openings within 900mm of the ridge or highest point of the roof space and the rest provided by eave vents.

How should the roof space be ventilated?

To determine the ventilation requirements for the roof space you firstly have to calculate the respective ceiling area. The question then becomes:

What is considered as the ‘respective ceiling area’ – is it just the immediate ceiling above the room with the exhaust or is it the whole ceiling area of the house?

There is no definition of respective ceiling area and HIA has sought clarification on the matter. While there is no definitive explanation to refer to it has been suggested that it should be taken as the total ceiling area located directly under the roof space, irrespective of the shape of the house or where the exhaust fan/s are located.

An alternative interpretation would be to be able to justify to the building surveyor/certifier based on the building’s design that that portion of the roof space would not be affected by the ingress of the exhaust. An example could be an L-shaped house, where there may be a part of the ‘L’ that does not have any exhaust system in it and the ceiling space is, in effect, isolated from the remainder of the house.

How to apply the NCC provisions

Here are some examples of calculating the ventilation requirements where the exhaust is discharged into the roof space as opposed to external ducting:

Example 1 – House has a ceiling area of 200m² and a roof pitch of 25°

Calculate the unobstructed ventilation area = 1/300 of 200m² = 0.67m²
Ventilation required within 900mm of ridge = 30% of 0.67m² = 0.2m²
Ventilation required at eaves 70% of 0.67m² = 0.47m²

This would then require a minimum of three rooftop ventilators (whirly birds or similar) vents at the ridge area and at least 6 – 400mm x 200mm eave vents evenly distributed around the eaves.

Example 2 – House with 18° pitch roof – 200m² ceiling area with 450mm wide eaves.

Minimum ventilation requirement – 1/150 of ceiling = 1.34m² area
30% for ridge ventilation = 0.4m²
70% remaining for soffits = 0.94m²

This situation would require six rooftop ventilators at the ridge area and a least 12 – 400mm x 200mm eave vents evenly distributed around the eaves.

What if I don’t have eaves?

If there are no eaves one option could be to increase the height of the external wall above the ceiling line and install the eave vents into the wall as an alternate approach. This may need to be considered as a Performance Solution though as there is no prescriptive Deemed-to-Satisfy Provisions for this.

Alternatively, the exhaust fans could be connected to flexible ducts in the roof space and be joined to a single main duct that is then taken up through the roof cladding, which would meet the DTS Provisions.

Further information on condensation

See Section 7 of the ABCB handbook on the new condensation management provisions.

The Tasmanian government has also produced a useful guide to assist with design considerations Condensation in Buildings – Tasmanian Designer’s Guide – Version 2.

To find out more, contact HIA’s Building Services team.

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