Condensation Management
Historically it has been widely accepted that exhaust ventilation from kitchens, laundries, bathroom, and sanitary compartments vent into the roof space and allowed to escape via gaps, cracks and openings in the building envelope (i.e., if the roof does not have sarking or silver paper insulation and is clad with tiles. The National Construction Code (NCC) has made some recent changes in terms of ventilation of these areas in reaction to the increase in energy efficiency in homes meaning these gaps, cracks and openings no longer exist in new built homes.
Sources of moisture in the home
You may be surpriesd about how much moisture we produce in our homes, below are the 3 main areas in the home that require mechanical ventilation.
When these rooms are vented into the roofspace, we are not removing the moisture from the home; we are simply diverting the moisture to the roofspace which has the potential to create a dark, high moisture environment conducive to increased structual degredation and timber pest attack.
Venting to outdoor air
Having your licenced plumber install ducting to your extraction vents is an effective and relatively inexpensive way to ensure as much moisture is directed away from the home as possible, minimising the risk that comes with heightened levels of moisture in the home.
The Code
Below are some extracts from the National Construction Code in relation to condensation management in the home.
Part F6 / Part 3.8.7 Condensation Management
F6.2 / 3.8.7.2 Pliable building membrane (NCC Volumes One and Two)
(a) Where a pliable building membrane is installed in an external wall, it must—
(i) comply with AS/NZS 4200.1; and
(ii) be installed in accordance with AS/NZS 4200.2; and
(iii) be a vapour permeable membrane for climate zones 6, 7 and 8; and
(iv) be located on the exterior side of the primary insulation layer of wall assemblies that form the external envelope of a building.
(b) Except for single skin masonry or single skin concrete, where a pliable building membrane is not installed in an external wall, the primary water control layer must be separated from water sensitive materials by a drained cavity.
F6.3 Flow rate and discharge of exhaust systems (NCC Volume One)
(a) An exhaust system installed in a kitchen, bathroom, sanitary compartment or laundry must have a minimum flow rate of—
(i) 25 L/s for a bathroom or sanitary compartment; and
(ii) 40 L/s for a kitchen or laundry.
(b) Exhaust from a kitchen must be discharged directly or via a shaft or duct to the outdoor air.
(c) Exhaust from a bathroom, sanitary compartment, or laundry must be discharged—
(i) directly or via a shaft or duct to outdoor air; or
(ii) to a roof space that is ventilated in accordance with F6.4 / 3.8.7.4.
3.8.7.3 Flow rate and discharge of exhaust systems (NCC Volume Two)
(a) An exhaust system installed in a kitchen, bathroom, sanitary compartment or laundry must have a minimum flow rate of—
(i) 25 L/s for a bathroom or sanitary compartment; and
(ii) 40 L/s for a kitchen or laundry.
(b) Exhaust from a bathroom, sanitary compartment, or laundry must be discharged—
(i) directly or via a shaft or duct to outdoor air; or
(ii) to a roof space that is ventilated in accordance with F6.4 / 3.8.7.4.
F6.4 / 3.8.7.4 Ventilation of roof spaces (NCC Volumes One and Two)
(a) Where an exhaust system covered by F6.3 / 3.8.7.3 discharges into a roof space, the roof space must be ventilated to outdoor air through evenly distributed openings.
(b) Openings required by (a) must have a total unobstructed area of 1/300 of the respective ceiling area if the roof pitch is more than 22°, or 1/150 of the respective ceiling area if the roof pitch is not more than 22°.
(c) 30% of the total unobstructed area required by (b) must be located not more than 900mm below the ridge of highest point of the roof space, measured vertically, with the remaining required are provided by eave vents.
