There is a growing understanding that unventilated cold roof spaces can result in excessive condensation with resulting wet insulation and framing timbers. This is well understood in many countries where ventilation in the roof space is seen as essential for dry framing, insulation performance and to reduce the corrosion risk to metal claddings or components.

 Cold roof design is very common in New Zealand, the insulation is installed over the ceiling lining, and the roof cladding, underlay and support mesh rise and fall in temperature with sunshine hours and ambient external temperature. Very few residential or light commercial ceiling have a vapour blocker layer under the insulation to stop moisture diffusion into the roof attic space from the normal activities of occupants. Older down light designs also allow moisture to pass into the roof space. As the air in the attic space cools overnight or on cold days, moisture suspended in the air condenses out at the dew point and forms on the underside of the (cold- uninsulated) roof cladding. The roof underlay absorbs the condensation and releases it again back into the air in the attic roof space when the sun warms the roof cladding and attic space. If, however the moisture rich air in the attic roof space cannot escape, excessive condensation can form overwhelming the underlay and raining condensation onto the insulation and roof framing.

Masons Universal Ventilated Cavity Battens in 11 mm or 18 mm thickness may be used as part of a passive ventilated roof space design. Passive roof space ventilation makes uses the stack effect and wind movement to create air movement though the roof space carrying out moisture laden air.

• The design should provide for air to flow enter the roof space at the eaves, across any bulk insulation pressing against the underlay and roof cladding near the eaves, along the line of the underlay to exit at the ridge. Ridge and eaves vents must always be used in conjunction.
• Care should be taken when designing passive type roof ventilation in corrosion zones such as geothermal or salt spray. In these zones Passive- stack ventilation may need to be avoided. Attention should be given to blocking vapour entering the roof space, and or increased ventilation for the occupied space among other design choices.
• A ratio of vent open area to the insulated ceiling space of 1:300 or higher is recommended.
• The roof pitch and the wind at the buildings site should be considered when selecting passive roof space ventilation. Mono pitch roofs and roof pitches below 10 Degree pitch benefit less from the ‘stack effect’ and require more passive ventilation free open area – consider Masons Universal 18 mm ventilated battens, or 11 m battens double stacked.
• The amount of wind the roof is exposed to substantially affects roof passive ventilation
• Masons Redway cavity closers may be used to provide additional wind baffling on lower double pitched roofs or in wind zones High and above.
• For roof pitches above 10 degrees use Masons 11 mm Ventilated roof battens.

• Insect proof — 4mm vents prevent ingress of nesting insects
• Made from recycled materials