New builds are designed to be energy efficient, sealing in warmth and keeping drafts at bay. But in doing so, they trap moisture in unexpected places, especially in the roof space. Moist air from daily activities like cooking and showering often passes through ceiling gaps and ends up in the cavity above. When that moist air hits cold metal roofing at night, it condenses and can cause mould, and damage over time.
BRANZ advises that space above insulation must be ventilated to manage moisture and protect durability. A minimum 25 mm gap between insulation and the underlay is essential to let air flow and prevent moisture build up. Without it, even well insulated homes can suffer problems in the roof space over time.
BRANZ: BU630 Roof space ventilation | BRANZ
Houses today are far more airtight than those built decades ago. This slows down natural drying and increases the risk of hidden moisture. BRANZ found signs of mould on new construction roofs with no ventilation, showing how airtight envelopes can backfire.
DriSpace NZ offers smart passive ventilation systems tailored for modern roofs. Their stepped system, combining breathable underlay, airtight membranes, and dedicated vents like VB10 battens and ridge vents, reintegrates airflow safely and effectively. These systems rely on natural air movement. Warm moist air exits at the ridge, fresh air enters at the eaves, keeping the space dry and needing no upkeep.
Masons NZ also offers a proven system built around the same logic. Eaves inlet vents, ventilated battens, ridge vents and a critical gap above insulation. This keeps warm moist air flowing out while pulling dry air in, much like drying clothes on a breezy day.
If your house has a cold roof over an insulated ceiling, especially with long run metal or tight ceiling linings, passive ventilation is a smart choice. It’s not just for new builds. Even recladding or reroofing jobs can benefit. Contact us today.
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