Above Sheathing Ventilation (ASV) is a smarter approach that repositions the ventilated cavity above the roof underlay rather than beneath it. By creating an air gap between the cladding and the membrane, ASV allows for continuous airflow driven by thermal buoyancy and wind pressure. This moving air helps to:

Remove heat before it reaches the insulation layer.

Dry out condensation and incidental moisture.

Protect membranes from prolonged exposure to heat and moisture.

Reduce the stress on insulation and vapour control layers.

In essence, ASV relocates the first line of defence to where it is most needed—between the cladding and the structure—allowing the roof system to manage both temperature and moisture more effectively.

Insights from sources like Green Building Advisor and JLC Online, echo the same principles: roof assemblies perform best when they control moisture at the source, provide robust thermal insulation, and manage air movement intentionally.

Attic Ventilation: Research shows that while attic ventilation can help manage some moisture and heat, it cannot compensate for poor air sealing or insufficient insulation. Likewise, ASV is not a magic bullet; it must be combined with good air barriers, vapour control layers, and proper insulation placement.

Over-roofing Techniques: Adding continuous insulation above an existing roof ("over-roofing") improves thermal performance by shifting the dew point outward, similar to how ASV manages temperature and vapour loads before they can impact the main structure.

To maximise the benefits of Above Sheathing Ventilation:

Use a durable, vapour-permeable membrane that can safely manage any incidental moisture at the sheathing level.

Ensure adequate cavity depth (typically 20-40mm) to promote effective airflow between cladding and membrane.

Design for intake and exhaust ventilation (e.g., at eaves and ridges)to support consistent air movement (the 1/150 or 1/300 with 40% high and 60% low rules).

Seal the conditioned side of the roof with an airtight, continuous vapour control layer to prevent indoor air leakage into the roof system.

Pro Clima advocates for this type of ventilated cavity approach because it aligns with the physics of how moisture, air, and heat behave in complex building envelopes—not because it’s trendy, but because it works. Their extensive range of products makes designing and building warm, dry, healthy homes the new norm here in New Zealand.