Air Cavity

The selection of air cavities within Roof Systems and there impact on design need greater consideration within NCC 2019 – Vol 1.

While there are several Standards for Roof Systems covering air cavity selection, there is no established design process that is used to decide on the ventilation provisions that will work best with all roof systems. Via the introduction of AS/NZS 4859.2:2018 - Thermal insulation materials for buildings - air cavities within Roof Systems are identified by the following categories:

Unventilated

An airspace—

(a) in which there is no express provision for airflow through it; and

(b) having no insulation between it and the external environment, but with small openings to the external environment where the openings are arranged so as not to permit airflow through the layer and such openings do not exceed—

(i) 500 mm2 per metre of length (in the horizontal direction) for vertical airspaces;

or

(ii) 500 mm2 per square metre of surface area for horizontal airspaces.

Slightly Ventilated

An airspace in which there is provision for limited airflow through it from the external environment by limited openings of area (Av) within the following ranges:

(a) For vertical airspaces: >500 mm2 but <1500 mm2 per metre of length (in the

horizontal direction).

(b) For horizontal airspaces: >500 mm2 but <1500 mm2 per square metre of surface area.

Well Ventilated

An airspace in which the openings between the airspace and the external environment are equal to or exceed—

(a) 1500 mm2 per metre of length (in the horizontal direction) for vertical airspaces; and

(b) 1500 mm2 per square metre of surface area for horizontal airspaces.

Via the nomination of either an unventilated, slightly-ventilated or well-ventilated cavity, Speckel enables a quick review of ventilation options alongside other factors that may impact the Total R-value assessment, such as the nomination of reflective membranes.

Step 1 – Let’s first reuse the Roof System we created in the Roof Systems Tutorial 1 – Getting Started video - Suspended Slab (Internally Insulated).

Step 2 – Adjusting Roof ventilation could not be easier. Simply create an air layer and then an internal finish. Here you will find access to different ventilation options, that enable you to change the properties of your roof in line with AS/NZS 4859.2:2018 calculation requirements.

Step 3 – If you would like to change the ventilation options entirely, simply click on the ventilation name, and you will be provided with three ventilation options as per the definitions above. If your air layer is interior to the insulation layer, ensure it is facing to the inside.

Step 4 - When your happy, simply click Performance Solution and download your Report.