The Benefits of Above Sheathing Ventilation

Above Sheathing Ventilation (ASV) is the superhero of roofs that work. It’s a solution to a few of the problems with roofs.

Over the next couple of months, a series of articles will be published exploring the back story of ASV and why it’s a really good option for New Zealand roofs. Here are the cliff notes, complete with spoilers:

Problems with roofs

The standard way roofs are designed and constructed in New Zealand typically involves fixing the roof cladding directly onto an underlay or membrane. The membrane is there to help protect the underside of the cladding. It’s also required to absorb the inevitable moisture that will form under the cladding. A big portion of this liquid comes from condensation of moist air moving up from the attic and the living space below.

Another problem is overheating. Temperatures in excess of 90°C have been recorded under roof cladding where this is no ventilated cavity (Cullen, 1992). Some of this heat will transfer into the rest of the building, making it either uncomfortable or expensive to keep comfortable. Combined with extreme cold (as much as 10 degrees below ambient air temperature due to night sky radiation), large diurnal and annual variations in moisture and temperature cause excessive stress on materials.

Sheathing

A little word on terminology. ‘Sheathing’ is not a common term in New Zealand. In North America sheathing usually refers to boards or some form of rigid backing that supports a membrane.

In New Zealand, it might be more accurate to describe ASV as ‘above membrane ventilation’ because it’s the space between the cladding and the membrane that’s key, just like in a ventilated wall cavity. But there are good reasons to align with common international terminology, not least of which is to accept that some tried and tested methods for building healthy, efficient and durable buildings from overseas, may actually be useful to consider in New Zealand.

Roofs are walls on an angle

What is a wall, tilted on an angle? It usually becomes a roof.

As a result of the $47 billion (Peter Dyer, 2019) ‘leaky building’ disaster, New Zealander designers have become very aware of the benefits of drained and ventilated cavities in walls. Why should roofs be any different? The benefits of ventilation between cladding and underlay are the fundamental principles of ASV.

ASV design and construction

The secret of ASV is to design and build roofs similar to walls, with a well-drained and ventilated space between the weather-resistive membrane and the cladding. The best way to achieve this in a roof is with vertical counter battens directly on top of the membrane, followed by horizontal battens for securing the cladding.

By avoiding the build-up of moisture and moderating the temperature, ASV is a great way to create a drier, healthier, more comfortable and durable building.

For more on this topic, stay tuned for upcoming episodes of ‘Above Sheathing Ventilation’.

References:
Cullen, W.C., (1992) Temperature Variations caused by solar heating and radiative cooling, Professional Roofing
Dyer, P, (2019) Rottenomics – The Story of New Zealand’s Leaky Buildings Disaster, David Bateman Ltd., Auckland

INTELLO PLUS Intelligent Airtightness Membrane

pro clima

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The pro clima airtightness system prevents uncontrolled air infiltration or leakage through the building envelope, and assists the drying capacity of the building through its unique moisture control properties.

Declare LBC Red List Free

Declared

Post-Consumer Recyclable Content

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Suitable for all construction and where there is risk of interstitial condensation
The protection of insulation from air movement
Preventing uncontrollable air flow though the building envelope
An optional continuous cavity for electrical and plumbing services can be created on the inside face of the external wall by fixing battens horizontally over the INTELLO membrane before installing the wall lining
The main advantage of a service cavity is to shift the plumbing and wiring out of the insulation layer
A service cavity also reduces the penetrations through the airtightness membrane allowing easy and fast installation of services
This also eliminates pressure points on inner lining (eg. plasterboard) from insulation expansion

The NZBC requires only that airtightness be considered, not measured. INTELLO directly assists with meeting the following requirements:

Clause E3.1 INTERNAL MOISTURE: Buildings must safeguard people against illness, injury, or loss of amenity that could result from the accumulation of internal moisture
Clause E3.2 INTERNAL MOISTURE: Buildings must avoid the likelihood of fungal growth or the accumulation of contaminants on linings and other building elements
Clause E2 EXTERNAL MOISTURE: Concealed spaces and cavities in buildings must be constructed in a way that prevents external moisture being accumulated or transferred and causing condensation, fungal growth, or degradation of building elements
Clause E2 EXTERNAL MOISTURE: Building elements must make due allowance for the following: (a) the consequences of failure, (c) variation in the properties of materials and in the characteristics of the site
Clause E2 EXTERNAL MOISTURE: Interior linings with all joints stopped require barriers to airflow
Clause H1 : ENERGY EFFICIENCY: The building must be constructed to limit uncontrollable airflow
Clause H1 : ENERGY EFFICIENCY: Account must be taken of physical conditions likely to affect energy performance of buildings; (c) Airtightness of the building envelope.
Clause B2 : DURABILITY: Performance B2.3.1(a), not less than 50 years, B2.3.1(b), 15 years and B2.3.2

BRANZ Appraised

SOLITEX MENTO 3000 Roof Underlay

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SOLITEX MENTO 3000 roof underlay is a three-layer roof underlay designed to be used with most typical roof cladding systems. While providing temporary weather protection it can remain exposed for up to 90 days.

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Declared

SOLITEX MENTO 3000 has been appraised for use as a self-supporting roof underlay on buildings within the following scope:

the scope of limitations of NZBC Acceptable Solution E2/AS1, Paragraph 1.1 with regards to building height and floor plan area; and,
with masonry tile roof cladding; and,
with metal tile and profiles metal roof cladding; and,
suited to NZS 3604 Wind Zones up to, and including Extra High.

In the opinion of BRANZ, SOLITEX MENTO 3000, if used, designed, installed and maintained in accordance with the statements and conditions of the BRANZ Appraisal, will meet, or contribute to meeting the following provisions of the NZBC:

Clause B2 DURABILITY: Performance B2.3.1(a), not less than 50 years, B2.3.1(b), 15 years and B2.3.2 SOLITEX MENTO 3000 meets these requirements. See Paragraphs 9.1 and 9.2 of the BRANZ Appraisal.
Clause F2 HAZARDOUS BUILDING MATERIALS: Performance F2.3.1 SOLITEX MENTO 3000 meets this requirement and will not present a health hazard to people.
Clause E2 EXTERNAL MOISTURE: Performance E2.3.2 When used as part of the cladding system, SOLITEX MENTO 3000 will contribute to meeting this requirement. See Paragraphs 12.1 and 12.2 of the BRANZ Appraisal.

BRANZ Appraised

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