Not always easier from the inside

// the building envelope

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You only get one chance to make the building meet the airtightness levels you hoped for, so choose the best membrane for its intended purpose and install it where it can protected from follow on trades and future occupiers.

Airtightness is increasing in importance as designers seek to improve their energy efficient measures without the need for extra thickness insulation thus loosing valuable foot print dimensions especially in city centre commercial buildings.

The A. Proctor Group always strive to provide construction professionals with alternative, cost effective and easier to install solutions and the introduction of their Air barrier systems are exactly that.

With over 10 years’ experience of providing the harshest climates (both hot and cold)in North America the group is promoting the external application of the airtightness strategy membranes in the UK.

The UK has up until now assumed that sealing the building from the inside was the best route to meeting increasing airtightness regulations, in the main this has included sealing up the Vapour Control layer or Vapour Barrier as some refer it to. This can make sense from a building physics point of view and help reduce condensation but the actual application can be fraught with problems when striving for low levels of airtightness. Some building designers, especially domestic, simply do not want low levels of airtightness (sub 3) as this would require a ventilation system which incurs additional costs, maintenance issues and one that most homeowners would be resistant to. However most commercial buildings will have adequate mechanical ventilation systems that alleviates these resistance factors to low airtightness so would benefit greatly from striving for the lowest possible airtightness results meaning lower fuel consumption as less cold air infiltration and warm air escaping.

The application of either Wraptite-SA or Wraptite externally as the airtight layer allows for a more continuous air barrier with less protrusions and difficult detailing to that of internal air barriers. There is less chance of the membrane being damaged by following trades so making the designer more confident of his airtightness strategy being implemented as intended from drawing board to site. There is no requirement for difficult accessories around joists, pipe penetrations, light switches or electrical cabling.

From the earliest examples of weather boarded barns to modern skyscrapers, rainscreen construction has been part of the built environment for hundreds of years. Today, with increasing energy performance requirements, and intense competition between designers to produce unique, aesthetically innovative and eye catching structures, the flexibility offered by rainscreen systems is more important than ever.

In a typical rainscreen curtain wall construction, the insulation is placed continuously over the structural frame and overlayed with a secondary weatherproof layer. The outer cladding is then fixed on rail systems, giving a drained and ventilated cavity between the insulated structure and the façade. This gives several advantages, not least fully insulating the frame and substantially reducing cold bridging compared to in-frame insulation. This setup also allows for increased design flexibility, as the outer leaf can be lighter than traditional brick facades, and can therefore adopt more unusual layouts and configurations that would be difficult or impossible to achieve with traditional façade materials. It is also comparatively easy to retrofit insulated rainscreen systems to existing structures, giving a simple path to improving both appearance and energy efficiency.

In most rainscreen systems, the outer cladding panels are not fully wind and watertight, making the performance of the secondary weather protection membrane more critical than with brick or blockwork outer leaves. While most such membranes have adequate air barrier performance, they are heavily dependent on quality of installation to ensure the overall air leakage rate targets are met. Failure to meet the specified targets can cause substantial increases in the energy performance of the completed building, and this “performance gap” has been identified in many studies of predicted vs actual building energy use.

Positioning an air barrier on the outside of the insulation rather than internally simplifies this process considerably, as there are less building service and structural penetrations to be sealed, however careful consideration of the specification and installation of this barrier remains critical during design and construction.  While such construction is not widely used in the UK, the A. Proctor Group’s Wraptite external air barrier system provides a simple and robust solution that can be easily incorporated into existing site practices.

Wraptite-SA, fully vapour permeable self-adhesive air barrier, is available either as a membrane or as a fully permeable tape, with a unique vapour permeable adhesive backing. This allows a simple and fast installation procedure, with the fully adhered membrane sealing to all common substrate materials. Its microporous film core ensures a low air permeance of 0.01m3m-2hr-1 while maintaining a high permeable to water vapour, with an sd-value of 0.039m. This versatile, high performance material can be used in full wall applications, at floor cassette or corner junctions or as an adaptable site-formed airtight flashing around door and window penetrations, providing an airtight seal without compromising moisture movement.

In tape form, Wraptite-SA can be used to seal panel joints in airtight substrates such as OSB, or used to seal penetrations from services or structural elements.  Its excellent adhesion characteristics also make it an effective method of jointing conventional air barrier membranes, thus maintaining a high vapour permeability envelope across the entire building.

The second component of the Wraptite system is the Wraptite vapour permeable air barrier.

Designed to cost effectively replace conventional breather membranes, Wraptite provides similar vapour permeability and air barrier characteristics to self-adhered membrane, but installed on or off site in the traditional manner.

Used together, the components of the Wraptite system enable a simplified and more robust method of achieving low air leakage rates, particularly when installed on site, as is typical with large scale rainscreen construction, or where the sealing of junctions between prefabricated building components is critical. By reducing the likelihood of failures to meet design air leakage rates, the Wraptite System helps ensure “as designed” performance is achieved, helping to narrow the gap between design and actual energy performance.

For nearly 30 years, the A. Proctor Group have been at the forefront of construction membrane technology. From the revolutionary air/vapour permeable Roofshield underlay to the low emissivity Reflectashield timber frame breather membrane, the A. Proctor Group continually work with suppliers and research organisations to ensure our products meet the most advanced technical requirements while providing exceptional value for our customers. For product information, case studies, and technical documentation, please visit us at www.proctorgroup.com or follow us on twitter @proctorgroup.

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