Windows of Opportunity

Successfully meeting performance requirements for door and window installations can be a complicated task.  ‘Window guru’ Gary Smith from the AWA outlines the key issues associated with installing windows.

A couple of decades ago, there were no real performance requirements for windows.  Provided they opened, closed and had 3mm float glass in them, everything was fine.  However, due to the continued evolution of standards and regulations, improvement to construction methods and greater consumer expectations, windows and doors now have to meet complex performance requirements.

Wind and rain resistance, sound attenuation, compliance with the National Construction Code (NCC), bushfire resistance, energy efficiency all need to be considered in window installation.  It is important to have a firm understanding of how to ensure that your windows meet all the requirements.  Having a sound knowledge of the existing requirements and tests will inform your decisions when specifying, selecting, purchasing, certifying and installing window systems.

When deciding the type of window and door system that will be used in a project, the correct wind load for the construction site will be a decisive factor.  Every building site in Australia should be assessed for wind load requirements according to AS/NZS 1170.2 (Wind loads for buildings) or AS 4055 (Wind loads for housing).  The two loads are Serviceability Limit State (SLS) and Ultimate Limit State (ULS).

Wind loads are provided in AS 2047 or calculated from AS/NZS 1170.2 or, if the construction is housing, AS 4055 can be used to obtain the N or C ratings.  For commercial or multi-residential projects, site specific loads should be calculated by the project engineer.

The requirements for windows also change as the construction type changes.  Have you ever considered why windows in commercial buildings have such big sections?  This is because windows in commercial buildings are not permitted to deflect as much as windows in a house, additionally commercial buildings often experience higher wind loads than houses.

Whether you are building a detached house, a residential development or a commercial building, the construction typology will determine the installation. The Australian Window Association website has a key message ‘AWA Building Classifications’ that explains the different types of constructions and window requirements in more detail.

After the site requirements have been met, the windows that are selected need to be tested for a number of conditions that will impact on their performance and durability.  The tests completed measure window deflection, confirms that the opening force is within the set limits, measures air leakage and water penetration, and confirms that the window meets its ultimate strength requirements.  Windows are tested in accordance with the requirements of AS 2047.  The performance results dictate where the windows and doors can be used based on the requirements for the type of construction and site.

Energy efficiency provisions and thermal comfort requirement will inform the window selection.  The BCA has technical requirements that dictate energy efficiency for all building classes, and there are also variations from state to state.  Compliance is achieved by the prescriptive measures found in the BCA, or through performance-based solutions using simulation tools such as AccuRate, FirstRate, BERS Pro and BASIX.  Either option will help specify both the window performance for total window U Value (Uw) and total window Solar Heat Gain Co-efficient (SHGCw).

In general the lower the U value the better the performance.  The SHGC value however needs to be determined based on local climatic conditions and the relative size of glazing areas for the building, cold climates may need a high SHGC, whereas hot climates may need a low SHGC.

The BCA requires that external glazing performance data must be determined in accordance with the guidelines of the Australian Fenestration Rating Council (AFRC).  The Window Energy Rating Scheme (WERS) follows the guidelines of the AFRC.  Performance requirements from energy efficiency reports are given to the window supplier, who will match or exceed the requirements with their WERS-rated products.  In some cases it will be easier to select the windows first then supply the performance data to whoever is rating the building.

Many construction sites now fall within bushfire-prone areas.  This has a significant impact on the types of windows and glass that must be used.  AS 3959 – “Construction of Buildings in Bushfire-prone Areas” is the Standard that specifies the requirements to assess the site, then specifies building requirements.  It is important to give the BAL rating to the window supplier so they can ensure they are providing you with a compliant product.

Ensure flashing is installed correctly

 

Understanding the walling system and material selection, including finishes and hardware, also dictates how windows are to be selected and installed.  In general full masonry and masonry veneer construction provides a drainage path so that if the interface between window and wall leaks a small quantity of water, this will not dramatically increase the risk of failure and moisture damage, unless flashings are incorrectly installed or absent within the construction.

Irrespective of construction type a sound recommendation for masonry veneer and lightweight construction is to have a suitable weatherproofing wall wrap / membrane (sarking) to protect the building frame, insulation and internals from water damage.

Recently, with the growing popularity of lightweight wall cladding systems issues have arisen around Australia in regards to how the penetrations of these systems are selected and installed. In 75% of cases, poor installation is the reason for water leakage in window systems.

There are a few simple principles that, if followed will provide a good performing installation in lightweight wall claddings. Some of these principles also apply to full masonry and masonry veneer construction.

  • A ‘Z’ shaped metal head flashing extending beyond the width of the window, between 25-50mm.
  • The window should be fitted with poly flashing to jambs and sill. Jamb flashing should extend 100mm above and below the window jamb height, and the sill flashing to extend to the outside width of the jamb flashings.
  • All flashings must offer weathering overlap ie. the sarking above the window must overlap the metal head flashing. The head flashing must overlap the window jamb poly flash. The jamb poly flash must overlap the sill poly flash and both jamb and sill poly flash must overlap the wall sarking.
  • There should be provision for drainage below the window, ideally along the bottom of the cladding for the full length of the wall.

It is important to remember that many wall cladding materials are used in residential applications, were traditional finned windows are used. With this window type, the waterproof line is the fin, and not the front of the window. Sealing or attempting to seal to the front face of the window is destined for failure.

In the event of a face sealed wall cladding system, all windows must drain to the external of the window sill, with no possibility of water entering the wall cavity.

There are several issues to look out for when installing windows in lightweight wall systems. There is often a lack of flashing, and it is common for the flashing not to extend high enough behind the external cladding and sarking. This is further complicated by the fact lightweight cladding systems often don’t provide a cavity. In addition, a significant portion of wall sheeting systems rely on external sealing between the window frame and the wall cladding. Many acrylic & polyurethane sealants, are subject to deterioration under UV. Additionally the window and wall sheeting may expand and contract at different rates, placing stress on the sealant. Often people seal up the perimeter of the window to the surrounding cladding, rather than relying on good flashing and draining principles.

It is important to speak to your wall system supplier and window supplier and follow the manufacturer’s proven practices when installing windows and doors. You must have good attention to detail when installing these systems to ensure you do not have issues with water ingress – it is much easier to use the correct methods in the first place rather than trying to fix a problem later. Adhering to these requirements and using these methods will take more time initially; however, in the long run you will have happy customers and save lots of money on very costly repair work.


Subscribe to Building Knowledge Newsletter: