Timber to aluminium windows like-for-like

By - , Build 91

Replacing timber windows with aluminium requires a building consent since the replacement windows are significantly different and not likely to be considered a like-for-like replacement. This means weathertightness details will need to be considered.

Figure 2: Extended facings with new window to provide sufficient cladding cover.
Figure 1: Preparation of existing sill trimmer and cladding after original window removed.

Often building owners want to upgrade or modernise the appearance of their dwelling by replacing timber windows with aluminium ones. While this can be readily done, it does present some difficulties in terms of ensuring that the completed renovation work meets the performance requirements of the New Zealand Building Code Clause E2 External moisture.

When an existing window is removed the framed opening is likely to be consistent with construction methods current when the building was first constructed. It is also likely that the cladding underlay will not be wrapped into the opening, and it is most unlikely that modern innovations, such as the use of flexible flashing tape, will be incorporated. Although it is not mandatory to follow the Acceptable Solution E2/AS1, it provides guidance on how the opening should be treated before the new window is installed. The aim must be to ensure that external moisture is excluded.

Window details in E2/AS1 that apply to the new window will be those that incorporate:

• a flexible flashing tape across thesill trimmer (as shown in Figure 1)and up to the top of the opening

• a sill tray or partial sill trayflashing

• sufficient window flange cover to the cladding (a minimum of 10 mm)

• expanding foam or sealant air seals(over a backing rod) between thewindow reveal and the trimmedopening.

Figure 1: Preparation of existing sill trimmer and cladding after original window removed.

Depending on the cladding it may be necessary to select aluminium windows that have larger facings or incorporate a timber facing as shown in Figure 2. This is to ensure sufficient cover is provided to the existing cladding, and to give some fitting tolerance as the original opening may not be that accurately framed or trimmed.

Figure 2: Extended facings with new window to provide sufficient cladding cover.

Installing a flashing that follows the principles of E2/AS1 at the head of the window is more difficult when retrofitting an aluminium window. It may be difficult to maintain a secondary drainage path from the face of the wall underlay out over the head flashing to the outside without removing the cladding from above the window. One exception is bevelback weatherboards. In sheltered locations, such as within 300 mm of a 450 mm (minimum) wide eave, inserting the flashing upstand into the weatherboard lap should provide sufficient protection –this solution is not recommended foran exposed situation or for other cladding types. In these situations cladding will most likely need to be removed, the flashing installed and an additional layer of wrap or flexible flashing tape (to the flashing upstand) installed as shown in E2/AS1.

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Figure 2: Extended facings with new window to provide sufficient cladding cover.
Figure 1: Preparation of existing sill trimmer and cladding after original window removed.

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