Better thermal performance from aluminium window joinery 

The traditional face-fixed way of installing aluminium window joinery in new homes has a big drawback that has long vexed scientists at BRANZ and the Ministry of Business, Innovation and Employment (MBIE). This article combines information written for the BRANZ March 2026 monthly technical update, and a piece published in BRANZ Build 207 

The joinery sits outside the line of the external wall framing, which means that the glazing unit and thermal break in the frame are not in line with the insulation in the external wall framing (Figure 2). Cold air can circulate around the frame, resulting in unwanted heat loss and reduced thermal performance. 

 

The challenge has been to find a solution that not only improves thermal performance but also meets other Building Code requirements, especially for weathertightness and durability. 

With recessed joinery installation systems (Figure 3), the joinery aligns insulated glazing with wall insulation. A manufacturer of proprietary recessed systems claims this approach delivers a 20% improvement in thermal performance. 

 

This alignment of the elements in the thermal barrier increases the thermal performance of the joinery and ultimately also of the building’s exterior envelope. 

This reduces the potential for cold external air to circulate around the joinery unit, creating the thermal bridges contributing to the heat loss through the building envelope, which can occur with traditional face-fixed joinery. 

Some manufacturers claim up to 20% increase in thermal performance for recessed joinery installations. Recessed external joinery also changes the aesthetic of the building. 

Centrally fixed aluminium joinery systems 

There are three main systems of recessed aluminium joinery offered by manufacturers. 

These include two similar systems (A and B) where the aluminium frame extrusion incorporates an integrated aluminium facing that covers the wall recess at the head, sill and jambs and flashes out over the exterior cladding. The main variation in these systems is in how the joinery is fixed in place. 

The third system (C) is primarily a flashing system for the wall recess that allows the installation of standard joinery in the flashed recess. 

System A window installation 

In this system, the aluminium window frame incorporates a fully integrated facing around the perimeter, with a separate fixing leg. The frame is fixed to the wall framing with screws through integrated continuous fixing fins around the perimeter that fit hard to the outer face of the wall framing over the (flexible or rigid) wall underlay. The fixing fins are then taped to the face of the wall underlay around the entire perimeter. 

The fixing position puts the window glazing and thermal break in a warm aluminium window frame within the line of the exterior wall framing, aligned with the wall insulation. 

This fixing system retains the window facing outside the building structure, facilitating the installation of a range of cladding systems. A proprietary head flashing incorporating an integral cavity closure and end dams completes the window frame installation. 

If an internal timber reveal is incorporated, it is fixed to the wall framing to secure it in position – but it is not the primary means of fixing the window in place. Expanding foam is then installed to the gap between the reveal and the wall framing around the entire perimeter. The foam fills the entire cavity through to the back of the fixing fin – no PEF rod is required. 

A system like this has a BRANZ Appraisal and CodeMark certification. A CodeMark certificate is deemed to be a means of proving Building Code compliance. 

System B window installation 

This system is like system A in that the window frame incorporates a fully integrated facing around the perimeter with a window facing on the outer edge. However, the frame is fixed to the wall framing at head, sill and jambs, with screws through the internal timber reveals – the traditional method of aluminium window installation.  

The fixing position also puts the window glazing and thermal break in the window frame within the line of the exterior wall framing, aligned with the wall insulation. This system allows the window frame to be fixed in a range of positions within the wall framing to suit a wide range of exterior cladding system depths. 

A traditional head flashing with a Window & Glass Association New Zealand (WGANZ) end dam is then installed. Expanding foam on a PEF rod is installed to the trim cavity around the entire perimeter, completing the installation. 

Systems of this kind may comply with the requirements of E2/AS1. If they do not, they must demonstrate compliance through an Alternative Solution. 

System C window installation 

This is more of a hybrid system as it can incorporate components from different manufacturers. 

There are a range of designs that incorporate a facing around the window opening perimeter that encloses the recess in the exterior wall framing. This facing flashes the head, sill and jambs of the opening and integrates with the window frame. 

This system can facilitate the traditional installation of standard windows in a recessed position, allowing better thermal performance while retaining weathertightness. 

Traditional head flashings can be used and expanding foam on a PEF rod installed to the trim cavity at least to the head and jambs, completing the installation. Consideration is typically needed to ensure water is drained appropriately at the sill.  

As for System B, this system may comply with the requirements E2/AS1, although E2/VM1 and BRANZ EM9 test methods may be used to confirm E2 compliance. 

BRANZ testing 

MBIE has recently finished testing some solutions for the recessed installation of both traditional and European window systems at BRANZ, and results will soon be available. As part of this work, the BRANZ EM9 test for the weathertightness of recessed window installations and the E2/VM1 test method for claddings have also been further refined. 

These systems and this research are all contributing to better ways to install windows into exterior walls in order to maintain the weathertight performance and improve the thermal performance. 

 

This article was first published in issue 207 of BRANZ’s Build magazine. www.buildmagazine.co.nz. Words by Greg Burn, Building Consultant, Structure Limited. Supplied by BRANZ.  It was amended with information included in BRANZ’s March 2026 Update newsletter.