ALUMINIUM Extra
Aluminium ’ s Thermal Credentials
BY PHIL SLINGER - CAB CHIEF EXECUTIVE
There are some that think aluminium framed windows will not be able to meet the thermal demands of the upcoming changes to Document L of the building regulations . Whilst it ’ s fair to say that the proposed changes in 2025 do pose a challenge , not just for aluminium , membership feedback at our recent Spring Forum confirms that our systems company members are already prepared .
SO WHY MAKE WINDOWS OUT OF ALUMINIUM WHEN THE MATERIAL CLEARLY HAS NO THERMAL INSULATION CHARACTERISTICS OF ITS OWN ?
Aluminium is virtually 100 % recyclable , not just once but many times over . Since its commercial availability in the 1880s , 75 % of all aluminium produced is still in active service today . The current scrap value of aluminium is almost that of new aluminium as the material is in such demand , indeed there is only enough scrap aluminium available to meet 50 % of our current needs for the material . Aluminium alloy can be provided in a wide range of ‘ grades ’ which offer unique characteristics for a wide range of uses in transport and packaging as well as in construction .
Structurally , aluminium in 6060 or 6063 grade offers a high strength to weight ratio , allowing for slim frame profiles to be utilised whilst ensuring a high visible glass to frame ratio . This is achieved without additional structural components being added to the material .
Together with its recyclability credentials and its structural capabilities , a further consideration in its use in fenestration is the surface finish that it can carry . Anodised or powder coated , these finishes are some of the most sustainable finishes that can be applied to any
architectural products , often lasting the lifetime of the building itself . No wonder other fenestration materials can be seen using an external skin of aluminium in order to take advantage of these capabilities .
SO , TO THERMAL INSULATION , HOW IS THIS ACHIEVED ?
Clearly a ‘ thermal break ’ is needed within the aluminium profile to introduce a thermally efficient frame design , this is achieved basically in two ways , a ‘ pour and set ’ or a ‘ roll-in profile ’. The majority of thermal breaks used today are profiles of polyamide , a glass reinforced nylon , these offer excellent structural rigidity combined with thermal efficiency . There are also various polyurethane ‘ pour and set ’ systems in use as well , which offer excellent thermal characteristics .
Possibly the highest frame U values that can be achieved are tested and recorded by the independent Passivhaus Institute . Aluminium systems are shown on their website achieving frame U value as low as 0.7 using polyamide thermal breaks , clearly demonstrating that aluminium profiles can achieve excellent thermal insulation characteristics . Whilst these systems seem extreme for this year ’ s changes to Document L , they do demonstrate what is achievable as we move towards ‘ zero carbon ’ homes . Simply put , current aluminium framed systems for use in commercial and residential applications will continue to evolve to meet pending legislation .
Whilst frame insulation is a critical factor in the compliance stakes , cold bridging around the frame to the opening reveal also plays a big part in enabling the installed window to achieve its claimed U value . In fact LETI , ( the London Energy Transformation Initiative ) state that much of the thermal benefit of fitting a new replacement window can be lost by not
considering the likely cold bridging where the window meets the reveal .
It is clear to see that members of the association are working towards the uplift of thermal insulation values for all systems to meet or exceed the requirements of Document L being implemented this year . Installer members are also reviewing their installation methodologies to include improved thermal efficiencies around the reveal by introducing new materials and techniques . As an Association , CAB will continue to support and communicate new methodologies to ensure aluminium remains at the forefront of thermal efficiency in the fenestration market .
www . c-a-b . org . uk
28 AUGUST 2022 CLEARVIEW-UK . COM