Futures in Thermal Breaks

Up to the early 1980s aluminium was the only choice for residential window replacement alongside more traditional timber, and even then it was encased in a hardwood frame. With advent of ‘white painted timber’ lookalike PVC-U windows in the mid 80s, aluminium lost ground rapidly.

So why the interest in aluminium now and how can aluminium carry high levels of insulation which are necessary in today’s new build and replacement market?

In the 1980s we relied on the then new 3mm width thermal breaks using a simple ‘pour and cut’ process; a cavity in the profile was filled with a polyurethane resin and once it was set the profile was then de-bridged to form a single profile insulated with the now set resin. This method offered a good level of insulation and helped reduce condensation, but not to levels at which the then-emerging PVC-U and existing timber could offer.

Then in the late 1980s a new ‘polyamide’ system was emerging alongside other forms of polyurethane foam filled cavities in profiles, some with the addition of PVC-U strips. Polyamide, a rolled-in strip system made from glass reinforced nylon for stability, offered the advantage of allowing the inside and outside profiles to be finished independently. These strips have now widened over the last 10 years and are commonly used at 36mm in width on modern aluminium systems. With the new ‘clip and pour’ system emerging from Thermbridge the future is bright for aluminium in both commercial and domestic use. With today’s thermal breaks and high insulating double glazed units aluminium can successfully achieve the ‘holy grail’ of efficiency – a fenestration rating of a ‘class A’ window.

As the polyurethane filled profiles are now being phased out in 2006 as they can only just reach the new Document L standards, Polyamide is quickly becoming the standard for designers of systems in the UK and Europe. In 2006 the insulated profile market in metres supplied was split quite evenly into three sectors with a third being polyamide insulated imported from Europe, a third being polyamide insulated in the UK and a third being polyurethane ‘pour and cut’ insulated in the UK. Each third is roughly 5.5 million metres of profile.

Building Regulations changes, as a result of the UK being committed to reducing greenhouse gasses by adopting the Kyoto Agreement, have pushed for higher insulation standards. Half of all the energy usage in the UK is used in our homes and offices with the glazed area of the building often being the least insulated of all the building fabric. In 2006 the insulation level was reduced from 2.2 U value to a U value of 2.0. It is expected that this value will be reduced to a U value of 1.8 in 2010 when the document again shall be rewritten. The introduction of Window Energy Rating in the domestic market has the backing of the Energy Saving Trust, which is supporting a target rating of ‘C’ for windows. The competitiveness of the market is already driving itself towards windows that achieve a rating standard of ‘A’.

Insulation methods for aluminium profiles will continue to develop and keep pace with the increasing requirements of Document L, this will include even wider thermal breaks and new materials. There is an increase in the use of timber clad with aluminium in order to extend the life of timber fenestration products, whilst these are currently on the increase and thermally efficient a great deal of energy will be used in the refinishing of the interior wood finishes. Aluminium profiles will not need any ‘re-finishing’ over an expected life of 30 years plus when powder polyester coated to a high standard.

With Clip and Pour the aluminium profile is never stressed, so relaxation of the metal is not an issue

More polyamide machinery is being installed in the UK to cater for the increased demand for polyamide insulated profiles whilst existing polyurethane machinery loses out. This creates an ideal gap in the market to utilise this capacity and this is where the new ‘clip and pour’ system can be a distinct advantage. In 2005 metre for metre the capacity in the UK for both these systems was roughly equal.

Current polyurethane ‘cut and pour’ designed profiles are being phased out due to being inefficient with a ‘cut’ on average now being 5mm. Polyurethane is an excellent insulator and is very stable as it has been improved over the years since its introduction, wider pockets and cuts are achievable but few, if any systems have been designed using a wider cut. Polyamide seems to be the popular choice, but the rolling needs to be very precise in order to get a strong bond with the aluminium to polyamide strip. There is also the matter of the little known problem of the aluminium losing grip on the polyamide particularly after powder coating as the heat causes the aluminium to return to its original extruded shape thereby losing grip on the polyamide strip. There is also the problem of having to reduce the oven temperatures on powder coating polyamide joined material otherwise the Polyamide is irreversibly damaged and it begins to expand and become brittle.

‘Clip and Pour’ advantage

So what of this new ‘clip and pour’ system from Thermbridge as mentioned earlier? The ‘clip and pour’ system offers the advantages of using a very stable and good insulator, polyurethane, whilst allowing much wider thermal breaks to be achieved than ever before using this material. Best of all it uses that huge gap in production capacity that is emerging from the discontinuation of the existing ‘pour and cut’ process. So what is ‘clip and pour’? It is simply an alternative to polyamide but without stressing the aluminium. Profiles are designed to accept a strip profile in PVC-U which ‘clips’ both inside and outside halves of a profile together, again allowing different finishes on the inside and outside if required – no special machinery is required other than low cost entry updates available for existing ‘thermal break’ machinery. In order to physically bond the resultant profile together polyurethane resin is poured onto both sides of the PVC-U clip profile and bonds to both aluminium sections in typically a 3mm or 2mm thickness. The PVC-U section only provides a support of the very tough polyurethane skins, a little like an aircraft wing construction.

Advantages? Well the aluminium profile is never stressed, so relaxation of the metal is not an issue. Thermal performance of 3mm fill polyurethane more than matches that of polyamide and a 2mm fill level exceeds that of polyamide for a given width of thermal break. No special equipment is necessary other than low cost updates to equipment already in the UK and lead times for completed profiles will be similar to that of the ‘pour and cut’ process – usually a few days.

Whichever insulation system is used in the next few years the future of aluminium as the number one choice for specifiers remains and the interest from the domestic market is encouraging as hardware manufacturers continue to develop high quality hardware for the slim and efficient aluminium profiles.