Joined-up design

By SAS

With the new Document L just released with, as expected, a further reduced U-value down to 1.6W/m2K for compliance, the window industry continues to look for materials and designs which will enable glazed assemblies to become even more efficient.

There are three issues which are determining, and in some cases limiting, overall window U-values and Window Energy Ratings (WERs). These are centre pane values, double glazing edge sealing technologies and frame designs. When each of these areas of a window perform well, offering good insulation levels, they work in harmony with each other and the result is high performance windows that can achieve high overall U-values. With most modern framing materials available today, compliance with the new document L is entirely achievable, so the industry does not need to worry too much with a 0.2 drop – well not just yet.

Initially looking at centre pane values, triple glazed units can easily be manufactured take the value down to 0.7 U-value for a typical 4/10/4/10/4 configuration (4mm glass [soft coated] / 10mm airspace [argon]). Double glazed units in 28mm and 32 mm configuration can get down to a 1.0 U-value. These are exceptionally high performing glasses, and when we take solar gain into consideration we are moving very much in the direction of renewable energy from such units. Where we do start to encounter problems is the unit spacer bar used to create the triple / double glazed units.

The commercial glazing market generally continues to use an aluminium spacer, perhaps because this is seen as the most structural solution for such high performing commercial glazing. It is important to note however, in structural glazing the outer glass relies solely on the unit sealant to hold the glass into place on the building – not the aluminium spacer bar. While an aluminium spacer bar gives an excellent strength to weight ratio it also transmits heat 200 times faster that air. So while we may be able to produce a centre pane value of 1.0 or below, at the edge of the glazing it is a very different matter where measured insulation can be negligible. This can often be seen on cold days where condensation appears at the edge of the unit only, irrespective of framing material.

There are alternative materials we can now use for the spacer bar, these are known as warm edge technologies. Flexible and adhesive backed, these bond to the glass and work dynamically with the glass and allow thermal movement whilst offering exceptional levels of insulation.

Finally the frame design can take up quite a large area of the window. Here various materials have advantages and disadvantages when you consider the thermal conductivity of the material verses its strength, life expectancy and required maintenance, to name but a few of the material sustainability considerations. With sizeable thermal breaks now deigned in most metal profiles, compliance can easily be achieved with the new document L. By just adopting warm edge technology on a typical window, with any framing material, could improve U-value performance by as much as 0.2 required in the new Document L.

Overall any window will continue to be one of the weakest areas of insulation within building structures, the slimness of the product and the interface with much thicker structural elements of buildings will restrict any real improvement in these areas. One solution is to use double windows, as used in some very cold countries, two windows separated by a broad airspace. While this solution helps to minimise cold bridging with a wide structure it can prove to be impractical in the UK when we wish to open windows easily for natural ventilation – as anyone with secondary glazing will testify.

There is a real opportunity now to rethink the way we design windows, doors and curtain wall systems. We can use warm edge technology, we can reduce frame sight lines of windows which maximise the good centre pane values and we can achieve all this by centralising the design team taking all elements into the equation.

Senior Architectural Systems has begun the journey by bringing the technology of glazed unit manufacture within the group by acquiring a company in Scarborough, now named Senior Glass Systems. This is a first in the UK and the company is already seeing the benefits of being at the cutting edge of high performance glazed unit manufacture. Seniors has joined forces with Edgetech and now use the company’s commercial ‘TriSeal’ product as a standard solution for commercial unit manufacture. The dynamic nature of the spacer material allows for glass movement, reduces shear loads on the edge sealant and as a result offer units which can be guaranteed up to 20 years. Designed specifically for the commercial market the TriSeal product is one of the only warm edge technologies which is actually manufactured here in the UK. Seniors believes that it has the perfect partner in Edgetech to develop class leading systems for our secure and low-carbon future.

Senior Glass Systems also operates in its own right, and secures orders from a wide range of fabricators and installers across the UK. Anthony Shillingford is sales manager and he is keen to point out the specialism of the business. “We are delighted to have the support of Senior Architectural Systems and we have re-organised the business to be dedicated to the commercial sector,” he said. “We have specialists who can advise on the correct specification of glasses and while we have direct supply from two major glass producers, we can obtain virtually any glass for a project where matching is required. At SGS we run an automatic DG line with two robots and can manufacture and ship across the UK up to 500 units per day.”

Since joining the Seniors fold, SGS has seen a steady uptake of orders from across the UK fabricators and obviously from Senior’s own customer base. Development is now taking place to determine alternative methods of fixing high performance units to efficient framing systems to provide windows to sustain our low carbon future.

www.seniorarchitectural.co.uk
01709 772600

www.seniorglass.co.uk
01723 580010

Pictures: Anthony Shillingford – SGS sales manager; setting up the TriSeal robot; completed Seniors project.