09.08 Edge details and frames

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Categories: Advanced Glazings

At the present time a typical advanced glazing is usually a component with good thermal performance, mounted in a well-insulated wall, with a poor glazing edge and a poorly performing glazing frame.  Whilst improving the performance at the centre of a glazing system is important, because designers prefer to use as much glazing as possible, as the centre-glazing performance is improved the significance of the edge of the glazing and of the glazing frame becomes more significant.

Lowe and Olivier [1995] discuss issues relating to the edge of glazing, and rate glazing frames in terms of the ratio of overall U-value to centre-glazing U-value.  A ratio of 1.0 or lower indicates that the window frame is not compromising the heat transfer through the system.  However, the influence of solar gain is also important on the overall energy balance, and this simple performance ratio does not tell the whole story.

O’Catháin and Howrie [1994] indicate that for best solar performance the ratio of frame to opening should be minimised.  However, the effect of the window rebate should also be considered, as this can overshadow the edge of the glazing quite considerably.

Warm edge technology
A principal shortcoming in many multiple glazing systems is the heat transfer that occurs through the aluminium spacer used to keep the panes of glass apart.  To reduce this heat transfer a number of manufacturers now offer spacers with a significant insulating performance.  This can reduce the interaction between frame and glazing, and in a study by Harris [1995] the best warm edge spacers were found to reduce the heat transfer through a simple window by 7.5% with conventional air-filled double glazing, and by 11% with air-filled low-e double glazing (the improvement in performance gets better as the centre-glazing U-value is increased).

The other advantage of warm-edge technology is that the temperature of the warm-side glass edge is increased; the best case assessed by Harris [1995] showed a 3oC increase in the lowest glass surface temperature, for an inside temperature of 20oC and an outside temperature of 0oC.  In a typical UK winter this can mean the difference between visible condensation and clear glass.  In a colder climate, such as Canada and mid-USA, where outside temperatures might drop below -20oC, the warm-edge spacer can make the difference between condensation and ice!

Typical types of warm edge technology glazing spacer include, image:

  • aluminium spacers with pour-and-de-bridge resin thermal break (Azon ‘Warm Light’, Helima ‘Helitherm’)
  • thin-walled stainless steel spacers (Allmetal ‘SST Spacer’)
  • butyl sealant spacers with metal reinforcing strip (Tremco ‘swiggle strip’)
  • silicone foam rubber spacers (Edgetech ‘Super Spacer’)
  • extruded plastic spacers (Inex Spacer Industries ‘INEX’)

In some cases the spacer is produced with the desiccant incorporated into the matrix of the spacer material, and for some materials there are a range of colour options.  The Tremco ‘Swiggle Strip’ does not require any additional sealant, and the Edgetech ‘Super Spacer’ is supplied with self-adhesive tapes already in place.

Warm edge technology is also discussed by Blower [1996] and Watts [1996].

Most of the warm-edge spacers include some kind of polymer material, which raises the usual issues of colour stability, compatibility with sealants and reaction to UV.  The softness of some warm-edge spacers may also cause problems if glazing units are made with thick gas-spaces - the thermal expansion of the gas can generate significant pressures inside such units, which may damage the spacer.  However, the makers of such spacers are generally aware of these issues and can give guidance.

An important issue in the use of warm-edge glazing spacers is the significance that is now placed on the glazing-edge sealant.  The high thermal conductivity of a traditional aluminium edge spacer means that the heat transfer through the edge sealant is insignificant; the low effective thermal conductivity of a warm-edge spacer means that the heat transfer through the sealant may be a significant proportion of the overall heat transfer.  Sullivan and Wright [1995] made a series of measurements of heat transfer through glazing spacers, in which several parallel strips of each spacer were sealed between plates of glass, as shown in this image, and the heat transfer was measured per unit length of spacer.  In the case of a warm edge spacer which comprised a reinforcing strip in a sealant base it was found that increasing the width of the sealant by 50% increased the heat transfer by 25%.  Computer simulation methods are usually used to assess the impact of the glazing spacer on fenestration performance (the change in heat transfer may be less than the experimental error in a typical hot-box), and so it is essential that the correct amount of sealant is used (too much sealant means too much heat loss, too little sealant may lead to premature failure of the glazing unit) and that its thermal conductivity is accurately known.

Warm-edge technology glazing spacers are already used in Canada and the USA and a large amount of information is being gathered on issues such as durability.  It is probable that other types of spacer will be developed (this is essential if glazings are to be produced with even lower centre-glazing U-values).