Dear Tony
26th June 2012

Chris Mayne of the BFRC (in his letter to Glazine 19th June) reminds us that several factors affect a spacer’s thermal performance. It is not just thermal conductivity.

Crucially, performance needs to last. A spacer bar’s porosity – the degree to which it lets moisture in and gas out of the sealed unit – underpins thermal performance. A spacer bar with low thermal conductivity values will perform well to begin with. But if the spacer is permeable, so gas leaks and moisture penetrates the unit, that performance will be temporary. Over time the unit and the WER window will not perform as sold, and will fail sooner than it should.

To ensure long-lasting performance - the long life performance homeowners expect when they buy WER windows - spacer bars need a barrier that prevents moisture penetration and gas leakage. Spacer bars such as SWISSPACER have impermeable foils, which are a 100% barrier to gas and moisture. But not all backing foils are impermeable, and some porous spacer bars don’t have a backing foil at all. They rely on sealants alone to provide this barrier. They may be cheaper, and shave a few pennies off the price of a sealed unit and window, but how long will the sealed units last? How long will your reputation last when they start to fail?

Thermal performance is important, but the best spacer bars combine performance with long life so the energy saving windows people buy match their expectations of long life energy savings.

Yours sincerely

Vic De Costa
SWISSPACER Marketing & Sales Manager, UK and Ireland