Euro Inox - Stainless Steel Improves Thermal Insulation of Buildings
07 Mar 2014
Stainless steel plays an important role in taking buildings to low-energy or zero-carbon standards.
Balconies are a case in point. Traditionally, balcony floors were quite simply continuations of the floor slab onto the outside of the building. However, the relatively high thermal conductivity of concrete makes them thermal bridges. Even if up-to-date thermal insulation is retrofitted to a façade, the adjacent interior surfaces can become much colder than the rest of the room. In addition to an unacceptable loss of heating energy, the warm air in the room can cool down below dew point and condense. The result is a permanently humid zone, which is susceptible to mould formation. Fully wrapping the balcony floor is usually impractical. In new buildings and in the addition of new balconies to existing buildings, balconies are therefore fastened using stainless-steel-containing insulated connectors. Only a few pieces of stainless steel bar penetrate the insulation of the facade. Stainless steel has low thermal conductivity – the common EN 1.4404 (316L) grade, for example, at 15 W/m-K. This value is less than one third that of constructional steel, which is about 50 W/m-K.
Similar requirements of mechanical strength, corrosion resistance and low thermal conductivity also apply when natural stone or masonry is fastened to the structure of a building. Stainless steel fulfils them all. For coastal and other corrosive environments, higher-alloyed stainless steels are available, among them the so-called duplex grades, which combine exceptionally high levels of mechanical strength with further enhanced corrosion resistance.
Insulating materials in both renovation and initial erection need to be fastened to masonry or concrete structures. These connections will be exposed to humidity and practically inaccessible at a later point in time. They, too, are thermal bridges. Stainless steel wall ties are the perfect solution. These are intrinsically corrosion resistant and do not depend on applied metallic or organic coatings, which may corrode and fail over time without the practical possibility of repair.
The thermally insulating properties are also key in another application: the “warm rims” of double and triple glazing, in which the panes are held together by spacers. The space between the panes is filled with an inert gas of extremely low thermal conductivity, which is a key element of the overall insulating properties of the final window. The rims, however, can be the weak spot of the design. Traditionally, they were made from light metal, whose thermal conductivity is quite high – about 230 W/mK – and through which a noticeable amount of thermal energy was lost. In modern glazing, stainless-steel-containing spacers have become common. Although the material thickness can be as low as 0.15 mm, the strength of stainless steel and the stiffening effect of the profiling make them strong enough to ensure structural stability. Stainless steel is also gas tight and helps to keep the inert gas inside. At the same time, the stainless steel profile is so formable that corners with minimal radii can be bent. The combination of high ductility and low thermal conductivity makes stainless steel an ideal material for this application.
There are other areas in building and construction where stainless steel is used for its physical and mechanical properties rather than its visual appearance and corrosion resistance alone. Specifiers who want to use these properties for the optimisation of their designs can find them in an online database provided by Euro Inox, the European stainless steel development association. Also available as a printed brochure, it can be accessed on-line as an interactive database at http://www.euro-inox.org/technical_tables/index.php?language=en
Insulated stainless-steel-containing balcony fasteners reduce thermal bridges to negligible levels. Photo: Schöck, Baden-Baden (D)
Stainless steel masonry-support structures are available off-the-shelf or in customised designs. Photo: Fixinox, Paris (F)
Stainless steel wall ties are a durable option for the fastening of insulation materials. Photo: Ancon, Sheffield (UK)
Low thermal conductivity and excellent formability are the assets of stainless steel spacers for “warm edges” in double and triple glazing. Photo: Rolltech, Hjørring (DK)