The design alternatives available for subframing attachment systems include wood, steel, PVC/fibercement battens, or aluminum. In addition, there are also alternatives available for the use of either fixed system details that require supplemental shimming of the system to address any out of tolerance conditions in the substrate, or adjustable systems that use brackets that allow for internal adjustments to compensate for out of tolerance substrate conditions (see future posts and a link to ALLFACE Smart Fixing Systems in the product links section for more information).
So understanding the implications associated with the use of each of these subframing components allows the system designer to make an educated decision on appropriate material selection (based upon design and budget) and what the life expectancy for each will be.
For the sake of this analysis, we are assuming that the mechanics of the rainscreen system will follow suit with the physics of a rear ventilated rainscreen design. The intent of this system approach is not to keep moisture out of the cavity, but to limit the membranes exposure to moisture thru proper design. So how the subframing components respond to moisture is important for the longevity of the system, and sometimes the structural performance of the system.
So when considering products like wood battens, special care must be taken to assure that the elements are properly treated to prevent moisture related rot and warpage. As a minimum pressure treated lumber should be used, as well as full joint closures and cavity drainage elements.
The use of fibercement and PVC battens are typically only to provide sufficient cavity spacing (1/2 inch +/-), and are not intended to be structural elements. So for commercial and highrise applications, the decision really comes down to the use of steel versus aluminum subframing systems.
Conventional rainscreen design wisdom has gravitated towards the use of Aluminum subframing systems as the "material of choice" for various reasons. Notwithstanding, the long term benefits gained from its use far outweigh any additional initial costs that may be associated with its use.
The low weight and high strength, malleability, simplicity of fabrication, corrosion resistance and good ability to conduct heat and electricity are some of the most important characteristics of Aluminum. Also Aluminum is the most widely recycled product due to its abundant post consumer and post industrial stock available, its low melting point and limited energy required to recycle.
The density of Aluminum is approximately one third the weight of steel. So for sustainable design applications where the weight of the enclosure system can impact the sizing of the building structure or foundations, this can be an important factor to consider. But the weight of aluminum doesn't negatively impact the strength of the material as it relates to cladding systems where the limitation is actually with the spanning capabilities of the cladding material, and not the strength of the subframing.
But one important design factor to consider when using aluminum is the coefficient of thermal expansion of the material. Compared with other metals aluminum has a relatively large coefficient of linear expansion. So the system designer must take into consideration not only the thermal movements of the subframing, but the thermal movements of the cladding material at panel fastening locations. Otherwise an improper system design can lead to cracks at fastener locations, or cracks in the panels due to buckling or panel to panel contact.
As indicated by its position in the partial electromotive force series, Aluminum is a relatively reactive metal; among structural metals (only beryllium and magnesium are more reactive). Aluminum owes its excellent corrosion resistance to the barrier oxide film that is bonded strongly to the surface and if damaged reforms immediately in most environments. On a surface freshly abraded and exposed to air, the protective film is only 10 Angstroms thick, but highly effective at protecting the metal from corrosion.
The Swedish Institute for Metal Research into Corrosion has carried out open-air experiments with different untreated metals. These show the losses in weight of sheet metals with untreated surfaces after 8 years exposure for both inland and coastal (less than 1 mile from the coast) locations. In a mainland atmosphere of moderate saline content, the durability of aluminum is excellent. The following results were noted in these open air experiments:
Inland Location
Material : Weight Loss
Aluminium : 2g/m2
Copper : 31g/m2
Zinc : 61g/m2
Carbon Steel : 676g/m2
In strong saline atmospheres, it is possible that a small level of corrosion may appear on the surface. But generally the durability of Aluminum far exceeds that of either carbon or galvanized steel. The occurrence of salts, especially chlorides, in the atmosphere only slightly reduces this durability by comparison with other metals. The effects upon carbon steel are much more severe.
Coastal Location
Material : Weight Loss
Aluminium : 7g/m2
Copper : 57g/m2
Zinc : 133g/m2
Carbon Steel : 933g/m2
The average for the deepest corrosion on aluminum sheets after 8 years was 70μm (0.07mm) approximately 1/100 the loss of carbon steel and 1/10 the loss of galvanized steel (see zinc in the table).
So when considering the effects of corrosion on metals, the system designer must consider the two chemical processes involved: oxidation and reduction. The oxidation process takes place at an area known as the anode. The four essential components that are needed for a corrosion reaction to occur are an anode, a cathode, an electrolyte with oxidizing species, and some direct electrical connection between the anode and cathode. Although atmospheric air is the most common environmental electrolyte, natural waters, such as seawater rain, as well as man-made solutions, are the environments most frequently associated with corrosion problems.
Experience with the use of carbon and galvanized steel in rainscreen applications has shown that at areas where panels are fastened directly to the metal subframing, exposure to corrosive failure is concentrated at the exposed/cut edges of the steel elements and at fastening holes. So the system designer must take into account the potential for material density loss (oversizing of fastening holes) that can develop with steel subframing and what that means for the chance of future panel failures. Aesthetic issues also include corrosive streaking along the panel face due to the corrosion of steel subframing elements.
Similar problems have not been shown to be an issued with architectural grade aluminum.
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