Choosing the right building wraps
Most often called sarking, sarking-type material or wall wrap, these building wrap membrane products are installed within the building envelope of a given dwelling type to do one or more jobs. Doing these jobs well depends largely on the location of the building and, obviously, how well the membrane has been chosen, transported, handled, placed, affixed and taped. In Australia, getting this right is where standards like AS/NZS 4200 and AS/NZS 4859 come in, to ensure the requirements of the National Construction Code (NCC) are satisfied.
Some of the jobs a sarking or building wrap may need to do are:
The advantage of foil laminate membranes has long been the extra thermal resistance they can add in walls or certain roof spaces if installed facing a still air cavity. This is based on the ability of the foil to reflect rather than absorb the infra-red energy transmitted through the air. A whole extra R-Value, or more, can be achieved in carefully designed wall systems featuring multiple yet surprisingly narrow reflective air spaces. This approach is particularly valuable in roof-spaces during summer when the heat flow is intense and mainly downward. Reflective airspaces are calculated using the procedures found in AS/NZS 4859.2. Ametalin manufactures a range of superior quality reflective laminate membranes, including SilverWrapTM for standard sealing of houses, SilverSark® Micro-perforated for condensation management in cavity construction, and SilverSark® xR with double-sided reflectivity for additional system R-value.
When building nearly anywhere south of the Tropic of Capricorn, the vapour permeability of the wall system should be carefully considered. Building membranes are classified according to AS/NZS 4200.1 as vapour barrier (class 1 or 2) or vapour permeable (class 3 or 4), depending on the amount of water vapour which will pass through them under set temperature and humidity conditions. Ametalin Brane® VHP features the highest vapour transmission rate on the market, and installed properly within walls it will allow long-term balancing of humidity as activities like showering, cooking, and laundry generate indoor vapour. This is vitally important to avoid the absorption or condensation of moisture inside the walls themselves, leading to mould and other damage.
Some building wraps are manufactured from sheets of foam sandwiched between foil layers to form a much thicker product with very high tensile strength. These can function as sarking in roofs but are also recommended for increasing R-values within double brick construction. The other important role they fill is in steel frame housing, preventing thermal bridging through the highly conductive steel. The NCC requires at least R0.2 for the installed thermal break membrane for this purpose – Our Extra Heavy Duty product, Ametalin ThermalBreak 8 PLUSTM handily exceeds this value along with every other requirement of the relevant standards.
NOTE: ThermalBreak wraps and other vapour barriers may not be suitable under NCC provisions in climate zones 6, 7 and 8.