Control Layers
While thermal performance is an understandable front-runner in consideration of higher-performance building envelopes, control layers of other types are also to be considered.
A control layer is one of several layers that separates the inside environment from the outside environment, including everything from the exterior render finish on the outside of the wall cladding to the finished paint surface on the inside.
There are four main control layers in a building envelope:
Water Control Layers
The external water control layer is the most important, aiming to control the passage of liquid water. More formally, it is the continuous layer comprised of several materials designed, installed or acting to form the rainwater boundary.
In face-sealed barrier systems, this is the exterior-most face of the building envelope. It is a plane concealed behind the exterior face in concealed barrier systems. In drained systems, the water control layer is the drainage plane behind the drainage gap or drainage layer.
Air Control Layers
Air control layers reduce the risk of unwanted moisture leaking or diffusing into the building envelope. Potential failures occur due to air leakage from the building interior, carrying moisture into the walls and roof structures, where it can condense and cause long-term damage that can affect the durability of a system. A wall or roof systems are closed with no way of accessing them, damage from air leakage is concealed. It only becomes evident on internal or external surfaces in the form of mould.
Vapour Control Layers
Vapour control layers control how moisture transfers directly through materials not managed by the air control layer. A sheet of plasterboard board is a good air control layer. Still, as moisture can pass through it, it is important to ensure the vapour control layer resistance is appropriate for the building envelope system. The air and vapour control layers are often the same physical layer, but they do not need to be.
A good vapour control layer should manage moisture levels in the assembly to prevent condensation and allow increased drying potential. Buildings in colder climates face condensation risk mainly because of the need for higher insulation levels. Generally, an increase in the level of insulation in building assemblies translates to less drying potential due to the reduction in energy going through the assemblies. With many modern materials not allowing as much vapour flow as traditional materials, the risk of condensation in well-insulated assemblies has increased. We must consider adding drying potential to our building envelopes as we add thermal insulation.
Thermal Control Layers
Thermal control layers are the least important for building durability. Still, they receive considerable attention due to the obvious focus as an energy-saving initiative and a desire for increased thermal comfort.
In general, air and vapour control layers in heating climates are to be inside the thermal control layer. This prevents moist interior air from contacting cold surfaces due to being outside or between the thermal control layer and condensing.
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