While thermal performance is an understandable front-runner in the consideration of higher-performance building envelopes, control layers of other types are also to be considered.
A control layer is one of a number of 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 one of several materials that are designed, installed or act to form the rainwater boundary.
In face-sealed barrier systems, this is the exterior-most face of the building envelope. In concealed barrier systems, it is a plane concealed behind the exterior face. 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. As wall or roof systems are closed with no way of accessing them, damage occurring as a result of 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, but as moisture can pass through it, it is important to make sure the vapour control layer resistance is appropriate for the building envelope system. Often, the air and vapour control layer are 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. As we add thermal insulation, we must also consider the need to add drying potential to our building envelopes.
Thermal Control Layers
Thermal control layers are the least important control layer when it comes to building durability, but 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 of the thermal control layer. This prevents moist interior air from contacting surfaces that are cold, due to being outside or between the thermal control layer and condensing.