Putting It into Practice
At the design phase, the air control layer requires careful thought to ensure appropriate placement and continuity. There can be many interruptions to the layer, and to maintain continuity you must have a strategy for each of them, including electrical boxes and wiring, plumbing and HVAC penetrations, door and window openings, and intersections with interior walls and exterior elements like porches. Wherever possible, design for service cavities that minimize the number of penetrations.
Feasibility during construction must also be considered, since a continuous line on a drawing may not be able to be connected if the order of operations during building will hide or bury a crucial connection. build team must be aware of its placement in the assembly and the means of connection between elements. Intentional punctures and penetrations must be made conscientiously, as each hole can contribute significantly to the performance of the air control layer.
As soon as the air control layer is in place, test the building using a blower door. While the blower depressurizes the building, you will learn in real time if you have met your airtightness target, and you will also be able to find, and hopefully seal, any leaks that are detected during the test. It may be prudent to run this test again prior to occupancy, to find and address any issues with the air control layer that may have occurred during the remainder of the construction process.
Your air control layer is a critical component of your project. It is a consideration that must be determined early in the design process and then followed through the entire construction process by a person designated to carry this responsibility to ensure that your goals for occupant health, energy efficiency, and building durability are met.
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Vapor Control Layer
The vapor control layer is subject to a great deal of debate, and misinformation abounds about its role and function.
What Does the Vapor Control Layer Do?
The air inside and outside our buildings always contains water vapor in varying quantities. A vapor drive (caused by a gradient in humidity and temperature across the building enclosure) forces diffusion, the movement of water molecules through a solid material. Diffusion occurs without the movement of air through the building enclosure; it takes place through solid, airtight materials. The role of the vapor control layer is to manage the diffusion of moisture into the building enclosure.
You will notice that this definition does not insist that the vapor control layer must eliminate or prevent the diffusion of moisture into the building enclosure, but simply to manage it. Common building practice includes the use of a vapor barrier as the vapor control layer, in an attempt to prevent any vapor from diffusing into the building enclosure, but this is not always necessary, and may, in fact, create problems, as explained in this home design.