Understanding control layers: A fundamental of building science

Shelter is a human necessity. A place to be warm and dry. A place to feel at home These are the main purposes of every new home that gets built. If you dig into the different assemblies and individual pieces that make up a home, you’ll see each piece or combination of pieces serves a purpose. Just as the foundation supports the weight of the structure and keeps it tied to the earth, the control layers help separate indoors and outdoors thereby increasing our comfort and health inside the home. In this piece, I’ll explain the function of each control layer. 

The purpose of the water control layer is to keep bulk water out of the structure. It’s a system of several different products and assemblies working together to keep water moving down, out, and away from the building. It starts at the roof, moving to the walls, and then extending to and below grade. The exposed roofing materials and cladding or siding are the first line of defense with other materials under those finish materials adding a belt and suspenders approach to managing water. Felt and synthetic papers, and sometimes peal and stick membranes back up the exposed roofing materials. Felt paper and plastic membranes, also known as house wrap or water resistive barrier (WRB) are located behind the exterior cladding and protect water sensitive building materials in our walls. Other materials may also be present behind the finish materials, products that improve the draining and drying of water (rainscreen) are sometimes included to improve the building durability. 

After water control, the second most important control layer is air control. Small holes and cracks in our building envelope allow outside air (along with dust and critters) to leak in and conditioned inside air to leak out. This constant infiltration and exfiltration can affect indoor air quality, the durability of the structure and comfort. Operating costs, especially in hot and humid and cold climates can also be influenced by air moving through an assembly, as a matter of fact, up to 40% of the heating, cooling, dehumidification or humidification costs associated with creating comfort inside our homes can be attributed to air leakage (New air sealing fact sheet (energystar.gov) 

Air contains some level of moisture, which can be a double-edged sword when it comes to the wetting or drying of building materials. When air containing moisture comes in contact with a cold surface, the moisture in the air can condense on that surface. If not properly vented, this, over time, can lead to mold, rot, and affect indoor air quality and building durability. But, when air is dryer than the surfaces it contacts, air movement can promote drying. 

The vapor control layer, though much less of a concern than the water and air control layers, needs to be given consideration, especially in hot and humid and cold climates. Water in vapor form can move through many common building products. This process is called diffusion, we measure diffusion by a term called permeance. Permeance is how easily water vapor passes through the material. A permeance of less than 0.1 perm is considered vapor impermeable.  Materials in this class of permeability would be metal, glass and some types of plastics. Materials with permeance ratings of 10 perms or more are considered vapor open. Most fibrous insulations, unpainted drywall, and many house wraps have permeances greater than 10. 

Water vapor wants to move from someplace more, to someplace less. This can result in wetting of building assemblies, but the process can also help dry a material. In cold climates, vapor retarders are often installed on the interior or warm in winter side of wall assemblies to prevent humidity from moving into a dryer wall cavity where it can condense (freeze) on a cold surface. This same vapor retarder may restrict water vapor from drying inward, into the living space. This drying may be needed if a bulk water leak is present, wetting the cavity during a summertime rain event. The biggest consideration in vapor control is to be aware that much more water vapor is transported by way of air leaking into and out of the home than by vapor diffusion. Air control is more important than vapor control in most instances.