By Randy Williams

Understanding control layers: A fundamental of building science

Ready for a crash course in one of building science’s core concepts? Get to know the four control layers and how they relate to your construction plans and window and door choices in this piece by guest contributor Randy Williams.

a stone building with gothic architectural features, including pointed arches, decorative columns and steeply pitched roofs. The building has multiple levels, large windows and a mix of stone and wood elements

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.

Introducing control layers

As our species has moved from caves to stone structures to concrete and mass wood structures to the homes we live in today, our expectations for health and comfort have increased. We want our homes to stay dry — but not too dry. Temperatures are expected to be comfortable, consistent, and vary little throughout the home. Indoor air quality should be healthy to breathe. And we want our investment to last for a long, long time. The parts and pieces of a home that keep the elements out, help control the temperature and humidity, and generally the building performance we all expect are the control layers.

Building science has shown there is an order of importance to the four control layers:

Water control
Air control
Vapor control
Thermal control

Each of these layers should be identifiable in a set of construction plans.

A diagram of how control layers are layered

The water 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.

The air control layer

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

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.

The thermal control layer

The final control layer is thermal control. The primary purpose of the thermal control layer is to maintain consistent desired internal temperatures and help reduce heating and cooling costs. We do this by adding insulation to our floors, walls and roofs. If insulation is installed in the right location and made continuous, such as outside our walls and roofs, we reduce our risk of condensation forming inside walls and roof assemblies. Again, we are keeping moisture sensitive materials dry. We measure insulation effectiveness by its resistance to heat flow, or R-value. The higher the R-value, the more resistance to heat moving through an assembly.

The exterior of a home under construction. The wall is partially covered with insulation panels and the lower section in a light yellow color and the upper section in red. There is also a window framed in view

Windows and control layers

So, how do windows fit into the discussion of control layers? Well, windows are one of the parts and pieces that make up a building’s envelope, they need to be connected to all four control layers to operate correctly. When effectively connected to the water control layer, they help keep water out of the building assembly. They need to keep the wind from blowing into the home and help keep the air you just paid to condition in. And lastly, a window is part of the thermal comfort package of the home. Understanding the control layers is helpful to assure your window installations are effective with continuity with the control layers.