Airtight Construction

Air control layer

For most of human history, the idea of building airtight shelter was both unknown and technically impossible. 

Without technologies that could control the supply of fresh air and exhaust domestic pollutants, such as smoke from heating sources or cooking fires, airtight housing could have caused actual harm to its occupants. 

Yet the efficiency, sustainability and health benefits of living in a tightly-constructed home have helped push the boundaries of what’s possible. With advanced materials, technology, and know-how, builders can deliver the advantages of airtight construction without the drawbacks. Why is this goal so important, and how do you get there?

You'll Feel This One...

While other forms of energy transfer are important to understand and control when building, the movement of air–what science refers to as convection–is uniquely intuitive to most of us based on personal experience. An encounter with convection might take the form of the blast of hot air we feel when we roll down the window of our air-conditioned car on a sweltering summer day, or the silent chill of a “draft” moving across the kitchen floor on a cold winter day.

Most homes certainly look fully enclosed and—except for any doors and windows we open—appear to be airtight. 

But the simple truth is that typical construction methods and materials leave cracks and other openings between components such as foundations, walls, flooring, fireplace surrounds, recessed can lights, drywall and sheathing edges, doors, walls, rim joists, windows, roof penetrations like vent pipes, exterior trim, and soffits. 

The movement of air is not solely limited to openings between interior and exterior, but between collective levels of the interior, with energy traveling from the ground floor or basement, where the air is denser and cooler, to upper levels of the home.

This inevitable stratification or separation of air by temperature is made apparent in most homes by our natural desire to spend time upstairs on a cold winter day—where all the heat seems to have ended up—and on the lowest level possible on a hot summer day.

Even a well-insulated house can lose huge amounts of energy through this continuous cycle of air and heat transfer from exterior openings at lower levels, drawing in outside air, to exterior openings at higher ones, sending energy back outside—a process known as a convective cycle or convective loop. 

Dressing your Home for Comfort

The loss or gain of heat through convection obviously affects energy use, as the air we have already paid to heat or cool leaves the building. And it clearly affects our comfort as we adjust thermostats, put on warmer clothes, or cluster in areas of the home that through some confluence of factors just feel better. 

While it takes more than limiting air flow to create a comfortable home, the air control layer of a structure’s thermal enclosure is an essential component which building-science expert Jacob Deva Racusin compares to wearing a “wind-breaker” shell over a “sweater” of insulation. Though such layers may be reversed in buildings, they work together to create comfort and minimize energy loss.

Air Tightness and Durability

But unwanted convective currents affect more than energy use and occupant comfort: They can threaten the structural integrity of the building itself.

Air, especially warm air, can hold incredible amounts of moisture in the form of water vapor generated from normal human activities inside. In a cold climate, a leaky building that allows warm, moist air to flow through the walls, roofs, foundation, windows, and doors creates a big problem: When warm, moist air hits cold surfaces—often exterior plywood or other material in the framing cavity—the moisture condenses, turning from vapor to liquid like shower steam on a cold bathroom mirror, but in even greater quantities. Such condensation not only degrades organic materials like wood, but is one of the main factors that allows mold to thrive.

Air leakage with its attendant heat transfer and condensation effect can also help cause ice dams, a buildup of melted and refrozen water that can, over time, destroy roofing surfaces and the structures that support them.

Measuring Air Leakage

Foundation of a Controlled Atmosphere

While it’s impossible to create a perfectly sealed house, reducing the openings and airflow between exterior and interior through the use of an air control layer can help control the convection that creates so many problems in conventionally built homes.

The reality is that air control cannot be tacked on to a house built with standard methods. For example, products like “house wrap” may provide some protection from the infiltration of water past siding, but they don’t constitute a comprehensive air control layer.

Depending on the location, climate, and topography of the building site, as well as the style of home desired, high-performance homes incorporate a variety of construction techniques and materials to achieve something as close to airtightness as possible. 

What’s true in all cases–regardless of individual project variables—is that a successful air control layer strategy must enclose all six “sides” of a building, from the foundation up. 

To put it simply, building high-performance homes incorporating air control is a team effort requiring understanding and cooperation between architect, contractor, subcontractors, and the homeowner before the first shovel goes into the ground.