Figure courtesy Building Science Corporation

Not only do gaps allow air transfer, they also expedite uncontrolled migration of moisture.

The National Research Council Canada collected research data that illustrated how even small openings can affect overall air leakage performance. The amount of moisture transported through the building enclosure via an air leakage pathway at normal interior-to-exterior pressure differences is many times greater than the amount of water vapor that can pass through a permeable material due to vapor diffusion alone.⁴ The importance of a continuous air control layer to manage uncontrolled airflow, and the moisture it carries, cannot be overstated. For example, air leakage can carry about 90 times the amount of moisture through a one-inch square hole in a 4' x 8’ sheet of gypsum board when compared to the transport from vapor diffusion moving through the same 4’ x 8’ sheet (see Figure 9).

Figure courtesy Building Science Corporation

The impacts of airborne moisture transferred via vapor diffusion and through air leakage in a cold climate.

Most buildings require a continuous air barrier per the model energy code. This means the air barrier must be continuously detailed across the entire building enclosure, including at interfaces, to be effective. To achieve continuity, the air control layer requires much more than selecting a material or specifying a lab-tested assembly. Penetrations, changes in plane, and interfaces such as where the roof meets the wall must be sealed to prevent air leakage that could result in a discontinuity that can lead to condensation problems. This involves potentially multiple trades and manufacturers.

Putting a Halt to Vapor

Water vapor also enters or exits a building by the process of diffusion through building enclosure materials. To understand how this is possible, we return to the second law of thermodynamics mentioned previously:

• Hot moves to cold;
• Moist moves to dry;
• High pressure moves to low pressure;
• Heat, moisture, and pressure always equalize when possible.

Applying the second law of thermodynamics to building science sets forth the following equation: for a conventional building in a northern climate, warm, moist inside air moves outward during the colder winter months. Therefore, the direction of the vapor drive is from the interior to the exterior. This drive notably reverses during the summer, when exterior air is hotter and more humid. In that case, moisture vapor seeks to drive to the interior of a structure, where air is cooler and drier. Buildings in hot and humid climates face vapor drive from the exterior year-round. Regardless of the direction, the moisture vapor is working to get to an equilibrium of both moisture level and vapor pressure by pushing moisture molecules through a material. The amount of moisture that can diffuse through a given material is controlled by the material's water vapor permeance. Regardless of how moisture vapor moves, the water molecules need to reach a surface or location that is at or below the dew point temperature for condensation to occur.

Condensation, which is liquid water, can negatively affect the building in many ways. One example is if condensation accumulates in the insulation layer, it can lead to R-value loss due to water displacing air within the insulation. Additionally, condensation can cause premature degradation of many wall and roof system components, such as rotting wood or rusting metal (including structural components).⁵ It can also contribute to unwanted biological growth, such as mold. Good news: prevention of these negative effects is possible.

When analyzing the roof, for example, there are three strategies that can help prevent condensation within the assembly:

1. Stop the movement of vapor by way of air. The most important way to minimize vapor problems in a roofing assembly is to use and define the air barrier. If air cannot get in, it cannot deposit moisture. This includes detailing at all penetrations and interfaces.
2. Stop the movement of vapor by diffusion before it reaches sensitive parts of the assembly. One way to reduce vapor diffusion within a roofing assembly is to use a vapor retarder. Vapor retarders can also act as an air barrier when installed and detailed as such.
3. Control the dew point location within the assembly using the thermal control layer. This ensures that any moisture vapor that happens to enter the assembly will not condense in a location where it can cause issues.

Addressing Thermal Control and Continuity

The thermal control layer follows the same principles as the other control layers discussed. The thermal control layer is designed to resist the movement of heat from one side of the building enclosure assembly to the other. Not only is this thermal layer responsible for providing indoor comfort and energy performance, it is also a critical element in preventing moisture damage. Any breaks in the thermal control layer or areas of insufficient insulation will increase the risk of condensation occurring, whether it be from humidity within the conditioned space or uncontrolled water vapor entering the assembly from the exterior.

In recognition of the critical nature of thermal control continuity, the current national model energy code, International Energy Conservation Code (IECC), and ASHRAE 90.1 include basic prescriptive requirements for insulation in both walls and roof systems. Continuous insulation is unambiguous within IECC as well as in the ASHRAE 90.1 standard it references, requiring it on both walls and the roof in all climate zones. IECC 2024 Section C105.2.1 (IECC 2021 C103.2.1) requires the depiction of the thermal enclosure. The amount of insulation required is based on R-value.

Defeating the Thieves of R-value (subhead)

R-value is, in practical terms, the measure of how well insulation resists heat flow across the material in question. Three major thieves that can steal thermal performance, or R-value, from insulation are: air leakage, lack of continuity at interfaces, and thermal bridging such as studs, fasteners or other material attachment methods.

Air leakage carries with it heat in addition to the moisture vapor discussed previously. This is detrimental to the performance of the insulation. Air infiltration and exfiltration makes up 10-40 percent of heat loss through the building enclosure, with the higher numbers found in cold climates.⁶ This is a key reason why the International Energy Conservation Code (IECC) has required the use of air barriers in building enclosures since 2012.

Thermal control continuity at interfaces is important in order to avoid thermal bridging and risk of condensation at these locations. Thermal continuity is maintained at the roof-to-wall interface by connecting the roof and wall insulation, which can be challenging. Continuity is easiest when the insulation on the outside of the structure versus inside. One example of how to provide continuous insulation at a parapet is shown in Figure 10.

Figure courtesy of GAF/Siplast

Continuous insulation design for a parapet.