Section 2: Wall Cavities and Benefits

WRBs are part of a modern wall assembly, which may also entail window and door flashing and accessory materials for application to exterior building envelope substrates within a ventilated cavity. What are the benefits of starting with wall cavities?

With wall cavities, the wall system is divided into two separate parts with an airspace between them, creating a thermal break between the two distinct wall layers. They offer resistance to water penetration and air infiltration and can be filled with rigid insulation, which provides another thermal barrier. Cavity walls offer the potential for better thermal insulation than any solid wall because that space is full of air and reduces heat transmission. They are cheaper than other solid walls. The wall cavity is a good choice for many structures; it prevents more heat from escaping from the building and prevents as much cold air getting in. Cavity walls reduce their weights on the foundation.

The wall cavity can further perform as a drainage plane and a pressure equalizer for the siding.

What about controlling air movement in these walls? Benefits include improved air quality, good insulation values, moisture and temperature control and occupant comfort.

Improved indoor air quality (IAQ)

This involves being able to move some indoor air pollutants outside and the air within the cavity no longer being stagnant. Using a device—such as ventilation systems, fans, spot ventilators, make-up air, and heating and air conditioning systems—helps ventilate a building and/or distribute conditioned air throughout a building. Ventilation also helps further remove or dilute indoor airborne pollutants coming from indoor sources and reduces the level of contaminants.

Good insulation values

Air is a proven, free, and a good thermal insulation material; the origin and the destination of the air depends on climatic conditions, building orientation and HVAC strategy. Cavity walls offer a heat flow rate that is 50 percent that of a solid wall. Insulation reduces heat transfer or flow, so it also can moderate the temperature across the building envelope cavity.

Moisture and temperature control

Air sealing and moisture control are essential for building energy efficiency. Moisture can cause problems in attics, some foundations and walls, and the solutions to those problems vary by climate.

Air sealing and moisture control are essential for new buildings and their energy efficiency.

During the heating cycle in wall cavities, heat radiates from the sheathing to the foil, but is reflected back to the sheathing without warming up the foil from radiant heat transfer. During the air conditioning cycle, the exterior is heated, but with the foil, the heat is not transferred to the sheathing. So the cavity acts as a heat exchanger and exhausts the heat of the hot exterior, and the heat isn’t transmitted to the living or working space.

Occupant comfort

The controlled movement of air within a building can directly contribute to occupants’ comfort through achieving improved IAQ and moisture and temperature control and providing good insulation values. In winter, movement of cooler air often is noticed as unwelcome drafts. In summer, though, air movement from either convection currents or by mechanical means over exposed skin enhances evaporation, making occupants feel both cooler and dryer.

WRB qualities and considerations for wall cavities

It is imperative that wall assemblies have the ability to dry out to prolong the durability of the wall. The permeability is the amount of vapor transmission a WRB allows over time while minimizing the potential for moisture vapor accumulation. The higher the permeance rating or perms, the more vapor-permeable the material. This number is somewhat nebulous because of variations in lab and installed conditions, but the perm rating should be higher than 5. Still, a higher perm rating doesn’t necessarily guarantee that it’s better—WRBs with micro-perforations may allow the passage of more water vapor, but they also can make it vulnerable to bulk water leakage. Other high-permeability WRBs may allow moisture stored in the reservoir cladding to be driven into the sheathing and insulation.

A WRB permeance in the range of 1 to 10 perms allows inward-driven moisture while still enabling outward drying in climate zones 1 to 7. Very low-permeance WRBs (less than 1 perm) should not be used unless there is 1 inch or more of exterior insulation (and at least 2 inches in climate zones 6 and 7). High-permeance WRBs (50 perms) should not be used with vapor-permeable exterior insulation with reservoir claddings that are exposed to higher rain levels. Low-permeance interior vapor control results in elevated moisture content in the sheathing by capturing the inward-driven moisture; this should be avoided.

The biggest concern with vapor-permeable insulating sheathings is inward-driven moisture caused by solar drive hitting a wet moisture-storing cladding, which is a problem that occurs in all climates. When this happens, high vapor pressure builds behind the surface of the cladding, which can lead to outward drying but also create an inward vapor drive, particularly if the indoors is air-conditioned to a lower vapor pressure.

WRBs with an integrated rainscreen provide a continuous vented airspace over the entire surface area of the wall, providing better drainage and drying. Because many rainscreen products use a combination of plastic materials for the gap, they aren’t subject to saturation and decomposition that could compromise wood furring. These products are recommended in areas with wind-driven rain, high amounts of rainfall (40 to 60 inches annually), or those with high temperatures and humidity. New construction in coastal areas and those with hilltop exposures are key examples for locations where this technology should be applied.

There is an order for wall components in new construction. As far as placement in the wall assembly, the closer the wet materials are to the ventilated cavity, the higher their permeability should be.

Section 3: Properly Draining Exterior Wall Cavities

What is the importance of properly draining exterior wall cavities and how does that mitigate moisture?

Improper air flow can have severe effects on the health and safety of the people in a building by allowing mold growth, spread of pollutants and possible backdrafting of combustion appliances.

As we know from Section 1, as building assemblies have progressed, WRBs have assumed new functions—one of the most notable being to help remove trapped water from the building enclosure. WRBs need to stop moisture from the outside while also allowing walls to breathe and prevent vapor buildup.

The water can get inside in many ways, as mentioned in Section 1. Behind the exterior finish, every new building should have a drainage plane, a mechanism that redirects any bulk water that penetrates the exterior finish down and away from the wall assembly. Drainage features are widely accepted as one of the most effective measures for reducing the risk of potential damage from moisture penetration; it is a critical component in keeping walls dry. In the past, the use of furring strips that separate the wrap from the structural sheathing and framing helped with drainage.

New products with enhanced drainage and an integrated drainage plane reflect and address the needs of new construction and environment. The best WRBs, then, divert bulk water from exterior wall cavities and drain it away from the assembly, preventing the potential damage caused by mold and rot. It is the job of the WRB to offer this level of protection, to block air and moisture while allowing moisture vapor from the wall cavity to escape to the outside. This should contribute to dependability, durability and safety for a long time.

The most advanced WRBs offer integrated drainage gaps through creping, embossing, weaving, or filament spacers and eliminate the need for furring strips, streamlining installation. A WRB’s drainage efficiency is usually tested in accordance with ASTM E2273 but, considering the many drainable building wraps, how quickly bulk water is drained can vary significantly.

A few select WRBs offer a drainage gap and drainage efficiency per ASTM E2273—without sacrificing durability and ease of installation benefits that builders and contractors have come to expect—and they can be installed in any direction without affecting performance.

Of course, the most effective way to reduce air-transported moisture is to seal the building tightly against air infiltration or exfiltration, helping to keep damp air outside and enabling the building’s HVAC system to remove excess moisture from the air inside the building. Properly draining exterior wall cavities contribute to the durability of the building as well as to the health, safety and wellness of its occupants.