The Cavity Wall Conundrum

Complex, modern building designs require balancing the need to keep the building dry, airtight, thermally efficient, and code compliant. Photo © Vladimir Sazonov Shutterstock.com

A new e-book called the “Evolution of Building Enclosures”, published by Construction Specifier, offers a four-part series, including an article on what the magazine calls “the cavity wall conundrum”. Authored by Todd Skopic, a building science manager, the article provides an in-depth, technical look at the use of open-joint rain screens coupled with unconventional wall orientations. While these configurations can be appealing, they also pose a potentially dangerous combination; abating water ingress is an important issue to address, but these systems must be compliant with building codes, including those that test for combustibility.

Balancing the need to keep the building dry, airtight, thermally efficient, and code compliant can create what Skopic calls a cavity wall conundrum. As more architectural firms push the limits of building design, ensuring a safe and efficient building envelope is becoming more complex. The growing practice of combining open-joint rain screens with unconventional wall orientations, such as a backward-sloping configuration, offers a prime example.

In such structures, design teams want to prevent water ingress, but they also need to follow the latest building codes. Staying compliant with certain ones, such as the energy code, complicates matters by introducing certain materials that increase potential safety risks.

Managing water with building enclosures involves the three Ds: deflection, drainage, and drying. Open-joint rain screen systems offer an increasingly popular means to achieve the three Ds and behind every open-joint rain screen, is an air and moisture barrier to defend against water ingress. All of these solutions are subject to and must comply with an abundance of codes and regulations.

The 2012 International Building Code (IBC) requires buildings in Climate Zones 4 to 7 to have a continuous air barrier, which in most cases also takes the form of a water-resistive barrier. The 2012 International Energy Conservation Code (IECC), is also driving the use of continuous insulation (ci), which in some cases is combustible. It needs to comply with the National Fire Protection Association (NFPA) 285 – a standard fire-test method for evaluating the fire propagation characteristics of exterior, non-loadbearing wall assemblies containing combustible components.

In other words, today’s design teams are trying to design building envelopes that are watertight, airtight, thermally efficient to meet code requirements, and to be NFPA 285-compliant. Solving this ‘cavity wall conundrum’ is possible, but it requires some familiarity with the competing design challenges and different industry standards.

This in-depth, technical article discusses rain screen design, and the standards for managing air and water, in context of the codes for continuous insulation (ci), air barriers, and water-resistive barriers, as well as life safety issues related to combustibility. For instance, how do cladding attachments impact a system? What is the the value of a continuous insulation system with adhesive-backed sheet membrane that isn’t penetrated? What are the differences between sprayed polyurethane foam (SPF) and expanded polystyrene (EPS) when used as insulation in cavity wall assemblies, vis a vis thermoplastic extruded polystyrene (XPS) which is a thermoplastic foam rigid insulation board? And how do these compare with mineral wool or fire-enhanced polyisocyanurate (polyiso) mineral wool in performance and code compliance? And what are all the codes?

Solving the Conundrum

Building designers are increasingly aware of the competing requirements and standards involved in modern cavity wall design. They should know continuous air barriers and insulation systems, along with NFPA 285 code and other compliance issues, which must be balanced with the goal of keeping water out of a building. Achieving this balance will help designers create the safest, most effective building envelope possible and thus solve the cavity wall conundrum. And on the building materials front, manufacturers need to test all their products to ensure they meet the extensive industry standards and testing.

The other chapters in the new e-book cover the benefits of specifying complete masonry veneer wall systems, defining and testing construction tape and flashing durability, and moisture in new concrete roof decks.


Advanced Insulated Wall Systems that Exceed Expectations and Code

StoTherm ci XPS is a continuous insulation system, which provides air, thermal and moisture control without the connection and compatibility challenges that characterize other systems, while also offering multiple design and finish options.

Today’s architects, specification professionals, and owners are typically looking for an insulated wall design that not only meets but exceeds the nation’s increasingly demanding code requirements. Enter the StoTherm® ci XPS continuous insulation system, which provides air, thermal and moisture control without the connection and compatibility challenges that characterize other systems, while also offering multiple design and finish options.

As the building industry adopts more stringent energy codes (Title 24, IBC, IECC, ASHRAE 90.1), the need for external insulated finish systems (EIFS) is increasing. The StoTherm ci system is highly energy efficient, minimizing heating and cooling costs while reducing greenhouse gas emissions. The components prevent thermal bridging, thus lowering the risk of heat leakage and the attendant energy loss.

Other features that make the StoTherm ci XPS system a superior alternative to other systems include:

  • Durability and impact resistance (77% higher density and 250% higher compressive resistance than EPS)
  • Low Water Absorption (due to its closed cell structure)
  • R-Value of R5/inch (the higher the R-value, the greater the resistance to heat flow)

The system is also installation friendly; one installer and a single skilled trade person can make quick work of it. The low allowable deflection value makes for lightweight construction, which reduces overall project cost and weight per square foot.  These factors make for material and labor costs that are highly competitive, if not more economical than most other options.

A wide range of decorative and protective wall finishes (StoCreativ® Brick, granite, limestone) along with unlimited color choices make StoTherm ci XPS one of the most versatile and innovative products on the market today.


Sto To Present at D+D Material Selection Conference

VLC odor neutralizing coatings are one of the innovative products that will be featured including StoColor™ Climasan®  an interior wall and ceiling coating that will neutralize odors in the air when exposed to a light source. 

Durability & Design (D+D), the trade magazine focused on architectural coatings for the built environment, will be hosting a one-day Material Selection Conference, September 26,  2017 in Pittsburgh, Pennsylvania. The program is designed to help owners, architects, contractors and other building industry professionals determine the best coating systems to manage moisture intrusion into exterior walls, and the best liquid-applied air barrier designs to limit heat, air, and moisture transport through walls.

In each of two panel sessions, a panel of industry experts, including manufacturer representatives, will discuss how their specific brands perform in different climates and indoor environments, and on different substrates. Sto will be presenting in both sessions – one focusing on coatings and the other on liquid-applied air barriers.

Attendees should leave with a better understanding of the effect of permeance on exterior wall coatings; learn how to determine the performance, cost and cost-effectiveness of liquid-applied air barriers; and understand the advantages and disadvantages of specific coating brands that are candidates for use on their buildings.

Attendees will qualify for 8 AIA learning credits for full-day participation and 4 credits for a half-day.


An Award-Winning Project that Includes StoGuard®

Disney Springs won an award for its contractor and kudos for a key vendor on the project: Sto. Photo: Chad Baumer

Disney Springs won an award for its contractor and kudos for a key vendor on the project: Sto. Photo: Chad Baumer

KHS&S is an international design-assist specialty contractor with a portfolio that includes more than 5,000 casino resorts, hospitals, hotels, entertainment venues, retail facilities, theme parks, attractions and public works projects around the country and overseas. Founded in 1984, the firm is now the second largest specialty wall and ceiling contractor in the USA.

At the recent Florida Wall and Ceiling Contractors Association (FWCCA) convention and trade show in Orlando, the company picked up the award for Project of the Year for the work they did on Disney Springs, an elegant shopping, dining and entertainment complex at Florida’s Walt Disney World. A key partner in the KHS&S supply chain? Sto Corp, who provided wall components for two stucco buildings, that included products from its leading air and moisture barrier system: Sto Gold Fill® and StoGuard Mesh, Sto VaporSeal®, Sto TurboStick™ and Sto DrainScreen.

Sto worked with Disney’s architects on the specification details. Now that the work is done, the results speak for themselves.

The Disney Springs design team specified StoGuard for the KHS&S project. Photo: Chad Baumer

The Disney Springs design team specified StoGuard for the KHS&S project. Photo: Chad Baumer

StoGuard

The handsome brick facing was constructed using StoGuard Vaporseal for protection. Photo: Chad Baumer


Building Better Walls PART THREE: Vapor Control & Thermal Control

Nike World Headquarters in Oregon where Sto VaporSeal® was used as an air moisture barrier.

The fact that many vapor barriers also retard or eliminate airflow sometimes causes confusion about the functions of the ABS and vapor barriers. The function of a vapor barrier is to control  water/vapor diffusion and reduce the occurrence or intensity of condensation. As such, a vapor barrier has one performance requirement: it must have the specified level of vapor permeance and be installed to cover most of the area of an enclosure.

That being said, vapor control systems are either vapor permeable or impermeable membranes. Determining which type you need primarily depends on climate and wall design.

There are a wide range of products and systems available today that employ formulations suitable for any climate. These will protect buildings from water infiltration in high-rainfall regions, as well as from water vapor drive and unwanted air movement. (Example: STO VaporSeal®) Fluid-applied, vapor-permeable building membranes are especially versatile, and may be used under a variety of mechanically attached claddings, including cement board, wood, vinyl, brick, stone and metal panels.

Providing thermal comfort without overspending on excessive space conditioning costs is also one of the primary requirements of today’s building designs.. Therefore, thermal control is an important aspect in almost all buildings. Understanding heat transfer and the temperature distribution through building materials and assemblies is critical for assessing energy use, thermal comfort, thermal movements, durability, and the potential for moisture problems.

The key components for controlling heat flow in building walls requires insulation layers that aren’t penetrated by thermal bridges, an effective air barrier system, good control of solar radiation and management of interior heat generation.

So, in conclusion of our three-part series, a “clever” wall (we’ll even venture to say a “smart” wall), needs resilient design components that control air, moisture, vapor and thermal conditions. Get the wall right and it can make all the difference in creature comfort, energy efficiency and economics. Whether you’re retro-fitting or building from scratch, wall systems that can provide all four key controls are what you want.


Building Better Walls PART TWO: Air Control

The second part of our series on Building Better Walls focuses on air control and the basic requirements for air barriers systems in walls.

The management of airflow is important for several reasons: It controls moisture damage, reduces energy losses, and ensures occupant comfort and health. Airflow through a building enclosure is driven by wind pressure and the stack effect – movement caused by warmer air rising and colder air falling that create pressure differences. These, in turn, can lead to air leakage, unexpected airflows, and indoor air-quality problems. Mechanical air handling equipment such as fans and furnaces also impact air flow.

A continuous, strong, stiff, durable and impermeable air barrier system is required between the exterior and interior conditioned space to control airflow driven by these natural phenomenon. Wall air barriers provide critical protection in all buildings, regardless of region or climate. Controlling air leakage within the building envelope also enhances a structure’s energy efficiency.

Basic Requirements of Air Barrier Systems for Walls

Typically, several different materials, joints and assemblies are combined to provide an uninterrupted plane of primary airflow control. Regardless of how air control is achieved, the following five requirements should be met to achieve a proper air barrier system (ABS):

  1. Continuity.  Enclosures are 3-D systems! Continuity must be ensured through doors, windows, penetrations, around corners, at floor lines, soffits, etc.
  2. Strength.  The ABS must be designed to transfer the full design wind load (e.g., the one-in-30-year gust) to the structural system. Fastenings can be critical, especially for flexible non-adhered membrane systems.
  3. Durability. The ABS should continue to perform for its service life. Ease of repair and replacement, the imposed stresses and material resistance to movement, fatigue, ambient temperature, etc. should all be considered.
  4. Stiffness. The air barrier must reduce or eliminate deflections to control air movement into the enclosure; it must also be stiff enough that deformations do not change the air permeance (e.g., by stretching holes around fasteners) and/or distribute loads through unanticipated load paths.
  5. Impermeability.  Typical recommended air permeability values are less than about 1.3 x 10-6 m3/m2/Pa. In practice, the ability to achieve continuous insulation is more important to performance; the air permeance of joints, cracks, and penetrations outweighs the air permeance of the solid materials that make up most of the ABS. Hence, a component should have an air leakage rate of less than Q< 0.2 lps/m2 @75 Pa, and the whole building system should leak less than Q< 2.0 lps/m2 @75 Pa.

It is important to note that increased airtightness must be matched by an appropriate ventilation system to dilute pollutants, provide fresh air, and control cold weather humidity levels. Good airflow control through and within the building enclosure will bring many benefits including reduced moisture damage, lower maintenance costs, energy savings, and increased health and comfort.