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.

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