Q: Where can I find a design for a two-hour fire-rated exterior wall with a standard 7/8-inch portland cement plaster (a/k/a stucco) finish over 5/8-inch Type X exterior gypsum sheathing, using metal studs and two layers of 5/8-inch Type X gypsum board on the interior? I am looking for a rated assembly that allows me to avoid adding a second layer of 5/8-inch gypsum sheathing on the exterior.
A: For the moment, such a system can be found in the Foundation of the Wall and Ceiling Industry’s "Single Source Reference Guide: Portland Cement-Based Fire-Rated Assemblies.” There are several points to consider before simply pasting this design into a new set of plans, however. This document, published in 1991, contains more than 70 details that depict building assemblies that use portland cement plaster/stucco as a fire-resistant finish. It was created to provide a quick reference for the then-available fire-rated systems that included stucco by several of the members of Association of the Wall and Ceiling Industry who at the time were the most knowledgeable people on the topic. The AWCI technical committee responsible for the maintenance of this document recently agreed that it is due for revision; however, recent changes in the building and energy codes may hamper any effort to update the document. That is because the requirements for exterior cladding systems are evolving right before our eyes.
The building and energy codes require new buildings to meet energy conservation standards by one of several paths: the prescriptive path, which is laid out in the codes; a trade-off path that achieves targeted energy savings using a mix of energy efficient means; and the performance path, in which a building is specifically designed to meet or exceed energy efficiency/conservation criteria.
The prescriptive path in the building codes now includes a requirement for continuous insulation as part of exterior cladding/building envelope systems. All but eight states (as of October 2013) reference some version of ASHRAE 90.1 or the International Energy Conservation Code for government buildings, and most states extend those requirements to commercial and residential construction. ASHRAE 90.1 requires that—among other things—exterior envelopes include a layer of continuous insulation between the structural frame of a building and the cladding or the building’s interior. This layer of continuous insulation is intended to provide a thermal break between the building’s conditioned space and the exterior environmental conditions with a minimum of thermal bridges. To accomplish that, ASHRAE 90.1 allows only "fasteners” (bolts, screws, nails and similar hardware) to penetrate the layer of insulation.
This particular requirement has made the application of an exterior stucco cladding system a very interesting proposition. Stucco has historically been applied over a variety of substrates or open framing using metal lath attached with nails or screws. The distance between the lath and the substrate or building paper in those circumstances has been just enough to allow the basecoat to key into the lath and close enough to contact the building paper or substrate behind it, which has occasionally led to the need for some kind of bond breaker present to ensure that the stucco didn’t bond to the substrate behind it, lest unwanted cracking occur. Now, with a layer of insulation between the framing and the lath, designers and producers are coming up with an array of configurations that will support the 10-pound per square foot cladding and still allow room for the continuous insulation. The most common method to accomplish this involves attaching Z-channel to the framing and then the lath to the Z-channel, but that alone does not create the thermal break intended by continuous insulation. Other systems rely on longer fasteners and the compressive strength of the insulation, with the bottom of the stucco membrane resting on the foundation. And as the insulation between the stucco and the framing gets thicker, the successful attachment of the lath becomes more challenging. Will a particular configuration withstand seismic movement or fire? We shall see.
So how much insulation does the cladding system need to meet the prescriptive requirements? ASHRAE 90.1 and the IECC have divided the United States into eight climate zones, each with an insulation requirement that includes the R-value of the insulation between the framing members and the R-value of the continuous insulation. Cold-formed steel framed buildings in all zones are required to have R-13 insulation in the stud cavity; zones 1 and 2 (except flat roofed buildings) require R-5 continuous insulation over the studs (and R-7.5 for flat roofed buildings in zone 2). Zones 3 through 8 require R-7.5 continuous insulation over the studs. Different insulation materials have different thickness. EPS board is thicker per R-value than XPS board, which is thicker than polyiso board. But there are costs and other considerations that must be factored into the insulation board selection process.
Well, one might ask, what does all of that have to do with selecting a two-hour fire-rated assembly that’s been around for more than 20 years? So far to date, none of the new continuous insulation systems with stucco cladding has been fire tested. So, to meet the two-hour fire rating, one can use a rated stucco system and avoid the continuous insulation requirement by following the trade-off or performance paths mentioned above, or he can convince the building official that the inclusion of the continuous insulation will not compromise the fire-rating, or he can specify that the continuous insulation go on the interior side of the structural frame, or he can choose another system that has a two hour rating over which the continuous insulation and stucco cladding can be applied.
Lee G. Jones is AWCI’s director of technical services. Send your questions to email@example.com, or call him directly at (703) 518.1611.