AC38 and How WRBs are Tested

The International Code Council (ICC) has acceptance criteria for WRB properties titled ICC-ES AC38. This acceptance criteria is designed to give the industry a list of tests that best evaluate the necessary performance factors required of a building wrap. These tests are helpful in comparing products.

The required tests as per AC38 are:

• Water Resistance—AATCC Test Method 127: Hydrostatic Pressure Test
• Water Vapor Transmission—ASTM E96 Desiccant Method
• Ability to impede airflow—ASTM E2178 Air Permeance of Building Materials
• Durability and tear resistance—ASTM D5034 Breaking Strength and Elongation of Textile Fabric
• Cold weather flexibility—During this test a specimen of housewrap is conditioned to 32°F (0°C) then bent over a 1/16-inch-diameter mandrel, and to pass it must not crack
• Flammability and smoke developed—ASTM E84 Surface Burning Characteristics of Building Materials

• And there is one more test that is optional:
• Drainage Efficiency—The optional test for this is ASTM E2273. During this test a wall assembly is created with the WRB over the sheathing behind the cladding. Water is applied to the assembly with the amount of drainage-over-time recorded. AC38 minimum is 90 percent drainage, with some WRBs achieving up to 96 percent.

To specify a WRB that has been tested to the highest levels, make sure that it also passes the testing criteria for ASTM E2273, for its drainability capacity, which is currently voluntary but that could be adopted into future codes.

Final Assurance: Taping the WRB

To be fully functional, the seams of the WRB must be sealed. There are two issues at play here. If the WRB is woven, the cut ends can fray and thus be vulnerable to gaps when sealing is attempted. For best results, specify a non-woven WRB. Next, the tape used should be compatible with the product, thus ideally from the same manufacturer. A newer generation of tapes with extremely strong sticking power are double sided, which eliminates the critical need for a typical 6-inch overlap.

Of course, proper installation and standardized flashing details are key to ensure that the specification of high-quality building materials was not for naught.

With the moisture issue clarified for specification purposes, we’ll now focus on how the wall is insulated.

The Right Insulation for the Application

Though control of water, air, and vapor are necessary for a safe and durable wall system, the insulation layer brings big benefits noticed by owners and occupants: comfort and savings, with the latter including both operating costs and planetary resources associated with energy use. Some insulation also provides acoustic benefits.

The type of insulation used in the wall design is highly specific to the application. It must meet the codes and be compatible with all other components in the assembly, from the framing or structural supports to WRB to hardware and more. A century ago, we designed and built with a handful of materials and asked far less of our buildings. Today there could be a matrix of multiple dozens or even hundreds of combinations.

“It’s a bewildering mix of product and system choices to get into,” acknowledges Herbert Slone, the chief architect and senior manager of commercial building systems for a global building products company.

The best strategy, Slone says, is to determine the performance goals for a wall system, then specify the system as “a system” to achieve those goals, and from that flows the products that will make up that system. A basic understanding of insulation properties helps make sense of proper wall design.

Let’s look at three distinctly different types of insulation—extruded polystyrene, mineral wool, and fiberglass—and their basic properties.

Extruded Polystyrene (XPS) 
Extruded Polystyrene (XPS) insulation, a type of rigid foam insulation, is recommended for, among other things, exterior above grade walls, and foundation walls, applied either on the exterior or interior face. It typically has an R-value of 5 for a 1-inch thickness.

XPS contains hundreds of millions of microscopic closed cells filled with a captive, low-conductivity blowing agent to provide its legendary thermal control. Perhaps the most notable quality of XPS is that it is virtually impervious to moisture, thus preventing moisture absorption and loss of R-value. Non-structural XPS weighs quite a bit less than plywood, OSB or other structural non-insulation materials (which typically have much lower R-values) and is easy to install. Because it’s rigid, it can be scored and snapped, or cut or sawed, with common tools. It never sags or settles.

In some framing applications where allowed by codes, XPS can be used as an insulating sheathing, with OSB-reinforced corners, in place of full OSB sheathing. XPS is also commonly used as continuous insulation sheathing, either over an exterior gypsum board sheathing or applied directly to the steel studs, in independently braced steel-stud framing systems. This means more R-value and reduced energy loss in stud framing systems.

Wall System Fire Performance
To meet energy efficiency standards, commercial buildings often incorporate foam plastic insulation and air/water resistive barriers in the building envelope. All foam plastic insulation is combustible, including XPS, expanded polystyrene (EPS), polyisocyanurate (ISO), and spray polyurethane (SPF). Many WRBs are also combustible. Commercial buildings, because of their area, height, proximity to property lines, and/or the nature of their use, are often required to be constructed in whole or in part of noncombustible materials. Noncombustible construction “Types” are defined in Section 602 of the International Building Code (IBC). Types I and II are defined as essentially all building elements consisting of noncombustible materials. Types III and IV are defined as the exterior walls being constructed of noncombustible materials. Type V is wholly combustible construction.

Limiting Fire Spread, NFPA 285
The IBC requires the exterior walls of many commercial buildings to be constructed of noncombustible materials, as is the case in Types I, II, III, and IV construction. The ASHRAE 90.1 energy standard for commercial buildings prescribes the use of continuous insulation (ci) over steel framing to minimize energy-inefficient thermal bridging. It also requires air-/water-resistive barriers to minimize air leakage. As explained earlier, continuous insulation is often combustible foam plastic insulation, and WRBs are often combustible. To address the dual requirements of noncombustible walls that are at the same time required to contain combustibles, with some exceptions, the IBC requires wall assemblies that are required to be Type I, II, III, or IV construction, be tested to comply with the acceptance criteria of National Fire Protection Association (NFPA) 285.

To pass the NFPA 285 test, a wall system must demonstrate limited fire spread vertically and horizontally away from the area of fire exposure. The IBC imposes two additional criteria for exterior wall assemblies:

• Potential heat: The potential heat of foam plastic in walls, expressed in Btu per square foot, is limited to the amount that has been successfully tested in the required NFPA 285 full-scale wall test.
• Ignition: Exterior walls shall not exhibit sustained flaming when tested in accordance with NFPA 268. Walls that are protected on the outside with a minimum of 1-inch thick masonry, concrete or a minimum of 7/8-inch thick stucco, are not required to be tested for ignition.

Mineral Wool—Fire Ratings and More
In applications where fire resistance is required and is a primary concern to save lives, mineral wool is the insulation of choice. For instance, it is used extensively in One World Trade Center. (See case study sidebar.)

It makes sense that mineral wool has good fire performance; it’s basically made of rocks.

“Think about rock, you don’t think of it as something that will melt and go away,” says Angie Ogino, technical manager for a company that makes mineral wool.

Ogino explains how mineral wool was made initially of naturally occurring rocks. But today, some sustainability minded companies use up to 70 percent slag, which is a byproduct of the steel industry, what’s left over after the iron ore is extracted. Those are melted at 2,600˚F and essentially taken back to their original molten state. The next step mimics the making of cotton candy, where the molten rock is introduced into an airstream, creating strands of fiber.

Mineral wool’s extraordinary resistance to fire makes it an integral part of the fire safety system for commercial buildings, including tall buildings. The mineral wool fiber is used within the spandrel area and joint created where a non-rated exterior curtain wall intersects with a rated floor assembly. Compression-fit mineral wool within the joint allows for the movement that is unavoidable in tall buildings, and creates a barrier to keep the fire compartmentalized and confined to the room of origin for up to two and three hours, preventing it from spreading from floor to floor.

Mineral wool can also be used as continuous insulation in cavity wall and rainscreen applications. Especially in systems that include an open-joint rainscreen application where UV-resistant insulations are required.

Because of its temperature and fire-resistant characteristics, mineral wool is also used in such industrial applications as high-temperature ovens and sound enclosures for industrial rooms. It is not affected by ultraviolet exposure, which can degrade other materials.

For residential use, some multifamily structures need a firewall and some UL-rated assemblies call out mineral wool.

While the fire-resistant qualities of mineral wool are widely appreciated, there are also thermal and acoustic benefits. Sound control in particular, Ogino notes, “is becoming an ever-increasing requirement for sustainable buildings.” In terms of aesthetics, mineral wool’s natural dark color provides camouflaging in open-joint facades.
 
The choice of insulation is application specific. When all is calculated, the fire performance of mineral wool insulation is most important.

“That’s my number one priority,” Ogino says of her work, “keeping people safe.”

Fiberglass Insulation
Perhaps the most flexible insulation, both in application and installation, fiberglass in batts or loosefill can provide R-values from about 10 to almost 100 and comes unfaced or faced with either a kraft or foil vapor retarder. Batts can be manufactured in thicknesses from 3½ inches to 14 inches.

Fiberglass insulation can be used in a wide range of exterior wall and roof/ceiling applications. The product can be installed in wood or metal framing cavities, installed between furring strips, or pinned to surfaces.

Because of its range of R-values and thicknesses available, it can meet many thermal specifications that exclude other materials. As an example, some products provide excellent thermal performance in the limited space of cathedral ceilings. Additionally, fiberglass insulation enhances interior noise control by improving the Sound Transmission Class (STC) of walls and floor/ceiling assemblies.

While some insulation can compress in the wall cavity and leave thermal voids, the best-quality fiberglass materials on the market are in fact dimensionally stable and will not slump within the wall cavity. Due to its inorganic fibers, it will not rot or mildew and is noncorrosive to steel, copper, and aluminum.

The Installer’s Point of View
Architects carry the mantle of creating healthy buildings more than most others in the built environment industry, and that concern likely extends to the health of laborers and crafts workers. New generations of ecologically sensitive fiberglass insulation produce less dust than other fiberglass insulation products, and some are Greenguard certified and verified to be formaldehyde free. Insulation manufacturers continue to find new ways to be environmentally friendly. At least one brand has a minimum of 50 percent total recycled content.

Kraft and standard foil facings on fiberglass insulation exceed the maximum flame spread limits prescribed by the building code. Those facings must not be used in Types I and II construction, and must not be left exposed. Facings are available that have a flame spread rating less than 25 and that can be used in those situations.