Specifying rigid foam insulation for masonry cavity walls

by nithya_caleb | June 19, 2019 12:00 am

by Nicole Richard

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The use of masonry cavity walls dates back to ancient Greece. While this construction method is still thriving, it has evolved to meet the performance needs of 21^st century buildings. One of the modern adaptations is the use of rigid foam insulation to maximize thermal efficiency. To capitalize on the material’s benefits and to ensure code compliance, proper project specification requires the consideration of a few critical insulation performance factors.

Masonry, also known as brick- or stone-laying, has been the cornerstone of some of the most significant architectural wonders of the world, from the Pyramids of Giza and the Roman Colosseum to the more ‘recent’ Taj Mahal. As one of the oldest construction methods, masonry has shown its ability to withstand the test of time by adapting to support modern building standards. Take masonry cavity walls, for example—when they were first constructed by the ancient Greeks, the remnants were observed by archeologists as two stone walls set roughly 50 mm (2 in.) apart. Masonry cavity walls have now evolved to incorporate new technology and safety measures, provide better moisture control, and, of course, improve energy management.

As the Masonry Advisory Council[2] (MAC) explains, “Today, masonry cavity walls are used extensively throughout the United States in all types of buildings. The primary reasons for their popularity are superior resistance to rain penetration, excellent thermal properties, excellent resistance to sound transmission and high resistance to fire.”

Building masonry cavity walls in the 21^st century requires a contemporary approach that maximizes the inherent benefits of these wall types. One such progressive technique is the addition of continuous rigid foam insulation. By incorporating continuous rigid foam insulation in the masonry cavity wall, building teams can effectively improve a structure’s total thermal efficiency. However, to make sure the masonry cavity wall’s moisture and fire performance characteristics remain unimpeded, the specified insulation material must adequately resist moisture penetration and combustion.

Overview of masonry cavity walls

Masonry cavity walls consist of two separate masonry layers (wythes) with an air space between them. The two wythes are connected by corrosion-resistant wall ties. Wythes can be built of brick, structural clay tile, concrete blocks, and stone or other type of masonry materials. The double-wythe configuration results in superior moisture protection of the building. The outer wythe provides a critical first line of defense, helping prevent rain water from reaching the inner wythe. Moisture making it past the outer wythe drains down its inside face to flashings channeling it back out of the wall via weep holes. Since the air space between wythes is crucial for managing the moisture, the Masonry Society’s (TMS’s) 402-11, Building Code Requirements for Masonry Structures, (section 6.2.2.8.2) (Consult the Masonry Society (TMS) 402/602-16, Building Code Requirements and Specification for Masonry Structures, 2016.) calls for:

A 4-½ in. (114-mm) maximum distance between the inside face of the veneer and the outside face of the masonry or concrete backing shall be specified. A 1-in (25.4-mm) minimum air space shall be specified.

In addition to exceptional moisture management, the multilayer wall configuration aids in thermal performance. “Both wythes act as a heat reservoir, positively affecting heating and cooling modes,” notes MAC[3]. As an added benefit, the double-wythe air space configuration dampens sound, for a quieter indoor environment.

To maximize a masonry building’s insulating power without hindering its inherent moisture repelling and thermal properties, special care must be taken when implementing continuous insulation (ci).

Continuous insulation for masonry cavity walls

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The American Society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE[5]) 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, defines ci as:

Insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior, exterior, or is integral to any opaque surface of the building envelope.

To get the best performance from the cavity wall, ci is installed on the inner wythe’s outside face. In this configuration, insulation serves as the first line of defense against heat loss from the building to the outdoors. The air space and the insulation work together to improve the assembly’s overall thermal performance, without disrupting the channeling of moisture from the outer wythe to the wall’s exterior.

ASHRAE 90.1 was established in 2007. It requires ci in most new construction (choosing to establish energy compliance using the ‘prescriptive’ approach in lieu of the ‘performance’ method), as well as new portions of existing structures and their systems, except low-rise residential buildings, for almost every climate zone. According to the U.S. Department of Energy (DOE), as of December 2018, 39 states have adopted commercial energy codes that meet or exceed ASHRAE’s standard (U.S Department of Energy Status of State Energy Code Adoption: Commercial Buildings.). California’s Title 24 has taken energy efficiency regulation even further by calling for all new commercial facilities in the state to be zero net energy (ZNE) by 2030.

Specifying continuous insulation in masonry cavity walls

Of the ci options available, extruded polystyrene (XPS) is widely used for masonry cavity walls in the United States. However, building teams have successfully employed a range of insulations to meet ci standards, including mineral wool, sprayed polyurethane foam (SPF), as well as other rigid foams, including expanded polystyrene (EPS) and polyisocyanurate (ISO). In regard to the range of cavity wall insulation options, the MAC Design Guide for Taller Masonry Cavity Walls says, “a foil faced, polyisocyanurate insulation is the most beneficial.” ISO is known for its use as roofing insulation, but it is also recommended in masonry cavity walls for its thermal, moisture, and fire performance as well as its resistance to solvents.

Additionally, ISO offers increased R-value per inch versus mineral wool, XPS, and EPS insulations. The high R-value per inch of ISO provides high-performance ci, while enabling the insulation layer to take up less air space within masonry cavity wall assemblies. Benefits of a thinner insulation layer include added room for the mason to work when installing the exterior brick veneer and reduced risk of mortar clogging the airspace within the cavity. Masonry[6] magazine reports some building professionals are moving to wider masonry cavity walls. However, narrower walls reduce product and labor costs since fewer materials are needed in the building process.

To determine which insulation is well-suited for masonry cavity installation, design professionals must closely examine commonly used masonry cavity wall insulation. The following will evaluate the thermal, moisture, and fire performance, along with solvent-resistance properties, of multiple ci options.

Thermal performance

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As defined earlier by TMS 402-11, a minimum air space must be maintained within a cavity wall to allow for critical moisture management. Keeping this in mind, specifying insulation to maximize thermal performance without hindering the air gap in a cavity wall is crucial to creating a code-compliant, energy-efficient masonry building.

Since ISO offers increased R-value per inch, it provides effective ci while allowing for a thinner overall wall assembly. For example, the R-value of foil-faced ISO products is R-6.5 per inch and R-6 for other ISO wall products. Comparatively, other rigid foams are in the 3.6 to 5 R-values per inch range with mineral wool falling in the 3 to 4.2 R-value per inch range. By reaching higher, code-mandated R-values without increasing insulation thickness, ISO is able to provide flexibility in the wythe air space gap without increasing the wall’s thickness.

Moisture

If a masonry cavity wall is performing as intended, moisture or excess moisture vapor breaching the outer wythe will rarely reach the insulation attached to the inner wythe. In the event the insulation is compromised, either from damage to the outer wythe, seepage from the top of the wall, or other factors, it is important the insulation does not readily absorb water. When insulation is damp, its R-value is dramatically impacted because the water compresses the insulating air space between material layers. When insulation is compressed, the R-value is negatively impacted because there is less insulating air space. Additionally, if insulation is subject to continuous moisture retention, it is at risk of deteriorating.

Due to their closed-cell physical makeup, foam insulation boards—this includes EPS, XPS, and ISO—naturally resist moisture intrusion, to varying degrees. Their physical properties can provide a great baseline for moisture performance. However, there are other features to consider. For instance, foam insulation boards with facers have the added advantage of enhanced water resistance. Consider ISO products with coated-glass (CG) mat or foil facing. The Polyiso Insulation Manufacturers Association (PIMA) explains in its Technical Bulletin 401[8], “The foil face on the polyiso insulation is an impermeable material, which enhances long-term thermal performance.” This, in turn, protects the building’s energy efficiency in the event of outer wythe failure or other moisture intrusion.

Fire performance

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Several wall assemblies, including masonry cavity walls, must meet criteria under the National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load-Bearing Wall Assemblies Containing Combustible Components. NFPA 285[10] test results are either “pass” or “fail” based on an evaluation of vertical and horizontal flame spread and conformance with temperature limits within the assembly. With limited exceptions, International Building Code (IBC) section 2603.5.5, “Vertical and lateral fire propagation,” requires wall assemblies in Type I, II, III, and IV construction using foam plastic insulation to pass NFPA 285 testing requirements. Although the title says “nonloadbearing,” NFPA 285 also applies to loadbearing walls, so masonry cavity walls are not exempt.

Masonry cavity walls are inherently fire resistant due to the noncombustible properties of brick, clay, stone, and other components of the wall. However, since a cavity wall system may include flammable materials, it is critical to consider code-required fire performance when selecting insulation.

NFPA 285 is particularly rigorous in that it evaluates entire wall assemblies, not individual products. Thus, even if a given insulation product—or other component—passes NFPA 285 in a specific assembly it does not mean the material is automatically compliant in alternate assemblies. To be included in masonry cavity walls, an insulation type must pass NFPA 285 within a particular assembly.

While critical, code compliance is just one component of the specification equation. In the event of a fire, each insulation type will perform differently. Of the polystyrene foams (EPS and XPS), both of the insulation materials are classified as “thermoplastic.” Gregory Havel[11], a 35-year fire service veteran explains thermoplastics will “melt, liquefy, and run down into the bottom of the cavity, and burn as a flammable liquid,” when they are exposed to fire.

In contrast, a “thermoset” plastic, such as ISO, is manufactured with chemical bonds that irreversibly harden during a fire, providing a high level of fire resistance. In other words, during a fire, ISO does not melt as thermoplastic styrene foam. Instead, thermosets develop a protective char layer. As a result of this fire performance, ISO manufacturers have developed a wide selection of products that pass NFPA 285 testing in multiple wall assemblies. Further, if a building has combustible claddings in some sections and brick in others, thermoplastics like XPS and EPS might not always have approved NFPA 285 assemblies. However, ISO does. As such, it does not require building teams to change materials in different parts of the building.

Resistance to solvents

As with most construction methods, masonry cavity wall designs often incorporate materials with petroleum-based solvents. These solvents come in the form of adhesives, preservative coatings, and waterproofing to aid in building performance and increase construction speed. Unfortunately, petroleum-based solvents can damage styrene rigid foam insulations. While this is not a primary deciding factor for choosing among insulations, as there are specially formulated solvents that can accommodate styrene rigid foam insulations, resistance to solvents helps simplify construction. When the insulation does not need to be protected from common building materials, the need to buy alternative materials or exercise additional caution around the insulation is eliminated. ISO is unaffected by common petroleum-based solvents and eliminates the worry of chemically based deterioration of the insulation.

Putting it all together – Manor High School case study

To serve a fast-growing student body in suburban Austin, the Manor Independent School District (MISD) added a new high school on an adjacent campus.

Manor High School demonstrates the advantages of ISO insulation in masonry cavity walls. Designed by Perkins + Will, the new school building has walls with rock masonry exterior cladding in some areas and metal cladding in others. It is common these days to have buildings incorporating more than one cladding, and masonry exteriors are often combined with other materials. One challenge the architects faced was providing ci within the cavity, while also meeting fire requirements under NFPA 285.

“The project was originally specified with XPS insulation, but swapping that out for ISO improved the thermal performance, and met NFPA 285,” said Darren Butler, president of a product manufacturer representative firm. “The price did not change when switching to coated-glass facer ISO for the contractor and everyone was happy and code compliant.”

Conclusion

Masonry cavity walls continue to adapt to current building practices, thanks to modern advances in building products. An important aspect of this evolution is the use of insulation within the cavity. To maximize the wall’s overall performance, it is critical the insulation is chosen carefully. Although it is commonly employed in roofing applications, ISO is gaining popularity among specifiers for use in masonry cavity walls. ISO enhances the masonry cavity wall’s inherent performance traits because of its complementary thermal conduct, moisture, and fire and solvent resistance.

Nicole Richard is a technical service specialist with Hunter Panels, focused on the company’s roof and wall insulation divisions. Richard has more than 22 years of experience in the building insulation industry, including technical services, customer service, and sales. She can be reached at nicole.richard@hpanels.com[12].

Source URL: https://www.constructionspecifier.com/specifying-rigid-foam-insulation-for-masonry-cavity-walls/

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