Continuous insulation: Rethinking the correlation between density and compressive strength

by sadia_badhon | February 27, 2020 9:44 am

by Tiffany Coppock, AIA, NCARB, CDT, ASTM, RCI, EDAC, LEED AP

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Open, light, and airy. Repurposed for a new life. So much of a building’s design comes down to the feeling one hopes to inspire in the inhabitants and the confidence underlying the facility’s performance. Some insulating materials, such as rigid cellular glass insulation[2] described in the August 2019 issue of The Construction Specifier, are designed to support that confidence by providing a level of redundancy to safeguard against water infiltration on the roof in mission critical buildings. The feeling of a building can also be supported by the insulating material in the cladding. Today’s building designers look to continuous insulation (ci) options to deliver a breadth of performance attributes including thermal, moisture, acoustic, and environmental benefits.

Viewed through this broader lens of performance attributes, compressive strength becomes an important selection criterion. However, the decision on what material to specify often comes down to weight, or at least it used to. Many architects and specifiers are finding reasons to rethink the weight factor in light of new research suggesting weight does not always equal strength.

Density does not always mean strength

Compressive strength in ci is a critical consideration as it reduces risks of deflection and contributes to the ability of the wall assembly to resist higher loads, whether from the weight of the cladding itself or wind movement across walls. Higher compressive strength can even translate into less product damage during installation.

For decades, specifiers have relied on material density as an indicator of compressive strength, equating a higher density or weight with a material’s strength. Thus, the denser the material, the more compressive strength it was assumed to have. While there is some truth in this observation for both foam plastics and mineral wool, the two are not directly related as it was once thought. It is important to keep in mind, according to ASTM C578[3], Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation, Type V polystyrene insulations at a density of 3 PCF exhibit a compressive resistance of 100 psi (690 kPa). Conversely, a typical high density 8 PCF mineral wool ci touts a 439 psf (21 kPa) of compressive resistance. While the test methods for the two materials vary (ASTM D6122, Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems, for extruded polystyrene [XPS] versus ASTM C165, Standard Test Method for Measuring Compressive Properties of Thermal Insulations, for mineral wool), simply holding the two materials by hand reveals the density difference between them. Advances in mineral wool production, including patent-pending processes, are improving this strength-to-weight ratio even further. Lighter, stronger materials enable more flexible design options to create the building architects envision.

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Manufacturing advances

Among insulation materials, mineral wool insulation is made by spinning molten rock or slag into fibers that are then bound together to create insulating material. Mineral wool was one of the earliest forms of insulation to be produced, originating in Germany in the 1870s, and has been manufactured in the United States since the 1930s. Today, mineral wool is perhaps best known for its fire resistance, and increasingly, for its sustainability attributes (like recycled content), which can contribute to green building certifications. However, as the building science community learns more about the ‘chameleon-like’ properties of mineral wool, research has focused on how to deliver higher levels of compressive strength.

Studies have shown, at a microscopic level, the random arrangement of fibers resulting from the spinning process that creates mineral wool could impact its strength, depending on the direction of forces applied to the insulation (read The compressive strength properties of mineral wool slabs: Influence of structure anisotropy and methodical factors by Andrius Buska and Romualdas Mačiulaitis, published in the Journal of Civil Engineering and Management). New, proprietary processes are optimizing binder content and fiber orientation to maximize the compressive strength of mineral wool without adding to its weight. For example, newly engineered automated processes create a highly uniform base layer of material that is fed through specialized equipment, enabling manipulation of the fibers and overall product structure. The benefit of this technology is a high level of control over the mechanical and thermal performances of the finished product.

In tests of mineral wool ci manufactured using these new patent-pending techniques, high compressive mineral wool outperformed a heavier insulation and resulted in a weight reduction of 25 percent. Applied across a full-scale commercial project, such as a mixed-use building or high-rise structure, that could mean a significant reduction in overall weight and stress on the cladding. While every project is different, reduction in weight is likely to lead to a decrease in attachment system material, whether that is fasteners or the amount of metal needed in girts. This could translate to lower material and labor costs.

Beyond strength, chameleon-like properties

Many specifiers are already familiar with mineral wool for its fire resistance and high R-values, but the total profile of this material has made it an increasingly popular choice. The following are some of the benefits mineral wool delivers throughout the enclosure.

Thermal performance

Mineral wool provides an R-value of up to 4.3 per inch per ASTM C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. Mineral wool can be used to support performance goals such as mitigating thermal bridging risk, particularly when used in applications requiring high compressive strength where the cladding attachment is completely outside the ci, and only a small amount of screws penetrate the insulation to the wall below.

Moisture resistance

Mineral wool ci is engineered to absorb only 0.03 percent moisture by volume in testing simulating application in an exterior wall cavity. This is more than adequate for the intended application in an above-grade wall behind cladding. If it does get wet, mineral wool returns to its thermal properties after drying out.

Fire resistance

Mineral wool is fire resistant to temperatures above 1093 C (2000 F). Designed to prevent the spread of fire and smoke, mineral wool helps contribute to fire-resistant assemblies, protecting buildings and its occupants.

Acoustics

Mineral wool absorbs sound and per ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, rates above 1 (perfectly absorptive) at frequencies over 1000 hertz, at as little as 38-mm (1.5-in.) thickness (Figure 1).

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Air leakage resistance

Mineral wool can be installed using various hanging systems without compromising the integrity of the air and water barrier.

Vapor permeability

Mineral wool will not trap liquid moisture in the wall cavity where it can contribute to situations affecting air quality.

Sustainability

Since it is comprised from up to 75 percent recycled contents, the material can be removed from post-industrial processes and re-used. Like all insulations, mineral wool can reduce greenhouse gas (GHG) emissions by reducing the energy needed to heat and cool buildings and contributes to sustainable building practices.

While the features described above are widely recognized for making mineral wool a good option for interior insulation applications, the material is not always top-of-mind for ci, where insulating materials like expanded polystyrene (EPS), XPS, and sprayfoam are widely used. Foam plastics continue to be a valid choice in many situations with noncombustible claddings, and with advances in production technologies, mineral wool may now be considered not only a viable option for ci with combustible claddings, but also the preferred ci material for buildings. For example, mineral wool is particularly advantageous when working with claddings made of plastic and laminate where a noncombustible ci material is required to pass National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components.

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The case for higher strength at lower density

The change in mindset to higher strength at lower density is similar to the way people had to change their thinking about lightbulbs a few years ago with the shift to light-emitting diodes (LEDs). Most people were used to speaking about the brightness of bulbs in terms of watts, and it was hard to imagine we could achieve the desired level of light with fewer watts. However, when comparing the measure that really mattered—lumens—it was easy to see with the new technology, a 5-watt bulb could indeed produce as much light at a 60-watt lamp. Separating strength and density requires a similar shift in thinking, and can be a game-changer in design.

Less weight on cladding

Today’s variety of lightweight, attractive cladding options can help designers maximize the creative aesthetics and functionality of buildings. However, given the traditional density-to-strength equation for specifying exterior-grade insulations, designers have to weigh ci options (quite literally) against available cladding choices, always accounting for weight per square foot, deflection concerns, fasteners, and necessary cantilever capabilities.

By delivering a high compressive strength to comparatively lower weight and density, mineral wool insulation presents designers and specifiers with a greater range of options in pairing a building’s cladding with ci. Mineral wool is a practical ci option with traditional heavy cladding choices, such as masonry, as well as light materials, such as newer plastic and laminate claddings. The product can be used with brick, metal, stone, masonry, terra cotta, concrete, and architectural cladding options, including phenolic panels and high-pressure laminates. Other insulation options simply do not offer this level of flexibility under varying claddings.

With its noncombustible properties, mineral wool also helps designers and specifiers meet NFPA 285 and otherfire standards when working with new, lighter cladding materials that are more combustible than traditional veneers.

The ability to use a lighter weight cladding with mineral wool also provides some options in the construction process itself. Lighter cladding can reduce the need for additional workers and expensive equipment like overhead cranes. Handling lighter material is also less fatiguing and cumbersome for installers transporting the material and working on the jobsite. Additionally, an easy to handle building material can help support the number one priority on every job: worker safety.

More than compressive strength

While compressive strength always ranks near the top of the list when it comes to specifying materials, a product’s dimensional stability must also be considered. New, lighter, and stronger mineral wool ci options reduce risks of façade deflection under pressure and over time. Since the cladding is attached through the mineral wool insulation, a cantilever is established that helps to prevent this unwanted movement.

Not to worry, though, because even with higher dimensional stability than some other mineral wool products, these advanced mineral wool ci still have the ability to accommodate irregular surfaces and fit together tightly compared to many of their foam plastic counterparts.

Uncompromised thermal performance

While the density of this product is reduced, designers may be concerned about a reduction in thermal performance. Fortunately, reducing density does not sacrifice thermal performance of the material. These products are tested to exhibit a minimum R-value of 4.2 per inch thickness just like other mineral wool ci counterparts. Therefore, installing these materials into exterior walls does not require compromising thermal comfort or energy efficiency for the mechanical and weight benefits.

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Additional attachment options

The cladding attachment system is another key factor when it comes to choosing ci. Mineral wool ci may be attached with simple, inexpensive fasteners such as impaling pins, screws with washers, or even adhesives. It can also be secured between masonry ties or girt assemblies. Impaling pins can be installed prior to the air and water barrier (AWB), adhered to the AWB, or screwed through it. Mineral wool also works with clip and rail systems. Some ci offers its own hanger system as well, which is designed to simplify and streamline installation.

The high compressive strength of mineral wool ci enables the use of additional attachment options such as girt systems completely outboard of the insulation due to its ability to resist deformation.

Responsible use of resources

Whether the design and specification team are looking to meet a Leadership in Energy and Environmental Design (LEED) certification, or simply maximize the sustainability efforts of the materials used, mineral wool insulation is a responsible choice for ci and other insulating purposes. With its 70 to 75 percent slag content—one of the highest recycled contents among all ci products—mineral wool can help contribute points to various certifications. Mineral wool is also inorganic and does utilize blowing agents. Mineral wool can also resist exposure on a jobsite in the presence of extreme temperatures and ultraviolet (UV) light.

Better vapor permeability and moisture control

A building must be able to manage vapor flow as well as drain any liquid moisture out of the building. While many foam-based board insulations slow the transfer of vapor through the wall, mineral wool ci is exponentially more vapor permeable, so designers are given maximum freedom in selecting where or even if a vapor barrier is placed in the wall assembly. This makes mineral wool a versatile choice across various climate zones.

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Ci must also be able to withstand exposure to moisture, without retaining any incidental moisture that may reach the exterior wall cavity. Advances in technology allow mineral wool to be engineered to be hydrophobic and repel water as it reaches the insulation. Any moisture that is collected drains and the insulation dries completely. Once dry, the mineral wool’s R-value returns to previous levels.

Robust performance in harsh conditions

The Museum of the Modern Image (MOMI) in New York is a good example of a building project that illustrates mineral wool as a ci. The innovative open-joint system features mineral wool securely installed behind the cladding. Not only do these materials need to perform every day in the building assembly, but they were also exposed to jobsite challenges while the project was being completed. Over a nine-month period, the materials stood up to water, snow, ice, and plenty of UV light. While it is always important to protect materials on a jobsite, the reality of Mother Nature and the nuances of production schedules mean materials can be left exposed. In the case of the MOMI project, only a few minor pieces of mineral wool needed to be replaced, even after three seasons of exposure. Mineral wool proved to be robust, able to withstand a heavy load and the conditions on the jobsite.

Better acoustics

Lower-density insulation does not have to mean a noisier environment. As noted earlier, mineral wool has an impressive sound attenuation profile even at a minimal thickness. While this is often seen as an advantage with interior walls and floors, when used in ci, mineral wool helps impede noise transmission. Today’s thinner, more conductive claddings increase opportunity for sound to transfer from outside to inside (or vice versa). Mineral wool can uniquely help these buildings achieve that elusive inner peace.

Specifying for strength

It is time to re-think the perception that a correlation always exists between higher weight and higher strength. As specifiers look to match cladding to ci, they should consider compressive strength separately from density. Research has demonstrated a lighter weight material, such as mineral wool, can certainly meet, and even exceed, mechanical and thermal performance expectations, while offering ancillary benefits to support fire resistance, sound attenuation, vapor and moisture management, green building goals, and ease of use on the jobsite. With new, light, strong options now available, specifiers may find an insulating material’s high density weighs a lot less heavily in their ci decision-making.

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Tiffany Coppock, AIA, NCARB, CDT, ASTM, RCI, EDAC, LEED AP, is a commercial building systems specialist for Owens Corning. She provides leadership and technical guidance in building science, testing, and documentation to design professionals and the Owens Corning team. She holds degrees from Texas A&M University and the University of Colorado, and is a registered architect with specialization in healthcare and historic preservation. She can be reached via e-mail at tiffany.coppock@owenscorning.com[10].

Source URL: https://www.constructionspecifier.com/continuous-insulation-rethinking-the-correlation-between-density-and-compressive-strength/

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