May 30, 2017
by Brian Chang
Growing use of continuous insulation (CI) across virtually all building types reflects its role as an essential means to achieving higher efficiency standards as well as sustainability goals. It also affects the design process. Applying CI demands that specifiers master this construction element, taking time to consider how to make the best possible choice for a given building project.
Basic but essential questions have sown confusion among too many project teams. For example, how much insulation is sufficient? And how much is too much? What is the best way to calculate the cost-effectiveness of the CI product system—is it lowest life-cycle cost? These are just a couple of criteria for evaluating and selecting the so-called ‘building blanket,’ but there are others, and the complexity only grows.
Essential to the application criteria and project decision-making, of course, are the other envelope requirements—the choice of cladding, configurations of air and moisture barriers, and window-to-wall ratio, among others. Depending on the jurisdiction, certain code mandates (and even utility or zoning incentives) should be factored into the decision. Also important is the type of CI. There are many choices of product, materials, and systems, but which offers the best match for a particular project?
Four specifier hurdles
Several factors help demystify the continuous insulation approach, establishing a coherent methodology for choosing the right CI solution. It begins with four essential requirements every specifier should review.
Continuous insulation is identified thusly in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2013, Energy Standard for Buildings Except Low-rise Residential Buildings:
uncompressed and continuous across all structural members without thermal bridges other than fasteners and service openings.
Per ASHRAE, CI can be installed on the enclosure’s exterior or interior, or it can be integral to opaque envelope materials. Wherever it is located, the CI layer must span across or through thermally conductive elements such as steel columns, metal studs, concrete masonry units (CMUs), and others. If not, thermal bridging through high-conductivity structural components can reduce insulation performance by up to 40 to 60 percent in metal-framed buildings, and up to 20 percent in wood-framed enclosures, according to studies by Oak Ridge National Laboratories (ORNL). (For more, see “A Review of High R-value Wood-framed and Composite Wood Wall Technologies Using Advanced Insulation Techniques,” by Jan Kosny, Andi Asiz, Ian Smith, Som Shrestha, and Ali Fallahi. It appeared in Energy and Buildings , published in 2014 by Elsevier.)
Thermal bridges have other detrimental effects. Most noteworthy, these hot and cold spots can cause condensation and moisture ingress in the enclosure, as well as occupant discomfort in localized interior spaces. Clearly, traditional insulation layers—for decades located neatly between steel columns or light-gauge studs—have been insufficient.
Code and standard requirements
In practically every situation, CI is essential to meeting key energy codes, including ASHRAE 90.1, the International Energy Conservation Code (IECC), and numerous state energy codes. To meet such lofty goals as net-zero-energy operation, Leadership in Energy and Environmental Design (LEED) certifications, meeting Passive House, or performing to other standards seeking reduced carbon footprints or greenhouse gas (GHG) emissions, CI is a necessity.
Some of these benchmarks offer prescriptive solutions. Using the project’s climate zone, the space’s conditioning category, and maximum allowable U-factor (i.e. rate of heat loss), ASHRAE 90.1 offers guidance on applying CI to achieve required performance levels. Using an example from North American Insulation Manufacturers Association (NAIMA), a metal-framed worship facility in Raleigh, North Carolina, could meet the required U-factor in 90.1 with an R-16 fiberglass batt layer and R-5.6 of CI for the roof, along with walls constructed with an R-10 fiberglass insulation layer and R-5.6 of CI.
Many project teams practice lean construction methods, seeking to optimize building systems for life-cycle economics. According to architect Herbert Slone, RA, insulation life-cycle cost (LCC) can be expressed as:
While first cost increases as R-value does, at the same time energy costs drop, reducing operations expense. In this way, optimal insulation can be defined for any given climate, building type, and facility usage patterns.
There are life-safety requirements that affect the use of CI. Most relevant for U.S. building teams is National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-loadbearing Wall Assemblies Containing Combustible Components. If the CI layer for exterior walls is made of foam plastic, or if the air and water barriers are combustible, the NFPA 285 fire test is mandated for walls of construction types I, II, III, or IV—even though these wall types are, by definition in the International Building Code (IBC), to be made with noncombustible materials. (There are some exceptions. The design professional should refer to 2015 International Building Code [IBC] Chapters 14 and 26.)
With the rise in use of CI and air barriers, NFPA 285 has become a ubiquitous requirement for building projects. Specifiers often check to ensure their proposed assemblies have already been subjected to the costly test—an impressive full-scale multi-story mockup subjected to a fire intended to simulate a blaze originating from an interior room. In the 2015 IBC, six sections (including 1403.5 for weather-resistive barriers [WRBs] and 2603.5.5 for foam plastic insulation) reference NFPA 285.
In general, project teams must focus on NFPA 285 early in the design phases as the market may not offer enough compliant assemblies for walls of greater than 12 m (40 ft) above-grade. Additionally, some rainscreens and other cladding types have yet to be tested with foam plastic insulations. In these cases, the design team can choose to employ a tested combination of products or, in some cases, have the proposed assembly tested.
Expectations with performance
With these four preconditions under the specifier’s belt, attention turns to the impact of CI on building performance, as well as its ramifications for cladding system selection. It turns out continuous insulation is a design mindset as much as a widely prescribed wall feature.
Among the primary benefits of continuous exterior insulation is it maintains the enclosure and framing elements at temperatures closer to those of the building interior. With additional R-value at the exterior, the dewpoint moves toward the outside and, in some cases, exterior to the insulation in the framing cavity, says Slone. This effect can also eliminate or reduce condensation in the enclosure—a pernicious source of moisture that can prematurely degrade structural materials.
Beyond helping with this situation, CI protects against the thermal bridges where structural components, substructure, anchors, and other penetrations reach through to the exterior. Uninsulated steel-stud framing in contact with exterior sheathing, for example, is an efficient conduit for heat regardless of how much insulation is packed between the studs. Adding CI across all the steel frame members dramatically cuts heat and cold bridges, boosting overall R-value and reducing the U-factor.
Other penetrations through the façade can cause thermal bridging and compromise the CI layer, due to inadequate detailing or misalignment of the thermal control layer. If the structure includes steel shelf angles without stand-offs, it will transfer heat. Exposed concrete floor slabs and steel penetrations for balconies or canopies can compromise the CI layer.
Other bridging challenges are windows and doors with thermal breaks that do not coincide with the opaque wall’s thermal control location, or where structural members hold off their lintels. Sometimes, parapet walls are incorrectly detailed, becoming a building-wide perimeter heat sink. Details matter—the enclosure design team should track possible thermal bridging paths. Properly designed, the CI layer cuts U-factor considerably.
Another effect that can be reduced using CI is moisture accumulation due to transport of water vapor through envelope materials such as brick or CMU. This mitigation is especially effective when a properly specified and installed WRB is also employed. Attention to climate zone, the type of wall system used, and the building’s intended use will help ensure proper enclosure function.
This raises a related point—the CI layer can also serve as part of an air barrier system and moisture barrier protections. For example, extruded polystyrene (XPS) insulation boards can be an effective air-barrier material, typically with taped joints and sealed penetrations using silicone- or latex-based sealants, which are compatible with XPS. To determine if the CI systems employed will perform as a code-compliant air-barrier assembly, specifiers can refer to manufacturer data per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies. (Specifications sometimes also refer to ASTM E2178, Standard Test Method for Air Permeance of Building Materials, or ASTM E1677, Standard Specification for Air Barrier Material or System for Low-rise Framed Building Walls.)
Using the CI layer as part of the air and moisture protection systems is an efficient double use of a building material—a sustainable combination.
Choice of cladding and CI implications
Before choosing the CI solution, building teams generally select or recommend their cladding system. Aesthetics, performance needs, and suitability to the application are among the drivers for choosing cladding. All things equal, are there types of cladding that make the most of CI? Possibly.
The reason for this is construction type, climate zone, and the building’s intended use would determine where the CI layer is placed in the enclosure assembly. Its location also affects construction sequencing and cost—that is to say, construction preferences may drive the choice of cladding so the CI layer is easy to apply, inspect, and, in some cases, to repair. When the CI insulation layer is outboard of the structural framing, for example, the detailing and construction tends to be much less complex than when it is interior to the structure, obviating such interruptions as floor slabs.
This leaves two typical options for CI location: behind the cladding or integral to it. Cladding options typically falling outboard of the CI layer include masonry and brick exteriors, as well as panelized metal systems such as aluminum composites, often used with mineral wool for the CI layer. As the CI materials must be supported, the façade attachment hardware can serve in some cases to secure both the cladding and the insulation; examples include clips, horizontal girts, and screws with sealing washers. Other types, such as impaling fasteners, are designed to support insulation alone.
Also located exterior to the CI layer are built-up façade cladding systems that require no penetrating fasteners to attach the insulation, further minimizing thermal bridging. Examples include traditional stucco and exterior insulation and finish system (EIFS) assemblies, which can use fully adhered insulation boards to offer good wind load resistance and protect against cracking caused by thermal expansion and contraction.
For many other cladding systems, the CI layer is integral. These can include precast concrete panels, tilt-up concrete walls, and masonry cavity walls such as brick veneer. XPS can be employed under various masonry veneer exterior finishes or over steel-stud framing, concrete, or masonry wall structures.
Similarly, CI is a feature of rainscreens and rain barrier claddings made with metal or aluminum composite panels, fiber-cement board, glass-fiber-reinforced concrete, or other materials. The chosen insulation must meet minimums for compressive strength and code requirements.
With many metal cladding systems and rainscreens, joint detailing can influence the selection of the CI material. Rainscreens with masonry veneer or open-joint panels are designed to admit moisture, which is then weeped or drained (or both), and dried through ventilation in the assembly’s cavity or air gap. If exposed to moisture, the CI material should be water-repellent. According to NFPA 285 testing, open joinery can expose the insulation to fire, so the CI material is chosen with adequate fire resistance in mind. In some cases, the open joints may also allow ultraviolet (UV) light from the sun past the cladding; therefore, exposed insulation and barrier materials should be rated for UV degradation.
What type of CI works best?
Several kinds of insulation, sometimes in combination with other building materials, can be used to achieve the desired CI performance. One of those is sprayed polyurethane foam (SPF). Four kinds of rigid or semi-rigid products are more commonly employed (noted with their R-value per inch):
In high-performance buildings that have better insulation, 100 mm (4 in.) or more of these rigid materials might be incorporated into the enclosure. However, a benefit of walls with CI is the elimination of thermal bridging enables them to be thinner than equivalently insulated walls without CI. This is good news, as the 2012 IECC calls for R-20 or R-13+5 for Climate Zones 3, 4, and 5; for Climate Zones 6, 7, and 8, the minimum is R-20+5 or R-15+10.
The insulation types noted above can meet these goals, and all are relatively inexpensive building materials. They also offer good long-term thermal resistance and the ability to reduce operating costs.
Also called ‘mineral-wool’ and ‘stone-wool’ insulation, mineral fiber is noncombustible and fire-resistant due toits high melting temperature, allowing fire ratings of one to two hours. It is resistant to water and moisture, which helps it retain R-value when wet. Chemically inert, this UV-stable insulation will not rot, cause corrosion, or support microbiological growth.
Most products made with mineral fiber for building applications are from natural and recycled feedstocks, and do not require fluorocarbons in manufacture. In typical applications, mineral wool allows for draining water and absorbing sound for acoustical properties in the envelope.
For nonresidential projects, this insulation is a medium-density or high-density semi-rigid board; it can be foil-faced, and works in cavity wall and rainscreen applications. Its fire and moisture performance make mineral fiber a good choice for wet cavity walls, as well as metal cladding systems or open-joint rainscreens.
Polyiso has recorded the highest R-value per inch compared to other rigid-foam-board insulation materials, according to Polyisocyanurate Insulation Manufacturers Association (PIMA); its R-value increases with board thickness. However, recent testing has shown losses of R-value over time, in cold temperatures, or both.
Some polyiso products have a higher level of inherent fire resistance, and the material has been used in various assemblies passing the following fire tests:
Polyiso’s foam core is moisture-resistant with some water absorption potential, and the boards are stable and compatible with most construction sealants and adhesives. A facer is applied in manufacturing, and may be used as a moisture drainage plane.
Foil-faced polyiso insulation is commonly used in masonry and rainscreen cavity walls, where its high R-value per inch tends to reduce the cavity depth needed. Polyiso experiences some change in R-value in cold weather, which may be considered when calculating CI performance.
Known for high R-value per unit cost, EPS is cost-effective, dimensionally stable, and commonly used for ground contact and below-grade uses, as it does not retain water. Faced boards also function as vapor retardant, though, when used as sheathing, EPS should be laminated or used with an air-and moisture-barrier layer. A versatile rigid insulation, EPS is useful in foundation applications and is typical in EIFS façades and integrated assemblies such as insulated concrete forms (ICFs) and structural insulated panels (SIPs).
The use of XPS takes advantage of its closed-cell structure and water resistance, offering a cost-effective choice for improved R-value. It is also recyclable—another benefit for green building projects. Specifiers often use plastic-faced versions, which can serve as vapor retarders. Like EPS, XPS is combustible, and its performance is affected over time by UV light, according to Slone. XPS may also absorb more moisture over time than other insulation boards.
For these three reasons, XPS is generally unsuitable for open-joint applications such as metal panels, terra cotta, or high-pressure laminates. Instead, XPS works well for barrier walls and closed-joint rainscreen, as well as most cavity-wall drainage systems.
As shown, there are benefits and drawbacks to each insulating material. For an EIFS project, EPS is ideally compatible. For brick veneer, the higher R-values of polyiso and XPS allow for thinner wall sections, meaning smaller shelf angles and lintels that help reduce thermal bridging.
While rigid-foam insulation boards have varying performance capacities, they all have excellent R-values per unit cost. All the materials noted can meet the core goals of a continuous insulation layer, and all are relatively inexpensive. Looking at first costs alone, insulation is a valuable performance element. In a CMU masonry wall with an installed cost of $370/m2 ($34.30/sf), the CI layer accounts for only about seven percent of that total. For a steel-frame assembly, the first cost is just over eight percent. By comparing their functional characteristics, specifiers can determine which insulation works best for a given CI application.
The material choice is one step in the complex yet critical process of wrapping the building in a blanket of continuous insulation. By adding to this process such variables as cladding choice, fire-safety rules, and building operations and life-cycle needs, the specifier team can make the best choice possible to meet the highest building-efficiency standards.
Brian Chang is director of product and channel management for Sto Corp. in Atlanta, where he is responsible for overseeing and leading the product management team of the company’s overall product lines to include profitability, market segmentation, positioning, and new product development. Chang comes to Sto from LG Hausys (LG’s Building Materials Division), where he served as senior product manager, responsible for their composite countertop division. He holds an MBA from Georgia State University. Chang can be reached via e-mail by contacting email@example.com.
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