Designing enclosures from the outside in

IMP_0711-8_DSC2088
Photo courtesy Demonica Kemper Architects and Steinkamp Photography

By Paul R. Bertram Jr., CSC, FCSI, CDT, LEED AP
There are well-established best practices for delivering energy-efficient buildings on the path to net-zero. After site selection and building orientation, perhaps the most important consideration is the building enclosure or envelope. Next, the energy conservation measures (ECMs) are determined based on the envelope’s predictive energy-efficiency performance; they also include lighting, HVAC equipment, and controls. The last step for high-performance, low-energy buildings involves designing performance contributions of renewable energy technologies.

Increased energy performance requirements for envelopes is being driven by building owners, National Energy Code of Canada for Buildings (NECB), Natural Resources Canada (NRC), and green rating programs like that of Canada Green Building Council (CaGBC).

The building envelope encompasses the entire exterior surface enclosing interior spaces, including walls, doors, and windows. The basic design criteria includes keeping moisture and air out, while providing efficient thermal control. New construction and retrofit projects require greater knowledge and application of building science in design, evaluation, and specification of enclosure materials, systems, and assemblies.

This demand for optimized energy efficiency has increased interest in high-performance, low-energy hybrid envelope assemblies beyond traditional site-built systems. The more new technologies and practices are adopted with proven performance, the better chance they become accepted as standard practice. By incorporating energy efficiency, renewable energy, and sustainable green design features into a building at the outset, the energy consumption can be controlled.

There are many existing and emerging hybrid envelope systems to consider. One possibility is insulated metal panels (IMPs) for walls and roofs. These products have proven performance of vapour and air infiltration barriers since there is no metal conductance from exterior to interior skin. IMPs with properly sealed joints have tested at 0.00 air infiltration based on ASTM E283, Standard Test Method for Determining Rate of Air Leakage through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen.

Off-site manufactured enclosures are tested as assemblies, rather than as individual components of traditional site-built systems. Therefore, IMPs can provide greater quality control for the specified performance outcome.

The building enclosure
In the Building Science Corporation’s (BSC’s) Building Science Digest 018, The Building Enclosure, enclosures are defined as a term preferable to ‘building envelope’ largely because it is considered both more general and precise, as it considers both exterior and interior loads. The building enclosure may contain, but is not the same as, the thermal envelope—a term referring to the enclosure’s thermal insulation.

BSC also states a building’s overall durability depends on the environmental loads to which its materials are subjected. In this regard, a material (or assembly) itself is neither durable nor non-durable; it is the interaction of the material (or assembly) in its environment that determines its durability. Therefore, certain building assemblies may be more suited to specific environments, uses, and/or occupancy loads.

The building science of low-energy enclosure design decisions are influenced by three key areas: support, control functions, and finish. Support functions include consideration regarding resistance, transfer, and structural loading (dead and live) from both exterior and interior environments.

Control functions include control of the flow of air, moisture, energy (heat), and sound. Finish functions include enclosure finish surfaces and interfacings with exterior/interior environments regarding relevant esthetics, along with wear and tear.

IMP_FIGURE 1 - USE
This graphic, from a Smart Market report produced by McGraw Hill and American Institute of Architects (AIA), focuses on “The Drive Towards Healthier Buildings,” highlighting interest regarding the health impacts of materials and systems compared to energy savings based on Material Transparency design practices. Design teams should consider how occupant health information is assessed, along with its impacts on envelope and whole building design. Data courtesy McGraw Hill Construction

The design process tends to begin with a focus on esthetics. The enclosure specialist works to the desired look that delivers the building science/physics, initial costs, functional performance, compliance, lifecycle costing, environmental impacts, durability, resilience, and service life.

Enclosure analysis also requires systems testing evaluation, including resistance against impacts, blasts and ballistics, seismic activity, and hurricanes. Properties testing evaluation include:

  • density;
  • shear strength;
  • tensile strength;
  • compressive strength;
  • humidity aging;
  • heat/cold aging, flash, and ignition;
  • thermal;
  • enclosure loadings (dead/live); and
  • creep/shrinkage.

Considerations specific to moisture control include:

  • bulk air movement (air leakage) through the assembly;
  • air movement within the assembly (convective loops); and
  • vapour diffusion into or out of the assembly.

From an owner’s perspective, energy savings is still the most important concern (Figure 1, pictured right).

The 2011 NECB calls for greater energy efficiency. It outlines the minimum energy efficiency levels
for all new buildings in Canada, providing the design team and enclosure specialists three distinct pathways for compliance—prescriptive, trade-offs, and performance—for greater flexibility in achieving code compliance.

In the BSC study, “Review and Analysis of Exterior Wall Assembly Design: Large Single-story Retail, Manufacturing, and Warehouse Buildings,” energy efficiency was identified as most critical for buildings where the interior environmental conditions are expected to be different from exterior conditions. This means enclosure performance is predominantly a concern in cold climates, as the temperature difference is generally much greater than in moderate or hot regions.

Systems such as IMPs with high R-values and more airtight enclosure performance are appropriate considerations for these instances. They can also be considered for buildings that require special interior environments—such as cold storage spaces, or manufacturing facilities that need specific environmental controls. In these locations, high-performance wall assemblies would be applicable (Figure 2).

Figure 2

 Weighted Overall Performance Summery Importance factor Insulated steel stud
with masonry veneer
Insulated steel stud with EIFS cladding Split-faced masonry block with interior insulated steel stud wall Tilt-up concrete panels with interior insulated steel stud wall Insulated precast concrete Insulated metal panels Prefabricated metal with draped
vinyl-face batts
Conductance 5 30 30 40 20 30 50 30
Infiltration 5 30 40 20 50 50 50 10
Unit cost 5 20 30 30 40 10 30 40
Installation speed 4 24 24 16 40 40 40 40
Weather installation sensitivity 3 24 18 24 18 24 24 24
Liquid water management 5 50 10 40 50 20 50 30
Air transported moisture management 5 10 10 10 20 50 50 10
Vapor diffusion management 3 6 18 6 6 30 30 30
Exterior impact/Abrasion resistance 3 30 12 30 30 30 18 18
Interior impact/Abrasion resistance 2 8 8 8 8 20 12 8
Mould growth resistance management 4 8 8 8 8 40 40 16
Total 240 208 232 290 344 394 256

Seven typical building systems were reviewed and assessed against a set of 11 separate criteria chosen by Building Science Corporation (BSC) for their relevance to building construction and sustainability. The criteria are listed in the overall performance summary table, and for the purpose of the white paper are further grouped into three broader categories of energy efficiency, cost, and durability—key aspects at the forefront of sustainable construction. Data Courtesy Building Science Corporation

The enclosure design encompasses wall, roof, fenestration, below-grade wall systems, and base-floor systems and requires detailed drawings of the joinery where the opaque assemblies meet fenestration or other substrates within the design. Currently, whole-building design building information modelling (BIM) drawings do not represent at a sufficient level of detail and needs supported with accurate 2D drawings, preferably with assistance from manufacturers. Some manufacturers are also offering upfront thermal, and WUFI (Wärme und Feuchte Instationär) modelling regarding performance information for the design team to make better informed decisions of various envelope systems.

Tools such as the National Institute of Building Sciences (NIBS) National Performance-based Design Guide for Buildings is useful to design teams when establishing levels of performance. Although a U.S.-based resource, the guide identifies levels of performance, which allows a design team to select and implement the best strategies to meet project goals based on a defined set of alternatives (Figure 3).

Figure 3

Attribute Baseline Tier 1 High Performance Tier 2 High Performance Tier 3 High Performance Verification
Measurements & Verification Plans & Specifications Calculations & Analysis Design Basis
of Design
Envelope – Protective Security
Blast Resistance ISC Level II ISC Level III ISC Level IV Site Specific Risk Assessment Blast Mockup Testing Site Planning Interagency Security Criteria ASTM F1642 Design Team Calculations Describe blast resistance level design requirements.
Attribute Baseline Tier 1 High Performance Tier 2 High Performance Tier 3 High Performance Verification
Measurements & Verification Plans & Specifications Calculations & Analysis Design Basis
of Design
Envelope –
Natural Hazard
Seismic Resistance Life Safety Reduced Damage Immediate Occupancy Operational Performance Mockup Testing IBC-2012 ASCE 7-10 FEMA 356 ASTM E2026 Design Team Calculations & Inspection Describe seismic resistance design assumptions

Although a U.S.-based resource, the National Institute of Building Sciences (NIBS) National Performance-based Design Guide for Buildings is useful to design teams when establishing levels of performance. Data courtesy NIBS (npbdg.wbdg.org/enclosure/html)

Designing the envelope as a high-performance, low-energy strategy involves complexities best managed through an integrated design process. According to the Whole Building Design Guide (WBDG):

An integrated design process includes the active and continuing participation of users and community members, code officials, building enclosure specialists, contractors, cost consultants, civil engineers, mechanical and electrical engineers, structural engineers, CSC specifications specialists, and consultants from many specialized fields and material manufacturers. The most important aspect of integrated design is that it is most successful when driven by Owner Requirements.

The outside ins of building enclosures
Thermal envelope performance is one of the principle functions of enclosure systems. Site-built enclosure constructions involve coordination of multiple trades to install the individual components making up the ‘end’ system/assembly. A project team could consist of the best integrated design team, a sound schedule, and budget co-ordinated with a contractor, but enclosure performance hinges on multiple trades correctly installing systems/assemblies. This is where the latent defects in the project are most often realized.

One advantage of IMPs and off-site factory manufactured assemblies provide is consistent installation. Factory-controlled production of envelope systems, walls, and roofs, are more likely to incorporate a quality management system such as International Organization for Standardization (ISO) 9001, Quality Management, to ensure specified performance requirements. IMPs deliver all three key enclosure functions in a single component for uniform thermal continuity. There is no construction waste, other than packaging and, on occasion, some field cuts. Off-site manufactured IMPs are also more resource-efficient than site-built constructions.

Connecting the building envelope at the wall and roof junctions can be a challenging design and performance issue. Overall enclosure performance can be optimized when IMPs are used for both walls and the roof as an integrated system. However, the panels can also be designed to integrate with other building systems when multiple fabrics make up the building design.

IMP performance
MasterFormat lists IMPs under 07 41 00–Roof Panels and 07 42 00–Wall Panels. The assemblies are composed of an insulated core material sandwiched between two pre-coated metal skins, using an interlocking sealed joint for a weather-tight vapor barrier and insulating system.

IMP_FIGURE 4
An insulated metal panel (IMP) has a flat smooth exterior steel skin with two-stage sealed joint and reveal. Image courtesy Kingspan Insulated Panels

IMPs offer various insulation choices with the majority being constructed with polyisocyanurate (polyiso) cores, providing one of the highest R-values per inch at competitive prices. The material is typically a closed-cell, rigid foam board insulation considered a thermoset ‘foam plastic.’ It is a polyurethane or polyiso plastic foam comprising polyol, isocyanate, flame retardant, blowing agent, catalysts, and surfactants. It is important to note some polyiso is now free of halogenated flame retardants. IMP insulation options include mineral wool for IMP fire panels; on a specified basis, expanded and extruded polystyrene (EPS and XPS) are also available. The thermal performance of the IMPs with polyiso cores generally range from R-7 to R-48 (Figure 4, pictured right).

Steel metal skins are specified as 24 and 26 light-gauge with options including zinc, stainless steel, and aluminum. In combination with the insulation core, IMPs range in width from 610 to 1067 mm (24 to 42 in.) and heights of 2.4 m to 16.15 m (8 to 53 ft) The combination of the steel and closed cell polysio that comprise IMPs provide greater span capability than many other systems, and lighter loads than concrete or masonry systems. This is also a resource impact reduction requiring less structural framing.

IMP_FIGURE 5
A 76.2-mm (3-in.) IMP Term model demonstrates the thermal performance of the product with the pressurized, sealed joint. Image courtesy Kingspan Insulated Panels

IMPs offer significant advantages in the pre-engineered metal buildings industry where R-value code requirements are increasing. Other insulations may have to use double layers or more-expensive, higher-density materials to meet the thermal code requirements. IMPs can achieve these increases in thermal performance requirements as specified (Figure 5, pictured left).

Tested IMP R-values include joints according to ASTM E1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot Box Apparatus. This test requires at least two IMP panels with testing across the joint. A therm model of an IMP joint demonstrates thermal performance across the joint. Continuous insulation is defined by ASHRAE 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, as being continuous across all structural members without thermal bridges (other than fasteners and service openings). Most IMPs perform well because the side joints have a natural thermal break between the outer and inner metal facers.

The IMPs join together with an engineered pressure moderation (two-stage joint) interconnecting sealed joint providing air and moisture control.

Off-site manufacturing
IMPs are manufactured using one of two processes: lamination or foamed-in-place (also known as continuous line). Laminated IMPs are primarily found in custom architectural lines where foam is injected between the exterior and interior metal skins where it expands to uniformly fill the insulated core per the specified R-value. The lamination process bonds ‘pre-cured’ and factory-shaped polyiso rigid boards to the metal skins meeting specified performance.

The foamed-in-place process is more common in commercial industrial applications and flat smooth architectural products. Typically, continuous line IMPs are made for high-volume orders. When specifying IMPs, the manufacturing process (laminated or continuous line) should not be called out; rather the specification should be based on the project’s functional and compliance performance requirements. Regardless of the process used in manufacturing, the off-site assembly construction becomes one of IMPs’ leading benefits with factory quality control as compared to site-built construction by multiple trades.

All coil steel used in IMP assemblies are typically pre-galvanized with a primer coat and a finish coat. These coatings are high-performance of siliconized modified polyester (SMP), solid polyvinylidene difluoride (PVDF) colours. These coatings have an oven-baked epoxy primer and a finish of an air-dried 100 per cent acrylic bonder with natural silica aggregate, minimum 305-µm (12-mils) dry film thickness (DFT), and finished to resemble sprayed stucco. This coating is factory-applied to pretreated coils after the panels are fabricated. All other coils come to the plants ready to roll onto the production line for assembly. Coating warranties can range from 20 to 40 years.

The high-performance finishes applied in production to IMPs require significantly less maintenance than many exterior finishes or claddings applied in the field with finish warranties of 20 and 40 years. In a lifecycle costing exercise with a predicted service life of 60 years, exterior refinishing might need to occur two times. Most IMP manufacturers offer finish options with solar reflectance index (SRI) cool roof ratings, which also increase energy-efficiency performance through heat island reduction.

IMP installation
IMP assemblies have the faster build speeds compared to traditional site-built wall and roof enclosure systems. This is extremely important in cold climates when fast track construction is needed to ‘dry-in’ the enclosure for work to continue through winter months. The installation of IMPs requires one subcontractor instead of multiple trades to attach to the structural framing.

IMP_figure7
IMPs are attached to intermediate support attached to the structural steel framing. The intermediate support is back-sealed before the panel installation. Panels connect with a two-staged sealed joint. Image courtesy Kingspan Insulated Panels

The IMP mounting systems employ concealed fasteners with an integrated gasket to reduce thermal bridging and eliminate the need for multiple fastener performance as specified in multi-component systems. IMP fastener systems also contribute to structural span performance.

The erector places and ties the pre-engineered single component panels together in a sequential process. Due to the spanning capability of IMPs, framing and foundation requirements can be efficiently designed with resource and cost benefits. For example, support girt spacing of 203 to 305 mm (8 to 12 in.) can be used on many projects because of the relatively high panel strength and structural integrity. This creates other project economies and benefits.

Studs can be added to the interior skin and finished with drywall or another kind of interior finish for more finished interior options. In some cases of application, such as cold storage or in extreme climates, radiant barrier material may be added (Figure 6, pictured right). From a standpoint of building speed and economy, IMPs can also provide benefits for the building’s interior such as warehouses where the interior metal skin of the IMP becomes the finished wall.

Conclusion
The increasing use of IMPs allow project design teams to create integrated building exterior enclosures with stunning appearances and flexible design options with high-performance, low-energy, and resilience. Designers have choices of many profiles and patterns, with a wide palette of colours and finishes.

If a metal finish is not the desired esthetic, there are options where a hanger rail is integrated into the IMP so virtually any material can be applied to the exterior. Selections include full-size brick, thin brick, ceramics, aluminum composite panel (ACM), high-pressure laminate (HPC), and other single-metal skins.

Combining these façade options allows the designer to have desired esthetics while the insulated metal panels perform the control and support wall functions expected.

Paul R. Bertram Jr., CSC, FCSI, CDT, LEED AP, is the director of environment and sustainability for Kingspan Insulated Panels North America. He represents the company on various U.S. Green Building Council (USGBC), American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE), ASTM, International Code Council (ICC), and National Institute of Building Science (NIBS) groups. Bertram is also a past-president of the Construction Specifications Institute (CSI). He can be contacted via e-mail at paul.bertram@kingspan.com.

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One comment on “Designing enclosures from the outside in”

  1. Each electronic product is made with the best designs in mind. This is a vital part of the overall appeal of the product. This is why the enclosure design including custom electronic enclosures and the chassis custom design of any product are of monumental importance and adds greatly to the success of that product.

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