Designing with insulated metal panels

by Katie Daniel | November 27, 2015 10:06 am

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By John Straube, PhD, P.Eng.
Insulated metal panels (IMPs) have become increasingly popular in the last decade as a lightweight, prefabricated, total building enclosure system. Their strong, stiff, metal skins provide excellent water and air control. However, the joints between panels and the transitions to other components—especially windows, doors, roofs, and foundations—demand close attention. Discontinuities at joints and transitions are one of the most common causes of building failures, and this is especially true for designs using IMPs. Employing an appropriate approach to rainwater control will help significantly reduce this risk of failure.

The building enclosure is the component separating the indoor and outdoor environments. To perform its function, an enclosure must offer support, control, and finish functions. The control functions with the most impact on durability and performance are provided by the water, air, thermal, and vapour control layers. Ensuring these control layers are identified and provided in a continuous plane around a building by all involved in the design and construction process is a key to successful enclosure performance.

IMPs are composed of two thin layers of coated sheet metal wrapped around a rigid foam core to form a stiff composite. The metal skins act as the water, air, and vapour control layers, (In most cases, the interior steel sheet acts as the air and vapour control layer. However, in special circumstances [e.g. a cold storage building in a warm climate], the exterior sheet may be defined and detailed as the vapour and air control layer.) and, in many cases, the bare or coated metal also acts as the finish (Figure 1). Just as for any other enclosure, the most important control function is rainwater, followed by air flow, thermal, and vapour. For IMPs, the strategies most successful for rain control will also successfully control air flow and vapour, and each of these topics will be briefly discussed in turn.

Rain-control strategies
As controlling rain penetration is the most important function, it must be addressed first. There are three recognized design strategies to control rain penetration within and through the enclosure. (See this author’ piece [co-written with E.F.P. Burnett], “Rain Penetration Control: Applying Current Knowledge,” which appeared in the July 1999 Journal of Thermal Insulation and Building Envelopes, published by Canada Mortgage & Housing Corp.)

As shown in Figure 2, these approaches are:

• storage (or ‘mass’);
• drained (or ‘rainscreen’); or
• perfect barrier.

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Building enclosures have been, and continue to be, designed and built successfully using all three strategies.

Storage
The oldest strategy, storage, assumes some rainwater will penetrate past the outer surface of the enclosure. It requires the use of an assembly of materials with enough moisture storage capacity and moisture tolerance (that is, sufficient ‘mass’) to absorb all rainwater that is not drained (i.e. shed) from the outer surface.

In a functional storage/mass wall, this moisture is eventually removed by evaporative drying from both the inside and outside before it reaches the inner surface of the wall as a liquid. Some examples of storage systems include adobe walls, thatched roofs, and multi-wythe brick masonry.

Drained enclosures
In the drained enclosure approach, it is also assumed some rainwater will penetrate the outer surface. However, rather than storing it, the assembly is designed to remove it by providing drainage. (The term ‘rainscreen’ is applied to some drained systems (since the cladding ‘screens’ the rain), but the term is imprecise, as it means different things to different people.)

Drained systems have justifiably received a lot of attention from researchers and practitioners. For many cladding types (e.g. brick veneer, siding, stucco), drainage is not just the most practical strategy, but it has also been the most successful system of rain penetration control.

While this approach can also accommodate a range of other claddings and backup systems, drained systems are not always an appropriate strategy. Insulated metal panels—as well as existing multi-wythe solid masonry walls, glazing panels, large-format precast panels, and low-slope membrane roofs—are examples of common enclosure systems that perform better when a rainscreen drained system is not used.

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Perfect barrier
In a perfect barrier system, all water penetration stops at a single plane. These assemblies may be face-sealed (where the perfect barrier is at the exterior face) or concealed (where the barrier is protected behind other materials).

Some concealed barrier systems, such as protected membrane roofs (PMRs), have a long record of good performance. Others, such as stucco and adhered veneers applied directly over building paper, have shown disastrously poor performance in many applications. The difference in experience is directly related to the likelihood of a perfectly waterproof barrier being achieved in construction and maintained over the desired lifespan. It is difficult to build and maintain a perfect barrier with many materials, but some systems, usually factory-built, provide wall elements that are practical and durable perfect barriers.

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Joints
Joints may be classified in the same manner as enclosure elements—in other words, a joint may be designed using a storage, drained, or perfect barrier approach (Figure 3).

It is important to note enclosure elements and the joints between them can use different approaches. The joints between perfect barrier elements should almost always be designed as drained joints in the form of two-stage sealant joints or similar.

Rain control for joints and penetrations
Unlike some other systems, drainage or rainscreen approaches are not recommended for the overall design of an IMP wall. Insulated metal panels are inherently a perfect barrier (water obviously will not leak through steel) and should use a perfect-barrier rain-control approach. However, water can and will enter the enclosure through joints and penetrations.

IMP wall joints are made onsite (increasing susceptibility to workmanship errors) and comprise materials more likely to degrade (due to ultraviolet [UV] radiation and high and low temperatures). It is therefore critical these elements be drained.

The concept of a drained joint, or two-stage seal has been promoted for almost 50 years,  (See two G.K. Garden articles—“Joints Between Prefabricated Components,” Building Note, No. 40, p. 5, Feb. 01, 1963; and CBD-97, “Look at Joint Performance” from the Division of Building Research, NRCC, Ottawa, 1968. See also American Architectural Manufacturers Association’s (AAMA’s) 1971 publication, The Rainscreen Principle and Pressure-equalized Design.) based on solid research, (See Sven Svendsen’s “The Principles of One-stage and Two-stage Seals” and T. Isaksen’s “Rain Leakage Tests on Through-joints”—both from the 1967 Weathertight Joints for Walls: Proceedings of the International CIB Symposium, Oslo, Norway. See also R.E. Platts and J.R. Sasaki’s “Rain Leakage Tests on Vertical Through Joints,” a Division of Building Research Internal Report (No. 23) from 1965.) and has been widely used with great success for almost as long in the precast concrete and curtainwall industries. In contrast, face-sealed polymer sealant joints (i.e. a single line of exposed sealant) have a poor record of performance and cannot be recommended for controlling rain entry.

Even exposed gaskets, often used to create the joint between insulated glazing (IG) units and the window frame, tend to shrink and crack over time. When these seals fail, as field experience shows is likely, significant water penetration occurs. (For more, see M. Lacasse and H. Miyauchi’s “Water Penetration of Cladding Components : An Overview of the Vulnerability of Sealed Joints to Water Penetration,” from the Proceedings of the 12^th Canadian Conference on Building Science and Technology [CCBST], held in Montréal in May 2009.) For these reasons, a drained approach is recommended at joints for many systems, such as between the glass and frame of windows, between panels of precast concrete, and between IMPs and windows, doors, and other IMPs.

The interior seal in a two-stage joint acts to ensure both air control and water control continuity (Figure 4). The outer seal is a protective screen that ensures little water enters the joint, and the inner sealant is protected from UV radiation and temperature extremes. The inner seal can be installed from either the interior or the exterior and be formed of many different materials, although metal flashing and polymer-based sealants are the most common. Gaskets, unless mechanically compressed, do not usually provide sufficient tightness for the inner seal, but are often preferred for outer seals.

Generally, the inner seal should be held back about 25 mm (1 in.) behind the backer rod, gasket, or overlap of the outer seal to create a well-drained air gap. The gap can be deeper, if convenient, and often is in thick wall systems. Weep holes, with a minimum dimension of about 12 mm (½ in.), or continuous unsealed overlaps, must be provided to facilitate drainage. Weep holes are almost always installed at the base of vertical joints, and protected unsealed laps are used at horizontal joints on top of flashing. It is ideal, especially in enclosures highly exposed to wind-driven rain, to provide some protection for weep holes by adding exterior drip edges or extra layers of internal baffles to interrupt the direct kinetic energy of raindrops.

In addition to joints, window and door penetrations are critical to controlling rainwater. All enclosures should use a drained approach for window and door penetrations, regardless of the enclosure’s overall rain control strategy. Sub-sill flashings of various types are widely available to create a drained opening for windows and doors. For drained wall systems, the sub-sill flashing can drain into the drainage gap. For perfect barrier (e.g. IMP) and mass systems, the flashing must drain water to the exterior face of the assembly—that is, the water control layer of the rough opening must be made continuous with the water control layer of the wall.

Finally, fasteners must be considered. IMP systems must be fastened back to a primary structure to safely and effectively transfer the collected loads. This is usually achieved using fasteners and clip systems. These discrete connectors vary in shape and size amongst different IMP manufacturers. They may occur only twice along the length of a panel or every few yards. The design of these details, and the construction of the IMP enclosure, must account for these special penetrations or leaks of air and rainwater may occur. Through fasteners can either be hidden (and hence, protected) from direct rain impingement or exposed. In the latter’s case, they should be set in sealant or be provided with a pre-installed gasket and washer.

Air control in IMPs
Insulated metal panels meet all the requirements to be part of an air barrier. Sheet metal is clearly airtight, and hence is listed as an air barrier material in codes and standards such as American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, without need for any additional testing.

Further, air leakage condensation cannot occur within the body of the IMP, even if one of the metal skins is breached, because all materials are completely air-impermeable and there are no empty voids to allow air flow. However, excessive air leakage at joints may be a problem if air leaks across the air gap and contacts a cold surface on the other side.

Therefore, as with rain control, air flow control in IMP systems depends primarily on the joints, penetrations, and transitions. Construction documents should detail how an uninterrupted, strong, and airtight transition from the inner sheet steel layer to the adjoining curtain wall, roof, canopy, foundation, etc., will be achieved while accommodating dimensional construction tolerances and in-service movement. Drained and vented joints should be used to allow incidental air leakage condensate and vapour to leave a joint, and will control condensation for small air leaks in most applications and climates. Of course, the vented outer joint cannot be part of ensuring air barrier continuity. Hence, the inner seal provides the continuity of the air barrier across joints.

For panel-to-panel transitions, high-quality, gun-grade sealants are preferred—compressed foam or butyl tapes often cannot accommodate the range of joint sizes demanded by dimensional tolerances on the jobsite, and polymer gaskets rarely remain pressed tightly to the metal skins. Protected interior seals made of quality polymeric sealant materials can usually provide service lives of 30 to well over 50 years. Mechanically clamped gaskets and membranes are usually practically difficult or too labour-intensive to install between panels, but are the preferred solution for joints between an IMP and another building component.

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Vapour control in IMPs
The steel skins of an IMP are perfect vapour barriers (i.e. 0 perm). At joints between panels, achieving a low-permeance vapour barrier is not very important, except in demanding applications with very high average differences in water vapour content (e.g. freezer warehouses, swimming pools, special-purpose factories). However, some vapour resistance should be provided.

In many cases, the backer rod and sealant used as an air barrier seal will also provide an adequate vapour barrier.
For high-interior-humidity environments such as swimming pools, however, open-cell foam backer rods and silicone sealant are usually too vapour-permeable. Urethane and butyl-based sealants over closed-cell foam backer rods can provide low-permeance vapour barriers. In all cold climate applications, a vented outer sealant is recommended to allow any moisture that may enter the joint from the interior to leave to the exterior.

Thermal control
The thermal control layer is clearly provided by IMPs in the form of the continuous foamed insulation layer. Thick panels, 100 mm (4 in.) and more, can provide high levels of thermal control. However, thermal bridging is a concern as it is in many enclosure systems. Care must be taken in the design of metal flashings, door frames, overhangs, and the like to limit the full-wall penetration of metallic components. Research for B.C. Hydro has shown even metal flashings can significantly decrease the overall thermal performance of systems in which flashing is used often or without thermal breaks. (The research for BC Hydro’s “Building Envelope Thermal Bridging Guide,” was conducted by Morrison-Hershfield in Vancouver, and published in 2014.) A 76-mm (3-in.) thick nominal R-21 panel would have an actual R-value of R-15.8 if horizontal flashings were located every 2.7 m (9 ft) on centre.

Conclusion
Insulated metal panels can provide all the components of a functional building enclosure in one component. Understanding the control layers used, and the rain control strategy employed, is important to achieve the best performance and avoid potential pitfalls. A vented, drained two-stage joint provides a single interior air and vapour barrier, with the outer seal acting as a vented rainscreen with little air or vapour resistance.

Connecting the control layers of other enclosure components with an IMP system in this manner can provide reliable, durable performance. As with most systems, achieving continuity at the joints between panels and between different components is critical.

[6]John Straube, PhD, P.Eng., is a principal at RDH Building Science Laboratories, and a cross-appointed faculty member in the School of Architecture and the Department of Civil and Environmental Engineering at the University of Waterloo. He is also a prolific writer and a noted public speaker. Straube’s leadership as a building scientist and an educator has been recognized with multiple awards, including the Lifetime Achievement Award in Building Science Education from the National Consortium of Housing Research Centers (NCHRC). He can be reached at jfstraube@rdh.com[7].

Source URL: https://www.constructioncanada.net/designing-with-insulated-metal-panels/

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