The Ins and Outs of IMPs

Armed with the various design options offered by insulating metal panels, together with some best practices for project coordination and installation, architects will be best equipped to deliver successful IMP wall and roofing projects
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Sponsored by Metal Construction Association, IMP Funders Group

Understanding Thermal Bow

One characteristic of IMPs is the fact that the metal will commonly expand, particularly when exposed to heat. This will frequently occur with IMPs or any metal panel material, for that matter.

Because IMPs are made up of two metal skins with a closed-cell foam-in-place core, as the exterior skin heats up, it will bow up slightly between the girts/purlins as the interior metal skin typically remains at a close consistent temperature.

In more extreme situations, the thermal bow not only causes installation difficulties, but the likelihood of oil-canning will increase. Furthermore, “once the panels are installed, the constrained skins coupled with the disproportionate expansion of the exterior skin can definitely yield an undesirable aesthetic,” warns Hooper.

Thermal bowing will be even more prevalent in cold-storage applications—where there is a pronounced temperature difference between the interior and exterior—and with darker exterior finishes highly exposed to the sun where the panel surface can reach as high as 180 degrees Fahrenheit. Inevitably, this buildup of excessive heat on the exterior panel skin relative to the cooler interior skin causes the exterior metal to grow disproportionately to the interior skin, thereby ‘crowning’ the panel.

Fortunately, there are some things that project teams can to do mitigate this effect. For example, specifying lighter exterior colors, utilizing shorter panel lengths, and limiting the usage of dark color IMPs on elevations with high exposure to intense sun can all help.

“Relief cuts at tall panels, corners of large openings, or across ganged openings can also help reduce the effects of thermal bow,” adds Haugh.

This stress relief cut is created by field cutting the interior face along the line of the intermediate support. When this is required, the interior face must be secured above and below the cut, instructs Lehr. An angle or flange may also need to be added to accommodate this fastening of the panel.

Manufacturers will also provide load span tables and recommended panel lengths to best assure panel performance.

Relevant thermal bow information can be found in the Metal Construction Association’s Selection Guideline for Insulated Metal Panels (available as a free download). MCA points out even though the thermal expansion may not be aesthetically pleasing and will affect the installation process, thermal bow will not adversely affect performance of a properly designed system, as panel analysis calculations and best practices make allowances for it.

Lifting, Cutting, and Framing Alignment

When it comes time to lift the individual panels, cut them—if necessary—and align them, it’s important to lift them on edge, if this is being done by hand.

Imp in a red color and shades of gray covering the Exeter YMCA in Exeter, New Hampshire.

Photo courtesy of Metl-Span

An IMP in a striking red color and several shades of gray clad the Exeter YMCA in Exeter, New Hampshire.

“Do not carry or lift the panels in a flat orientation, parallel to the ground,” cautions Lehr. “This can cause the panel to flex or bend in the middle, which can delaminate the metal skin from the foam and cause a horizontal ‘blister’ to form at the edges of the panel.”

Most manufactures will allow the use of a vacuum lift, depending on the profile and finish used. Otherwise, a rope pulley system with a lifting end clamp suspended from the roof can be used. Another option is an end clamp with the use of a crane, adds Lehr.

On the issue of cutting, DPR Construction generally takes steps to try and avoid this; for example, utilizing 3-D modeling in place of 2-D shop drawings. “That way, if there’s a complex angle, we want to model it accurately so that the fabrication is precise before it arrives on-site, ensuring exact and quality installation,” Robertson says.

At the same time, if the tolerances are too tight, it will be necessary to field measure the panel or the wall behind the structural component where the IMP is mounted. In this case, DPR Construction plans the schedule and sequencing to take these measurements before ordering the panels.

“If we do have to cut on-site, it will undoubtedly be in a controlled environment,” she says. “We would prefer to enlist a certified IMP contractor who would have the capacity to take the material back to their shop to wrap whatever cut was made with metal panel material to keep the finish cohesive to help address vapor and water barrier concerns.”

For contractors undertaking this themselves, it’s important to never cut the panels with a torch, a reciprocating saw, or any saw using a composition abrasive blade. Some manufactures recommend cutting one side of the panel at a time, then using a knife to remove the foam for cutting holes.

When it comes to framing the panels, this is critical, as excessive variation from the plane of the wall can cause undue stress on the wall panels, joint, and connections, thereby creating an undesirable aesthetic and increasing the potential of oil-canning.

“The maximum deviation of the plane of the steel substructure should not exceed 3/8 inches +/- in any 20 feet length, horizontally or vertically. The deviation cannot vary more than ¾ inch on any building elevation and transition areas, such as corners and soffits, should not have girt steel within +/- 1/8 inch of the girt plane,” instructs Lehr.

educational facility covered in IMPs

Photo courtesy of Metl-Span

Predominantly covered with high-performance IMPs, this 155,000-square-foot educational facility is state of the art inside and out.

The surface of the girt surface itself must be free of bolt and rivet heads, excessive welds, or any other obstructions which could prevent proper bearing, she adds.

More Details

When dealing with details such as cut backs, end laps, penetrations, and roof curbs, Robertson’s biggest piece of advice is “planning, planning, planning.”

By finalizing these details prior to placing the order for the panels, this lessens the chances of encountering mishaps in the field. “We try to model everything in addition to confirming these details and the shop drawings, and often utilize a third-party skin consultant to review these details.”

Robertson adds that the architect should always account for vapor and water barriers.

When an end lap joint is required, which is typically the case, the interior facing skin, called a liner, is usually cut. Once the liner is removed, a knife should be used to cut away the foam core, using a brush to clean away the foam from the back of the exterior skin. This will serve as the overlap onto the panel below, explains Lehr, the length of which will be determined by the manufacturer’s recommendations.

The end lap joints are then caulked and fastened, again according to the roof manufacturer’s recommendations. For wall panels, the use of a stack extrusion in place of an end lap joint is often used.

For penetrations and roof curbs, there are many possible variations and addressing these will depend on what the specific penetration and roof curbs look like, explains Elking. “To make sure the possible variations have been addressed, and to ensure the highest quality, we typically hire a third-party consultant to evaluate the many different environments coming into the building,” he says.

Overall, Patrick T. Johnson, PE, LEED Green Associate, CSI, CDT, Kingspan Insulated Panels, Columbus, Ohio, recommends a field mockup that encompasses a number of crucial conditions used on the project. “The mockup is normally tested per established air and water test standards, such as AMA 501.2 or ASTM E1105. The test mockup can be a separate structure or a portion of the finished work that is designated to be tested prior to proceeding.

Similarly, Robertson is a fan of modeling all major penetrations to identify exact locations. “This allows us to appropriately plan the design and fabrication, and weave these openings into the model and shop drawings to avoid cutting penetrations in the field.”

In Conclusion

An ideal solution for a growing variety of building types, IMPs are a win-win, combining aesthetics, energy efficiencies, and a full building enclosure solution.

“They are a simple, effective way to meet energy code requirements,” restates Haugh. “It’s a single product/system that provides an exterior skin, insulation, an air and vapor barrier, and an interior facing installed by a single trade.”

As more and more projects seek to leverage the technology’s benefits, well-informed designs and expert installations will continue to “lend credence to an industry that is capable of not only adding unique design elements to structures, but also being a building block of building envelope construction,” concludes Hooper.



Metal Construction Association logo. The Metal Construction Association’s Insulated Metal Panel Funders Group comprises leading manufacturers, resellers, and suppliers who are dedicated to growing the use of insulated metal panels (IMPs). www.metalconstruction.org

 

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Originally published in Architectural Record


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