Insulated metal panels (IMPs) have recently risen to the fore as an outstanding prefab option for the construction of building envelopes, due to their low mass as well as the outstanding water and air resistance they can provide.
The panels themselves consist of stiff foam cores sandwiched between thin layers of coated sheet metal, which due to their impermeable nature serve as an excellent means of preventing the ingress of rainwater, vapour or moisture in the air.
While insulated metal panels themselves excel at preventing the intrusion of rainwater and other environmental elements that can severely compromise the performance of the building enclosure as an insulting barrier, their effectiveness nonetheless remains highly contingent upon proper usage and installation.
In particular, failure to properly address the presence of discontinuities in between the panels, such as joint, transitions, windows and doors, can significantly undermine the effectiveness of the panels, given that they inherently lend themselves to the creation of “perfect barriers” that enhance the performance of building enclosures by completely preventing the ingress of moisture of air.
Building enclosures are generally designed to prevent the intrusion of rainwater by means of one of three main design approaches – storage, drainage or the creation of perfect barriers.
Both storage and drainage designs are based on the premise that some rainwater will penetrate the exterior surface of the enclosure, and that any accumulated moisture will subsequently be expelled by means of evaporative drying or systems that effectively facilitate the outflow of water.
Perfect barrier systems, however, are designed to create an exterior building layer which is fully impervious to the penetration of water by means of either a concealed or face-sealed surface. For this reason, it’s imperative when using insulated metal panels that the joints and other discontinuities in the building exterior, which themselves serve as enclosure elements, possess a robust level of moisture resistance.
Drained joints with two-stage seals, the usage of which has been advocated by members of the building industry for nearly half a century, are an excellent option for perfect barrier systems. They have long been employed with precast concrete and curtain walls to tremendous effect.
The joints contain two seals for enhanced resistance – an exterior seal that serves as the initial layer of prevention against the ingress of moisture, and an interior seal that serves to create continuity in terms of air and water control.
Fasteners also warrant consideration when employing insulated metal panels, given that they serve as a key structural element that connects the panels to the main structure and ensure that collective loads are safely and properly transferred. The fasteners should be used sparingly – no more than twice along the length of each panel, and either concealed from direct exposure to rain or secured with sealants, gaskets or washers if left exposed.
The other major form of discontinuity in the building exterior is window and door penetrations, both of which are vital areas for controlling the penetration of moisture or air. Even if the building enclosure employs a perfect barrier approach to rainwater resistance, window and door penetration should still employ a drainage strategy for the removal of moisture.
Sub-sill flashing that channels water to the exterior assembly surface is an excellent drainage option for the window and door penetrations of perfect barrier building enclosures, as they create a level of continuity between the water control layers of the opening in the exterior and the rest of the wall face.
Achieving air control with perfect barrier building enclosures employs principles similar to those for shoring up rainwater resistance with respect to joints and other discontinuities. It’s important to create robust, airtight connections between the panels themselves, as well as between the inner layer of sheet steel and adjacent structural elements such as canopies, curtain walls, foundations and roofs.
Membranes or mechanically clamped gaskets serve as an effective means for attaching the insulated metal panels to other structural elements, while high-end, gun-grade sealants are the best option for connecting the panels themselves to each other.