The perfect wall is an environmental separator; it has to keep the outside out and the inside in. To do this, the wall assembly must control rain, air, vapor and heat. We need four principal control layers. They are presented in order of importance:
To meet all the performance demands, the perfect wall requires four control layers
- a rain control layer
- an air control layer
- a vapor control layer
- a thermal control layer
The best place for the control layers is to locate them on the outside of the structure to protect the structure (Figure 1). When we build out of steel/metal, we have to address the conductivity of the structure. Thermal bridging is a big deal.
The cladding may change, but the wall every steel/metal commercial building should use is in Figure 2. The base wall that our infrastructure should depend on has a conductive structure: metal studs. All the insulation should, and must, be located on the outside. You can build it anywhere in any climate.
There is more. We have learned that we need to have a continuous water control layer, a structurally sound and continuous air control layer (air barrier), a thermal layer (insulation) with no major conductive penetrations, and a vapor control layer. And all of this must be on the outside of the structure.
One of the variations of the perfect wall is an insulated metal panel (IMP). IMPs are a premanufactured version of a perfect wall (Figure 3). With IMPs, the vapor profile is slightly altered from that of a traditional perfect wall. Drying to the exterior occurs from the exterior face of the IMP in all climate zones. Drying to the interior occurs from the interior face of the IMP in all climate zones.
Note that the exterior face of the IMP functions as the water control layer, air control layer and vapor control layer.
Now, for the twist. The interior face of the IMP also functions as an air control layer and vapor control layer. This dual location of both an air control layer and vapor control layer allow this assembly to function successfully for refrigerated buildings and cold-storage facilities in all climates. This is the reason IMPs own this segment of the business.
It is important not to forget the thermal bridging issues that can occur where wall assemblies meet foundation assemblies. Figure 4 provides an example of how to address this issue. Note that a wood plate is used as a thermal break.
The traditional cladding is omitted, and the exterior face of the IMP becomes the cladding. With this approach, IMPs channel precast concrete panel systems. The results are impressive (Photo 1).
From our previous look at the perfect wall, we noted that the same approaches can be applied to roofs, the argument being that similar loads and the same laws of physics apply (Figure 5).
What about retrofit? Can you use an IMP as a retrofit strategy for uninsulated masonry buildings? Yes. With retrofit approaches it is common to apply a fluid water and air control layer directly to the masonry (Photo 2) and have the IMPs only provide the thermal control function and cladding function (Photo 3). See Figure 6.
JOSEPH LSTIBUREK, B.A.Sc., M.Eng., Ph.D., P.Eng., is a principal of Building Science Corp., Westford, Mass., and an adjunct professor of building science at the University of Toronto. He is a building scientist who investigates building failures and is internationally recognized as an authority on moisture related building problems and indoor air quality. He is an ASHRAE Fellow. For more information, go to buildingscience.com.