Continuous Insulation – Part 2
When we left of our last post, I was explaining that the 1.5″ or more of Continuous Insulation (CI) required by most current energy codes is causing problems with approved cladding attachment systems. In this post, we will outline some solutions.
Solution 1: The U-Factor Compliance Method via The Energy Code
The IECC 2009 and most similar energy codes provide two methods to meet the insulation requirements. The first [Table 502.2(1)] is the “Prescriptive Method,” which simply gives R-values for various construction types and elements. For instance, a metal-framed wall in Zone 4 would require “R-13+R7.5ci.” This essentially means R-13 batts or other insulation between the studs, and R-7.5 of continuous insulation. As long as everything is installed correctly, it’s quick and easy and you can move on. But as we have seen in the previous post, getting everything installed easily is not that easy. The second method is called the “U-factor compliance method” (Table 502.1.2). The basis for this method is calculating the entire wall assembly for compliance with an overall “U-value.” (FYI – U-value is inverse of R-value — U=1/R). The actual construction method is not as important, as long as you have the data you need to calculate the overall value on the assembly — which is not always easy. For the same Zone 4 Metal Framed wall, the requirement is U=0.064 (R=15.625). (Which is roughly equivalent to R-13+R7.5ci, so you suddenly see how weak R-13 batts in metal studs really is => R~8.125.)
The positive of this method is that all you have to do is add more insulation until you overcome the weak points created by thermal bridges such as metal framing. The biggest problem is that you have to be careful as to where the dew-point is going to fall at all points in the assembly, and provide necessary vapor and air barriers to prevent condensation, mold, and other similar issues.
The one time I suggest this method is when you choose to forgo the “R-13” batts in the studs and go entirely with Continuous Insulation . . . provided you can get it to work structurally and get it sealed properly, it is likely to be the most-efficient, most air-tight, and least vapor-problem-creating system you could choose. Many insulated metal panel systems work this way, as can EIFS. Some of the rigid foam board manufacturers are developing systems as well.
Solution 2: Secondary Cladding Support System
The second solution is likely to be used in a larger variety of projects with a larger variety of cladding systems. The basic idea is to create some sort of secondary structural attachment system outside the CI to attach the cladding. The problem is, there needs to be a way to support this secondary system without creating thermal bridges through the CI back to the primary structural system or studs. There’s really only two obvious ways to do this:
- A self-supporting system. On a low-rise building, this may be tenable, but probably not suggested. It is also probably not very cost-effective. The idea is to mimic the advantages of a masonry veneer. Masonry veneer works with continuous insulation, because it only needs lateral ties through the CI. The weight of the material is carried by itself down to the foundation or other supporting structure. With this in mind, it would seem possible to create a secondary structural wall, such as a second layer of metal studs, outside the CI, supported directly on the slab, foundation, or other member below, with minimal ties back through the CI for lateral support. For extremely heavy veneers or extremely thick layers of CI (over 4″), this method may actually be necessary.
- A non-bridging wall-supported system. The idea with this method is to create a lightweight attachment system outside the CI that is supported through the CI to the primary structural system with some combination of connections that meet the definition of “fasteners” described in the previous post. I believe this method has the most merit for the widest variety of projects, cladding, sheathing, amount of CI, contractor preferences, etc. The problem is, finding a way to make it work. Before I even began my research, my first thought was, “There HAS to be a way to use furring outside the CI to get this to work. But currently, the code doesn’t have provisions, nor do most cladding manufacturers. But, as it turns out, others have thought the same way, and solutions are starting to show up . . . In the interest of long reads, we’ll pick up there in our next post . . .
Please contact me at Curry Architects for all of your design needs.