If you want to kill the thermal performance of your insulation, you can take your pick of several excellent ways to do so. Of course, you could just not install insulation. (It happens, even in new construction.) You also could install the insulation unevenly. Or you could interrupt the insulation with more conductive materials, like wood. But if you want to take your insulation-killing skills to the next level, use steel.
The benefits of exterior continuous insulation
My article on series and parallel heat flow in walls shows that you don’t always get the R-value on the label of the insulation you install. For example, let’s say you build a 2×6 wood-framed wall with studs spaced 16 inches on center. Then you put R-19 fiberglass batt insulation in the cavities with no other insulation. Cover it with drywall on the inside and sheathing and cladding on the exterior.
Do you have an R-19 wall? No, when you average the effect of the wood studs and add in the other materials (e.g., drywall, sheathing…), you end up with R-15.8. That’s a 17% reduction from what it says on the label. When you add in the effects of the other framing (e.g., top and bottom plates, corners, rough openings…), the net thermal resistance drops even lower.
One way designers improve the thermal effectiveness of wall insulation is to add continuous insulation to the outside of the wall. If it’s truly continuous, that improves the thermal performance tremendously. Instead of putting R-19 in the stud cavities as above, let’s say you put R-19 continuous insulation on the outside of the wall. That results in an average R-value of 22.9, an improvement of 21% over the labeled insulation R-value and 45% over the R-15.8 we got above.
But how does the attachment system for exterior continuous insulation work? And does it degrade the thermal performance? Let’s look at the three most popular methods.
What got me thinking about this topic was a building under construction near the Energy Vanguard office. The lead photo above shows a closeup of part of the wall. You can see the blue extruded polystyrene (XPS) rigid foam insulation. You also can see the vertical metal strips every 16 inches apart. Those are called z-girts.The photo above shows what the z-girts look like before the insulation goes in, and the diagram below shows the whole assembly. In both photos above, it’s a continuous piece of galvanized steel from outside to inside. That makes it a superhighway for heat transfer. So, yeah, congrats on the idea to put in exterior insulation, but they get an F on the attachment system because it kills the thermal performance. As you’ll see below, vertical metal z-girts are the worst way to attach exterior insulation. The exterior insulation will provide only 20 to 40 percent of the thermal performance of the insulation. That’s the word from RDH Building Science, an engineering firm based in Canada but with offices in several cities in the US, too. You can find the link to download their paper at the bottom of this article.
Installing the z-girts horizontally gives you a bit of improvement, but it still has a thermal efficiency of only 30-50 percent. Using crossing z-girts, where you do one layer vertical and the other horizontal, is another step in the right direction, with an efficiency of 40-60 percent. Going with fiberglass or other nonmetallic materials for the girts gives you a bigger boost.
A second way to attach exterior insulation is with a clip-and-rail system. This reduces the contact at the wall because the clips don’t run the whole length of the rails. Also, the clips, rails, or both can be made of fiberglass or other nonconductive materials. Or they can be made of less conductive stainless steel.Depending on which type of clip-and-rail system you choose, the thermal efficiency could be between 40 and 90 percent according to RDH.
Using long screws is probably the most common way exterior insulation is attached to walls in new home construction. The screws go into wood strapping, girts, or hat channels on the outside, then all the way through the insulation, and then into the structure. The photo below is from the RDH report and shows how that system works with wood strapping and three layers of mineral wool insulation on the exterior.One question you may have is, Will those long screws be able to hold up the insulation and whatever cladding you install over the insulation? Joe Lstiburek faced this question in 1981 when he was building a house with exterior insulation in Ontario. They did some testing to see if this system would be able to support the wood siding. It worked.
Building science labs are still testing various configurations and attachment systems. The photo below shows a test of long screws through two layers of 2 inch thick rigid foam 8 feet high. Hanging from the bottom of each is a bucket of rocks. A deflection gauge measures how much the weight pulls it down. The answer is that if you do it right, it works.
Now, the screws aren’t free of thermal penalties. They’re made of metal and thus conductive. But there’s a lot less area for heat to conduct through with screws. And then there are two kinds of screws: galvanized steel and stainless steel. It turns out that stainless steel is great for structural use, especially where corrosion can be a problem. And they’re also great at reducing the thermal penalty because they’re not nearly as conductive as galvanized steel. Of course, the penalty with stainless steel is that it’s more expensive.
The thermal performance of various attachment systems
The chart below is from the RDH paper. As you can see, galvanized steel vertical z-girts—like the ones I saw yesterday—are the worst. The two best, at greater than 90 percent thermal efficiency, are stainless steel screws and fiberglass clips with no screws. Everything else falls somewhere in between.The takeaway here is that exterior insulation by itself isn’t going to be a game changer. You’ve got to pay close attention to the attachment system. Also, the building codes now give a lot of information about fastening schedules and other details you need to know.
Download the RDH paper: Cladding Attachment Solutions for Exterior-Insulated Commercial Walls
Also see papers on thermal bridging from Building Science Corporation, like A Bridge Too Far, by Joseph Lstiburek, PhD, PE.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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