RDH performed thermal analysis on an innovative new building product which can be used to improve the R-value of exterior insulated non-combustible wall assemblies. The product is a patented fiberglass girt spacer which is used, like a clip, to support cladding in exterior insulated wall assemblies. Our study found that its use allows for reduced wall thickness while still meeting ASHRAE Standard 90.1 requirements.
Here's how it works: the spacers are temporarily clipped onto metal z-bars, laid up on the wall, and screws are fastened through the z-bar and fiberglass spacer together into the backup wall. Semi-rigid mineral fiber insulation is placed between the spacers without the need for cutting. Cladding is attached to the z-bars (which do not penetrate the insulation), like any other girt system. Any type of cladding can be supported with the spacers including fiber-cement board, stucco, metal panels, etc.
Since the fiberglass girt spacer and screw are the only penetration through the exterior insulation, thermal bridging is minimized. In fact, thermal bridging is reduced so much that an ASHRAE Standard 90.1 minimum prescriptive steel framed wall (R-15.6 effective) can be constructed with only 3 ½” of exterior semi-rigid mineral fiber insulation.
The R-value of the insulation with the screwed-in fiberglass girt spacer is approximately 85% effective.
When conventional continuous vertical steel girts are used to support cladding through exterior insulation, the exterior insulation R-value is between 30% and 40% effective, meaning that 3 ½” of R-15 insulation only provides between R-4.5 and R-6 (far short of the ASHRAE Standard 90.1 minimum prescriptive requirement). Crossing horizontal and vertical z-girts improve the R-value slightly, offering insulation that is between 50% and 60% effective. Using galvanized steel clips, the insulation R-value is between 60% to 70% effective.
All of these other wall assemblies will require significantly more insulation to meet the ASHRAE Standard 90.1 prescriptive requirements. For example, to get a comparable wall assembly that is R-15.6 effective with galvanized clips, 7” of semi-rigid insulation would be required (double that of the fiberglass clip) resulting in a thicker and more expensive wall.
When comparing costs to construction for alternate assemblies, the fiberglass girt spacer was found to be the most economical option, primarily because less insulation and steel is used compared to conventional vertical girt assemblies.