Independent Study Shows the Effects of Fasteners on Thermal Performance in Building Enclosures
Structural Insulated Sheathing (SIS) was found to be one of the most effective systems in negating the effects of fasteners and thermal bridging, states a new white paper by M. Steven Doggett, PhD.
Parameters of Thermal Testing
“Cladding Attachment Systems: The Effects of Fasteners on Thermal Performance” is a newly released white paper by Dr. M. Steven Doggett that tests the effectiveness of six cladding attachment systems on negating thermal bridging by using 3D modeling software. The six cladding attachment systems were as follows (shown also in Fig. 1.):
- Code-approved hybrid assembly
- Common U-factor alternative exterior continuous insulation (C.I.) assembly, which serves as the base model for cladding attachment strategies #4 – 6
- Structural Insulated Sheathing (SIS) assembly
- Horizontal Z-Girts cladding attachment strategy
- Punched Z-Girt cladding attachment strategy
- Bracket and Rails cladding attachment strategy
When assessing the cladding attachment systems themselves, four systems were considered; Structural Insulated Sheathing (SIS), horizontal z-girts, engineered punched steel girts, and a bracket and rail assembly. The first three systems were individually compared to Structural Insulated Sheathing (SIS), wherein the panel itself served as the cladding attachment substrate, not the studs (Doggett, 3). During testing, the Structural Insulated Sheathing (SIS) panel used either stainless steel or coated steel fasteners. Fastener quantities, dimensions, and placements can be seen in Table 1 below:
Results of the Study: Structural Insulated Sheathing (SIS) Outperforms
When compared to the other tested assemblies, Structural Insulated Sheathing (SIS) surpassed them all in terms of the high performance of its insulation continuity and low number of conductive elements. Both of these components factored into the effective R-Value of the panel, reaching a value of R-21.5, above the minimum requirements of an R-20 assembly. The comparative thermal data can be seen below in Fig. 3.:
The Effects of Fasteners on Structural Insulated Sheathing (SIS)
When testing the Structural Insulated Sheathing (SIS) panel, there was a fastener-induced thermal loss of 13% when using steel fasteners due to shank interfaces with the thermally isolated magnesium oxide board (Doggett, 5). This loss did slightly improve with the use of stainless-steel fasteners, decreasing to 8.7%. However, the effects of the fasteners were offset by the panel’s poured polyurethane foam insulation layer, which has a higher R-Value per inch when compared to other continuous insulation choices.
In addition, because the cladding attachment substrate is the panel itself in most cases, there were no appreciable losses expected from cladding fasteners (Doggett, 5). In Fig. 4. below, you can see the Structural Insulated Sheathing (SIS) model exceeds the one-dimensional R-Values of other alternative exterior C.I. assemblies:
Fig. 4. Thermal performances of wall assemblies. Reported efficiencies represent walls configured with all components.
Thermal degradation from fasteners varies based on fastener type, quantity, and cladding attachment system (Doggett, 7). In turn, this means that fasteners do have consequences when used on exterior insulating assemblies, and, as such, must be considered when creating an assembly that meets the minimum R-Value requirements. As stated by Dr. Doggett, “Thermal efficiencies of modern walls are determined by large pieces of metal that traverse the exterior insulation layer. Reduce these highly conductive materials whether by material type or component geometry, and you effectively improve performance. Remove them altogether, as with Structural Insulated Sheathing, and you effectively achieve the true intent of continuous insulation.”
In Fig. 5. seen below, the differences in R-Values can clearly be seen depending on what materials are chosen for the wall assembly.
Dr. Doggett asserts, “If we do not include fasteners in our analyses, how do we know their effects? Here, I have examined just a handful of conditions yielding some very interesting outcomes. There are countless others waiting to be explored. And what about future innovations – those not yet conceived? As currently written, the code seems content with exclusion, inviting virtually any contrived system to become a fastener much as our cladding attachment system morphed into the cladding fastening system. Why not simply close the fastener loophole once and for all?”
For More Information
As a result of this independent study, ArmorWall Fire-Rated Structural Insulated Sheathing™ is shown to be one of the most effective systems in the building enclosure industry, being a Structural Insulated Sheathing (SIS) panel. ArmorWall surpasses traditional systems by mitigating the effects of fasteners on thermal performance, as shown by the data in the white paper.
You can download the entirety of the white paper below:
To learn more about ArmorWall, click here.
Doggett, M. Steven. Tech. Cladding Attachment Systems: The Effects of Fasteners on Thermal Performance. M. Steven Doggett, PhD., 2021. PDF file.