How roof fasteners impact the R-value of roof designs—and what to do about it (2024)

May 16, 2024

By Elizabeth J Grant PhD, AIA, GAF

What’s wrong with the image above?

The pock-marks in the snow on this roof reveal the location of the roof fasteners holding its components in place. These highly conductive metal fasteners are thermal bridges through the roof’s continuous insulation. And the tighter and better-insulated a building is, the bigger the difference all these weak points in its thermal enclosure make. A range of codes and standards are beginning to address thermal bridging in general, but often ignore point loads such as roof fasteners.

The problem has been made worse in recent years because changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and ASCE 7-22 mean fastener patterns are frequently becoming denser. More fasteners lead to more heat escaping the building in winter and entering the building in summer.

How big an issue is this?

A team of researchers at Simpson Gumpertz & Heger Inc., Virginia Tech, and GAF designed a study using computer simulations validated by physical experiments to understand the thermal bridging effects of fasteners. First, we tested a simple 2’ x 2’ x 4”-thick polyisocyanurate board (ISO). Second, we added a ½”-thick high-density polyisocyanurate cover board (HD ISO). Third, we added galvanized steel deck to the 4” polyiso and took away the cover board, and fourth, we used HD ISO cover board and ISO over steel deck.

Then we added a #12 fastener, 6” long, in the center of each 2’ x 2’ assembly, with a 3”-diameter insulation plate and ran each test again. The fastener penetrated the ISO and steel deck, but not the HD ISO cover board.

What did the study find?

The fastener reduced the R-value of the 2’ x 2’ sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation. This proved that roof fasteners have a measurable impact on the R-value of roof insulation.

When the HD ISO was added, R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. This proved that the HD ISO cover board had an important protective effect against the thermal bridging caused by the fastener.

Next, we found that the steel deck acted like a radiator, exacerbating the effect of the fastener. With the fastener added, there was an R-value drop of 11% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.

Finally, the assemblies with all the components (HD ISO, ISO, and steel deck) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board), and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.

What can you do about it?

When extending these findings to the “real world,” results will vary based on building location, roof fastener densities, and roof assembly type, but we can make the following general suggestions:

Consider an induction-welded system that minimizes the number of total roof fasteners. The special insulation fastener plates are welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for membrane fasteners.

Consider eliminating roof fasteners or burying roof fasteners. Multiple studies have shown that placing fasteners beneath one or more insulation layers and adhering the remaining layers can reduce the thermal bridging effects of fasteners.

Consider an insulating cover board. They provide enhanced longevity and system performance by protecting roof components from hail damage, allow for enhanced wind uplift and improved aesthetics, offer additional R-value, and mitigate thermal bridging as shown above.

Consider increasing the R-value of the roof insulation. If fasteners diminish the thermal performance of roof insulation, building owners are not getting the full design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.

Read GAF’s Building and Roofing Science Roof Views blog for more details about the implications of these different roof assembly attachment methods in your design projects.

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How roof fasteners impact the R-value of roof designs—and what to do about it (2024)
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