Thermal Bridging Research: Soffits

One of our many in-house research efforts is a large, collaborative project. We investigated the Thermal Performance of Façades through the AIA Upjohn Research Initiative.

Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. These thermal bridges are most often caused by structural elements that are used to transfer loads from the building envelope back to the building superstructure. Though design professionals generally understand that thermal bridging is a concern, few can quantify the extent of its impact on building performance.

Small changes in designs can still lead to dramatic improvements in performance. With careful detailing and attention to the issues of thermal bridging, the design and construction industry can improve the performance of our building envelopes.

Today we’re sharing our findings regarding soffits.

Envelope transitions from vertical systems to horizontal ones and then back to vertical again require complex support structures, which are well known to complicate air and vapor barrier installations and often lead to complex thermal bridges that are difficult to eliminate elegantly. When looking at comparatively small areas, such as soffits, these complex assemblies take on a disproportionately large role in the assemblies in question. When they are evaluated on their own, the R-value of the assembly adjusted for thermal bridges can be quite low. Beyond just the structural complications of beam-to-column interfaces, these sorts of assemblies usually include material transitions that will often require blocking or other accessory installations for anchorage and fastening. Considering soffits in particular, we observed R-values with a 35-70% reduction in performance over baseline.

To improve the performance of soffits, we studied a number of different potential improvements. As with other envelope conditions, we looked at minimizing continuous elements such as Z-girts with intermittent stand-off bolts, or using non-conductive materials such as fiberglass. These strategies were found to increase the R-value of the soffit by 65%. While we typically find that structure is best when kept on the warm side of the insulation, soffits are a bit more complex as the exterior face of the area needs support. If the material is inherently insulative, such as a metal foam panel, this issue will not present itself. Otherwise, it can be advantageous to concentrate the structural thermal bridges in a few, carefully planned penetrations of the insulation system and then treat all of the soffit construction in the same manner as a rainscreen.

 As-built condition, R-17.2

Thermally-improved condition, R-28.3 (+64.3): Z-girts have been replaced with stainless steel bolts and plate out of the plate of insulation.

Related:
Thermal Performance of Facades: Final Report
Thermal Bridging Research: Curtain Walls
Thermal Bridging Research: Investigating Insulation Thickness for Renovations
Thermal Bridging Research: Masonry Veneer Walls
Thermal Bridging Research: Window Transitions
Thermal Bridging Research: Foundation to Wall Transitions
Thermal Bridging Research: Rainscreens
Thermal Bridging Research: Metal Panel Wall Systems
Thermal Bridging Research: Parapets
Thermal Bridging Research: Transitions between Wall Systems