Thermal Bridging Research: Roof Penetrations

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 roof penetrations.

By their very nature, roofs present a large number of challenges in terms of maintaining a continuous thermal boundary. While roof drains and plumbing vents may be unavoidable and largely immutable, the impacts of other penetrations can be minimized through careful detailing. As part of our exploration, we considered skylights, balcony railings and roof davits as common conditions.

Skylights are generally recognized to have inferior thermal performance, and this is demonstrated through the fact that energy codes allow significantly higher U-values for skylights than windows. Because they frequently omit thermal breaks in the frame and larger openings typically require additional structure, there is significant thermal loss around the perimeter of traditional skylight installations. Additionally, for drainage reasons, the thermal barrier of the insulated glazing unit in a skylight typically protrudes above the roofing assembly. Just as with vertical windows that are proud of the façade, this requires more complicated detailing to try to maintain a thermal barrier. Wrapping the skylight frame in an insulated metal panel and ensuring thermal continuity throughout the skylight support dramatically improves its thermal performance.

On roofs that require access, railings and the structure required to support them can create substantial thermal bridges. At one building that we studied, there was a continuous bottom rail of an all-glass railing that was supported by steel tubes down to the slab below. Because the tubing interrupted the insulation, this sort of installation duplicated the typical conditions we expect to see with a continuous relieving angle on a brick veneer wall. By changing the material of the bottom rail to stainless steel and introducing a thermal break between the rail and the steel supports, we were able to minimize thermal bridging. Our modeling showed that such an approach reduced the heat loss through the assembly over 50%.

Roof Davits

Roof davits and other structural elements like roof dunnage present a similar challenge. These often penetrate through the roof to be connected directly to the building superstructure. While these individual occurrences may be so small that they have no significant impact total heat loss, we found they are often worth considering in humidified environments due to the threat of condensation. Looking specifically at davits, we considered several directions for improvement. To begin with, we proposed to cover the exterior of the davit with an insulated “sock” that could be removed when in use. However, because the steel of the davit is still penetrating from outside to inside the thermal barrier, the improvement in interior surface temperatures was minimal. Creating a structural thermal break with two steel plates (one on either side of the insulation) led to significant improvements, however.

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
Thermal Bridging Research: Soffits
Thermal Bridging Research: Roof to Wall Transitions