Thermal Bridging Research: Beam Embeds and Slabs in Existing Buildings

One strategy for living a sustainable life is buyiOne 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 beam embeds and slabs in existing buildings.

Since existing buildings are typically insulated from the interior during a renovation, the structural members can cause substantial thermal bridges. In load-bearing masonry buildings, common in the Northeast, the bearing support for beams are often embedded in the exterior wall causing the steel beam to penetrate the insulation and causing a significant thermal bridge. This was observed in thermal images to decrease the thermal performance of the adjacent wall by 72%. In photographed details, Teflon spacers had been used where the angles connected the embedded steel to the beam web. Because the beam was still extending through the insulation, the spacers—which were intended to lessen the thermal bridge from the beam—did not have a significant impact.

While existing beams pose a difficult challenge to improving this condition, when new structural beams are added as part of the renovation, there is an opportunity to improve the façade’s thermal performance. An alternative option was investigated that employed a structural thermal break at the thermal barrier of the envelope. Many manufactures have thermal breaks for structural steel connections, however because structure typically needs to be fire rated, careful review of the thermal break options is needed as only a few met both the thermal and fire rating requirements for the connection. The thermal break can be coated in cementitious fire-proofing spray, along with the rest of the steel, and placed in-line with the insulation to reduce the heat flow around the surrounding assembly by 36%.

As-Built Condition, R-16.62

Thermally Improved Condition, R-22.66 (+36%)

Similarly to the embed beams, a floor slab will cause a significant thermal bridge since the renovation of existing buildings typically add insulation from the interior that spans from the top of the slab to the bottom of slab. The slabs decrease the R-value of the assemblies by approximately 45%. There may be little that can be done for existing slabs being insulated from the interior, however when a new slab is being added to an existing structure there is an opportunity to thermally improve the performance. By holding the edge of the new slab approximately 5” apart from the back of the existing wall, a gap is created that can be infilled with compressed mineral wool in an insulating fire-stop detail. The bolts connecting the slab to the exterior wall are still thermal breaks, but this strategy did improve the R-value 31%.

As-Built Condition, R-16.14

Thermally Improved Condition, R-21.10 (+30.7%): New slab installed 5” from back of existing wall with compressed mineral wool insulation infill at gap.

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
Thermal Bridging Research: Roof Penetrations
Thermal Bridging Research: Mechanical Louvers
Thermal Bridging Research: Seismic and Movement Joints