Thermal Bridge Mitigation

Moving Towards High-Efficiency Building Enclosures

 

In response to infra-red imaging surveys conducted by researchers with the Construction Engineering Research Laboratory that found significant thermal bridging losses in a variety of facilities in U.S. Army installations, a Thermal Bridge Mitigation Catalog was developed that portrays detailed drawings of good construction practices in order to achieve highly energy- efficient building enclosure systems.

  

 By Axy Pagán-Vázquez, P.E., PMP, M.SAME, and Justine Yu, LEED Green Associate

 


 

Energy loss attributed to inefficient build­ing enclosure systems has been a major issue in the United States during recent decades.

Key voices including the Department of Energy, the U.S. Green Building Council, and the American Society of Heating, Refrigerating and Air-Conditioning Engineers all acknowledge the importance of achieving better and more energy effi­cient buildings. As a consequence, these organizations have been providing recom­mendations to mitigate building energy losses throughout a facility’s lifespan.

In addition to novel and stringent build­ing energy saving methods, it is crucial to keep out rainwater, prevent excess of moisture transfer, and reduce undesired air infiltration through building envelope walls. This means having a durable build­ing enclosure that will guarantee a desired separation between its exterior and interior environments. However, keeping a consis­tent and durable enclosure and construc­tion is not a trivial task, particularly when a vast list of factors affecting the construction components must be addressed.

 

thermal heat loss

Fig. 1: The thermal bridge problem generally occurs due to construction with a variety of materials, in which different values of thermal conductivity creates a particular thermal path or “bridge” between the exterior and interior sides of the building enclosure. In this case, high temperatures outside create a path to the lower interior temperatures. IMAGES COURTESY ERDC-CERL


 

Dealing with the building enclosure layers as a whole might create a decision-making process in which one factor could be sacrificed at the expense of the other, or might end up in increased construc­tion costs if proper actions are not taken during the construction phase.

Energy, for instance, could be saved by adding insula­tion. But what would happen if insulation is added in an incorrect location, in combina­tion with super air-tight construction and an inefficient mechanical system that would not allow excessive moisture to leave the building?

The building then could develop mold due to condensation issues that in turn will end up in additive cost—first due to the wasted energy; and second because of subsequent repair or restoration of degraded building components.

 

THERMAL BRIDGE PROBLEMS

As a subset of the building envelope issue, it is of concern to emphasize the thermal bridge problem. This, as defined by the International Organization for Standardization, is a phenomenon gener­ally occurring due to construction with a variety of materials, in which different values of thermal conductivity creates a particular thermal path or “bridge” between the exterior and interior sides of the build­ing enclosure.

As shown in Figure 1, these can take place in junction between windows and walls, floors, or where high thermally conductive materials (usually structural) penetrate the building envelope.

It could be assumed (and in past decades has been) that the impact of thermal bridges is insignificant as compared to the rest of a building’s energy draining factors. However, professionals in the Building Sciences field in recent years have found the ther­mal performance of a building enclosure could be downgraded significantly enough through overall degradation of its R-value, or thermal resistance to the point where undesired side effects, such as discomfort, higher energy costs and mold, would need to be seriously taken care of. Even more alarming, studies carried out on buildings exhibiting greater amounts of insulation, but which do not appropriately address thermal bridge losses, have shown to bring more underestimated thermal losses than partially insulated constructions. European assessment studies similarly show that ther­mal bridges could easily bring a greater impact on high performance buildings.

And so, why in the military should we care about these types of bridges and not just the ones civil engineers usually deal with? In short, we must care because the U.S. Army, although moving towards very high energy efficiency standards, unfortu­nately has a large inventory of buildings with serious thermal bridge problems.

thermal bridge example in a structure

Thermal weak spots are visible through infrared scanning of an Army Reserve Center facility. These thermal bridges are prevalent in older facilities as well as new construction and can have a significant impact on the energy efficiency of a building. PHOTO BY JEFFREY “LAKE” LATTIMORE, ERDC-CERL


 

CATALOGUING SOLUTIONS

A few years ago, several researchers from the U.S. Army Engineer Research & Development Center - Construction Engineering Research Laboratory visited several installations to search for ther­mal bridging issues in a variety of facil­ity types. The results were interesting, although not surprising. Using infra-red imaging, it was easy to detect that thermal bridges were everywhere, including in new constructions.

The primary explanation for the deficien­cies is that the thermal bridge and energy problem in the past was not the priority that it is today. Instead, structural integrity and safety were the primary drivers of design and construction plans. In older, very energy inefficient buildings, thermal bridge effects were overshadowed by the poor performance of the building as a whole. In response, researchers worked towards an answer to this thermal bridge mitigation challenge. This resulted in the development of a Thermal Bridge Mitigation Catalog, with supporting illustrated architectural details and construction sequencing steps.

Currently, the catalog is a compilation of 30 different building details—mostly junc­tions of walls, fenestration components and building construction sections intrinsically containing high conductive elements that bypass the thermal barrier in the building’s exterior walls. Each catalog page contains an existing and recommended architectural detail drawing, construction material infor­mation and guidelines to implement the improved section.

Take for example, a thermal bridge problem emerging from a window inserted into a steel stud wall that has interior and exterior insulation. Although the wall may seem well insulated and may also have a thermally broken window, there still can be serious thermal bridge problems just at its connection. For thermal bridge mitigation, a rule of thumb is that the window thermal break should be aligned with the wall system insulation plane. The Thermal Bridge Mitigation Catalog contains many problems and corre­sponding solutions with significant detail.

A second tool for addressing thermal bridge mitigation and building envelope energy savings are illustrated step-by-step architectural construction detail sequenc­ing procedures. These illustrate how to upgrade an existing Army construction detail into a thermal bridge mitigated detail.

Earlier experience had shown that one of the most challenging aspects of dealing with thermal bridges was keeping a continuous thermal plane while maintaining continu­ous moisture and air barrier layers. Every small detail such as taping the air barrier to the window support angle brackets or using a plastic window sill flashing if it would be penetrating entirely through the building envelope, can impact the building’s overall energy efficiency and durable construction.

 

ACHIEVING ENERGY EFFICIENCY

The building envelope integrity plays an important role in achieving energy efficient constructions. For today’s highly energy efficient buildings, which maxi­mize the performance of their mechanical and electrical systems, the overall energy performance will be jeopardized if localized heat flow paths, or thermal bridges in the envelope, are not adequately addressed.

Having identified their impact and consistency in a variety of buildings within Army installations, it was necessary, in the context of energy savings, to devise effective means for prevention and remediation of thermal bridging effects.

The resulting mechanism, the Thermal Bridge Mitigation Catalog along with illus­trated sequencing, will serve to support initial and future efforts to create awareness of the problem and viable solutions.

 


 

Axy Pagán-Vázquez, P.E., PMP, M.SAME, is Mechanical Engineer, and Justine Yu, LEED Green Associate, is Research Architect, U.S. Army Engineer Research & Development Center – Construction Engineering Research Laboratory. They can be reached at 217-373-3451, or This email address is being protected from spambots. You need JavaScript enabled to view it." target="_blank">This email address is being protected from spambots. You need JavaScript enabled to view it.; and 217-373-4526, or This email address is being protected from spambots. You need JavaScript enabled to view it." target="_blank">This email address is being protected from spambots. You need JavaScript enabled to view it., respectively.