Thermal Bridging: what is it?
Simply described, it’s heat moving through high conducting materials. Like a metal spoon, in a hot pot of liquid where heat transfers from the pot/liquid to the spoon, heat transfers through many different materials.
What causes thermal bridging in structures?
Framing members of wood, steel or masonry (each have heat conducting characteristics) that are not “disconnected” from the interior to the exterior of a structure form a thermal bridge. When there is something that blocks the bridge from inside and outside of a building, this is referred to as a “thermal break,” where the bridge for heat to travel is truly broken.
There is no such thing as cold, only the absence of heat. Heat always travels to where it can equalize. In structures, inside heat moves through framing members from the inside to the outside in winter and then reverses and moves from outside a structure to inside in the summer….relentlessly trying to equalize. The term for this heat movement in structures, again is called Thermal Bridging. Heat also conducts through windows and doors, but for our discussion we’ll be focusing on framing members.
Building Code efforts to reduce thermal bridging
Thermal Bridging is a well known phenomenon by the Building Science and Building Code communities. In recent years new building code requirements have been adopted to reduce Thermal Bridging (heat transfer/escaping) through framing members, that result in high energy costs and discomfort for occupants. The 2018 International Energy Conservation Code (IECC), for both Commercial and Residential structures each have a Chapter 4 where provisions are stated to properly insulate foundations, floor slabs, walls and roofs. In these Chapters there are Tables giving minimum R-values and U-Factors for the various Climate Regions throughout the US. The R-value Method Tables prescriptively call out for minimum R-values, achieved by the means of cavity insulation and an overlay of Continuous Insulation (Ci).
The role of continuous insulation (Ci) in reducing thermal bridging
By definition and application, Ci overlays the outside of the framing materials, thereby creating an insulated thermal break (disconnection) between the inside and outside of the structure. The requirement for adding Ci is very effective for saving energy loss and increasing the interior comfort of the structure.
Although applying Ci is very beneficial to occupants of structures, many states, counties, and cities have not adopted this code requirement due to the negative push back from commercial/residential builders and building contractors. What is the basis of the objection from these builder groups?….Ci assemblies create installation complexities and additional labor cost, higher material cost and a higher final building cost to the consumer.
Challenges of continuous insulation (Ci) for Contractors
In a number of ways Building Contractor’s concerns about meeting IECC requirements for Ci are warranted. The installation of Ci does present a challenge to the contractor community….who installs it? The framer…the siding contractor? Once this is decided, who installs it, how does the Ci attach to the exterior sheathing? How is the Weather Resistant Barrier (WRB) integrated into the assembly? How does it terminate? And at the termination points – windows, doors, mechanical penetration – how is it water drainage managed? Further, in many regions of the county it is required that Rainscreen Drainage Planes are necessary.…how are they accomplished when Ci application is required by the building code?
Framing solutions to combat thermal bridging and that satisfy IECC Ci requirements
All this said, there is a widely available commercial and residential framing system that offers an eloquent and simple solution that meets the necessary insulation requirements of the 2018 IECC – they are: Structural Insulated Panels (SIPS). As discussed earlier, the 2018 IECC code allows for the calculation by the U-Factor Method to provide for proper insulation performance of walls and roofs. SIPs are a structural component of OSB inner and outer skins, adhesively laminated to a strong rigid foam core, that once installed to create insulated walls and roofs, need no further insulation overlays to meet IECC code requirements. The U-Factor Method considers all of the materials of the SIP component assembly and their collective R-values. SIPs and the simplicity of their composite make-up is all that is needed to meet IECC insulation requirements. Additionally, SIPs are easily detailed to accomplish Rain Screen water management systems where climate zones dictate.
Common framing methods – wood, steel, masonry – plus the addition of Continuous Insulation (Ci) result in application complexity and final higher building costs. SIPs, with their simplicity of composite make-up and ease of installation, needing no additional Ci appliqué, perform better at lower final building cost.
Explore more about the two key options for wood framing to comply with IECC Ci requirements here.
For more information about the benefits of the SIP building system, visit this page.