Thermal Bridging – Exterior Wall Assemblies
The Canadian construction industry is undergoing significant changes that will improve the overall energy performance of buildings. By 2030, the National Energy Code for Buildings (NECB) will aim to achieve net zero energy consumption for all new buildings in Canada. This means that all new buildings will have to generate at least as much energy as they consume every year.
The use of continuous insulation in buildings with a focus on limiting thermal bridges will then be critical to achieve this goal. No connection must exist between indoor and outdoor to prevent thermal leakage by conduction through materials. Realistically, it is impossible to design and build an exterior wall assembly without thermal bridging materials of any kind; some of them are essential, such as studs and fasteners for insulation panels and exterior cladding. The designer’s mission is to reduce the impact of these materials by limiting their quantity in the assemblies while ensuring structural integrity. The energy used to heat or cool a building should not be lost due to poor thermal insulation strategies, significant air leaks, or thermal bridges.
This article should help you better understand thermal bridges and how to reduce their impact on a wall assembly. First, here are a few basic concepts:
What Are Thermal Bridges?
Exterior wall assemblies are composed of various materials (such as insulation); some help improve thermal performance. Other structural components (like girts, studs, and blocking) have the opposite effect. Any material that extends through an assembly separating the inside from the outside of a building can create a thermal bridge, even more so if the material is a good heat conductor. Metals are among the best heat conductors; for example, steel is about 400 times more conductive than wood.
What Is the Impact of Thermal Bridges on an Assembly?
Not only do thermal bridges cause the loss of thermal performance of the assembly, but they can also increase the risk for condensation within the assembly. In winter, if these materials are exposed to the cold on the exterior of the assembly, they will also be cold on the inside. The moisture contained in the indoor air can then condense on cold surfaces.
How to Reduce the Impacts of Thermal Bridging:
By Using Exterior Continuous Insulation
Building codes require us to consider the performance of assemblies and buildings in general. There are many requirements regarding the thermal performance of various parts of buildings. One of the most recent Energy Code requirements is about the use of continuous insulation. This concept is very simple: we want to limit, if not eliminate, thermal bridges. By using an insulation material continuously (e.g., often on the exterior of a wall assembly), thermal bridges are broken. The goal is to make sure that there is no direct connection between the inside and the outside through heat-conducting materials, thus avoiding thermal leaks.
Here is an example of the effect of continuous insulation in a wall assembly:
By Using an Efficient Cladding Attachment System
In order to understand the impact of different types of attachment systems, Morrison Hershfield performed thermal modelling for SOPREMA. Part of the results demonstrated the impacts of using Z-girt, rail and clip, or brick-tie attachment systems.
Here is a summary of that study:
In Assembly 1, the continuity is interrupted every 16 inches by a piece of metal (the vertical Z-girts). This assembly used a nominal exterior insulation of R-25 and obtained an effective R-value (Reff) of only 11.9. Assembly 1 is therefore inefficient. In fact, the thermal bridges created by the Z-girts are so important that even if we added exterior insulation up to R-40, it would not be enough to meet the Code’s requirement.
If we take a look at Assembly 2, only by changing the attachment system with rails and clips instead of Z-girts, retention of the nominal exterior insulation of R-25 was 75% instead of 47% for Assembly 1. This is even more true for Assembly 3, which retained 84% of the nominal exterior insulation of R-25 using brick ties.
For more information, please see the SOPREMA Build Better ebook on the thermal performance of exterior wall assemblies
By Using Efficient Materials for the Cladding Attachment System
There are different attachment systems (Z-girts, rails and clips, or brick ties), and there are also different materials that can be used to produce those attachment systems. Some materials are more conductive than others. When choosing an attachment system, it is important to consider the material being used, and not only the type of attachment system itself. For example, a stainless-steel clip is more efficient than an aluminum clip, even if they both are clips (see the charts below).
To know more about the materials, please consult this technical bulletin published by RDH (No. 011, Cladding Attachment Solutions for Exterior-Insulated Commercial Walls).
Thermal Comparison of Systems: a Summary
To summarize the thermal performance of the various cladding support strategies presented, the range of thermal effectiveness of the exterior insulation is shown below. These percentages can be multiplied by the R-value of the exterior insulation and added to the back-up wall R-value to determine an approximate overall effective R-value for the wall assembly.