Adding insulation to building walls and roofs is a simple and straightforward energy saving strategy. By resisting the flow of heat through the building envelope, insulation keeps buildings comfortable while reducing the energy required for heating and cooling. Unfortunately, many wall assemblies, and details at, for example, corners, parapets, and windows, are designed in such a way that large amounts of heat can bypass the insulation by flowing through structural components and framing elements, leading to decreased comfort and increased energy consumption. Quantifying the impact of these thermal bridging details can be difficult, requiring precise three dimensional calculations and measurements of heat flow rates through assemblies under carefully calibrated conditions. Fortunately, a great deal of this work has already been done. In August of 2014, Morrison Hershfield released the results of a detailed and comprehensive study of thermal bridging in building envelopes. The study and associated resources can be downloaded from the BC Hydro website. The study builds on previous work carried out for ASHRAE which introduced the methodology used and catalogued the thermal performance of 40 wall assemblies and details.
It is astonishing to see that thermal bridging can cut the insulating value of a wall assembly by 30-70%. In a poorly detailed assembly, an R-20 wall can easily fall to R-6. The first part of the report considers what the authors call the “clear wall”: the basic wall assembly in the absence of details such as corners, balconies, windows, etc. One of the most common thermal bridging elements in modern construction is the steel framing used to support the exterior wall. In addition, insulation within or on the inside of the wall is often bypassed by concrete floor slabs and interior walls (such as concrete shear walls). In order to circumvent this problem, the use of exterior insulation is often promoted. Exterior insulation lies on the outermost surface of the wall and provides an excellent thermal break by sitting outside of many wall details. Exterior insulation needs to be protected with a cladding system and there are many different ways of supporting this cladding. One of the more common approaches is to use vertical Z-girts to attach the exterior cladding to the steel studs. Unfortunately, these Z-girts are usually made of steel, or aluminum, both of which are highly conductive materials. With vertical Z-girts, a wall with R-20 exterior insulation has an effective performance of R-11. In this case thermal bridging doubles the rate of heat loss through the wall. The study considers a number of alternatives to this approach. Horizontal Z-girts minimize the contact area between the Z-girts and the steel studs in the wall assembly, reducing the effect of thermal bridging and improving the wall performance from R-11 to R-13.2. Using crossed Z-girts (horizontal in one layer and vertical in the other) brings the performance up to R-15.4. There are also many thermally broken clip systems which can eliminate thermal bridging altogether and bring the performance back up to R-20. A number of examples, including thermally broken metal and fibreglass clip systems, are considered in the study.
Another commonly used element in a masonry wall with a brick veneer is the shelf angle bracket. A shelf angle bracket is a metal bracket which extends from a concrete floor slab through an exterior insulation layer to support the brick veneer. Shelf angle brackets are used to support brick walls, because brick walls are unable to support their own weight for more than a few storeys at a time. Unfortunately, by providing a highly conductive thermal bypass between the exterior wall and the interior floor slab, shelf angles also transmit a lot of heat. In an exterior insulated wall assembly with shelf angle brackets and brick ties supporting brick veneer and exterior insulation of R-20, the effective thermal resistance of the wall is only R-10.9. With R-25 insulation, the resistance increases by only 0.7 to R-11.6. Without addressing the shelf angle, adding insulation to this wall assembly is a game of diminishing returns. In going from R-5 to R-25 insulation, the performance of the wall increases by only R-2.9. On the other hand, using a spaced shelf angle bracket with R-10 insulation provides an effective R-value of R-11.1, which is better than the R-20 wall with a standard bracket.
Both of these “clear wall” examples illustrate that addressing thermal bridging elements can have a much larger impact than simply increasing the insulation thickness. This is a clear lesson of this study which we will expand on in a future post.