Envelop the Structural Steel!
8 March 2009
We all know steel is a good conductor of heat. And most of us know that steel that extends across the building envelope insulation causes thermal transfer, or bridging, between the interior and exterior spaces and reduces a building’s energy efficiency. But just how much energy loss does this represent? Unfortunately, more than most people think.
Infrared thermal imaging cameras can be used to illustrate and measure differences in surface temperatures across the exterior of a building. Images taken on a cold winter day will show the areas of greatest heat loss through the building envelope as warm areas. This image shows the warm horizontal lines across the brick facade of an insulated masonry cavity at the locations of the steel relieving angles.
In the United States, a material’s resistance to heat flow is defined as its R-value, and is measured in degrees Fahrenheit, square feet hours per Btu, (̊Fft²h/Btu). The R-value of carbon steel is approximately 0.0031 per inch of thickness, which is quite small compared to other construction materials such as wood (average R-value of approximately 2.5 per inch) or normal weight concrete (average R-value of approximately 0.1 per inch). Expanded polystyrene (EPS) rigid insulation has a nominal R-value of about 3.7 per inch. This makes EPS about 1200 times more resistant to heat transfer than carbon steel.
The inverse of R-value is the U-value, which is a measure of a material’s thermal conductivity, with units in Btu per degrees Fahrenheit, square feet hours (Btu/̊Fft²h). So, the U-value of one inch of EPS is 1/3.7 = 0.27, and the U-value of one inch of carbon steel is 1/0.0031 = 320. It may be obvious, but it’s worth being clear: Carbon steel conducts heat about 1200 times better than EPS.
In a steady-state condition, the total heat transfer across material with surfaces of different temperatures can be calculated as the algebraic sum of the U-value times the area of each material. So, in a wall where the insulation consists of a continuous layer of EPS that is bridged by carbon steel with a cross-section of one-tenth of one percent of the wall area, more heat is transferred through this small percentage of steel in the wall than across the entire plane of EPS insulation. This means that the effective R-value of the entire wall assembly is less than half of the R-value of the insulation alone, due to the presence of a small amount of steel bridging across the insulation.
Let’s consider a continuous steel relieving angle supporting a brick facade, consisting of a 1/4-inch- thick steel angle with a horizontal leg that bridges across a layer of two-inch EPS at each floor level, 12 feet on center vertically. The steel angle leg comprises (0.25) / (12 x 12) = 0.17 percent of the wall area. Using the above values, the U-value of two inches of EPS is 1 / (3.7 x 2) = 0.135, while the U-value of two inches of steel is 1 / (.0031 x 2) = 160. The steel transfers about (0.0017 x 160) / [(0.0017 x 160) + (.9983 x 0.135)] = 67% of the total amount of heat that moves through this section of wall. This makes the effective R-value of this wall Reff = (.67) (.0062) + (1 – 0.67) x (7.4) = 2.4, which is a 67% reduction from the 7.4 R-value of the EPS alone.
This is the actual detail of the relieving angle shown in the above infrared:
Certainly, this is an oversimplified analysis. The actual heat flow through a wall assembly is affected by other elements in the system, and the thermal mass of the masonry plays a significant role, as well. And of course a leaking air barrier can cause heat loss that renders R-value calculations meaningless. But the main point should not be dismissed: The potential heat loss caused by steel thermal bridging can be quite significant, and we should take this into account if we are serious about improving our buildings’ energy performance.
The above example shows that steel elements that pass through the insulating layer of a conditioned building can have a significant effect on the effective insulation value of the building envelope. What happens if we increase the amount of steel that bridges across the insulation?
• If the angle or plate is 3/8″ thick, Reff = 1.8
• If the angle or plate is 1/2″ thick, Reff = 1.4
The reduction in Reff caused by steel thermal bridging follows an asymptotic curve based on percentage of steel. See the chart below. Despite large amounts of insulation in these walls, walls with steel extending across the wall insulation frequently have less insulating value than window walls, due to the steel thermal bridging effect.
What to Do?
1. Detail and design any continuous steel plates or angles that must pass through the insulation layer to be as thin as possible, while maintaining structural adequacy.
2. Consider using stainless steel for all steel extending across the thermal insulation layer. Since stainless steel has a U-factor of about 110 per inch – about a third of carbon steel – the heat loss per square inch of steel bridging across the insulation is significantly lower. This may be an expensive solution, however.
3. Design discrete, noncontinuous steel support elements that extend across the thermal insulation layer. For example, if a relieving angle is supported by a 1/4″ thick discontinuous steel clip, 6″ long, 48″ on center, the reduction in insulation effectiveness is 21% – an effective R-value of 5.8. However, these clip assemblies can create a challenging condition to maintain continuity of the air barrier, which is essential for proper performance of a building envelope.
4. Develop details that incorporate insulating materials, such as structural fiberglass, across the thermal insulation layer, thereby cutting off the thermal bridge. The sketch below illustrates a conceptual detail that effectively uses a 1/2-inch thickness of fiberglass as a shim plate. Using this scheme in the above example, Reff = 7.3 – a reduction in insulation effectiveness of less than 2% from the R-7.4 continuous insulation.
For roof edge angles, the same principle applies: Minimize or eliminate steel passing completely across the insulation, while maintaining adequate structural performance. It is also important to facilitate the continuity of the air barrier as it transitions between the wall and roof planes. The sketch below shows a schematic detail that accomplishes this, while maintaining the steel continuity required of the edge of the roof diaphragm.
Steel lintels over openings in masonry walls that incorporate steel plate or angle continuity is another area where thermal bridging can occur. In fact, the amount of heat lost through such steel bridging can easily exceed or surpass the heat lost through the entire window assembly. When combined with the potential for discontinuity of the air barrier when the lintel is not coordinated with the entire wall system, the energy lost at the window head can be enormous. Below is a conceptual detail that incorporates a continuous fiberglass angle, where the vertical leg acts as a structural shim and the horizontal leg serves as a shelf for the fiberglass, a closure for the wall cavity, and a clean transition of the air barrier between the exterior sheathing or masonry surface and the window frame.
Other typical building conditions where care is needed to minimize the loss of effectiveness of the building insulation or air barrier include:
• Perimeter Columns and Spandrel Beams – Their sizes and positions must be coordinated so as to allow the building envelope insulation to pass by continuously. Also, if the steel elements are adjacent to the building envelope’s air barrier plane, a slight offset may produce corner details that complicate the proper execution of the continuous air barrier.
• Exposed Steel Columns or Posts – Being thermally connected to the structural steel frame inside the insulated building envelope, any steel support element that passes through unconditioned space should be completely insulated, including, when possible, around the pier foundation support.
• Balconies and Canopies – The amount of steel elements that structurally connect across the insulated building envelope and air barrier should be minimized. Discrete point connections, with minimal thermal insulation interruption and carefully detailed air barrier continuity, are much more efficient than continuous connections.
Much of any building’s environmental impact comes from the amount of energy that will be needed to heat or cool its interior spaces over the building’s service life. In many cases, this impact can be more significant than the entire environmental impact of the construction of the building. Structural engineers, therefore, should coordinate their work with the rest of the design team so that the exterior building envelope’s thermal insulation and air barrier systems perform well. As a building’s energy performance becomes more critical, details that allow heat energy to pass through the building envelope become more and more important to control.