Buildings are insulated to keep heat in, but even a small break in the insulation can reduce performance by almost 20% as heat rushes through the gap. The New York State energy code requires that insulation be “continuous,” but historically, code interpretation has allowed some metal and concrete heat-leaking components to penetrate the wall. Now things are changing. The current interpretation by the NY State Department of State, now enforced by the NYC Department of Buildings, means that for most buildings exposed slab edges and continuous shelf angles are no longer considered compliant with prescriptive code insulation requirements. That opens the possibility to prevent that 20% energy loss through walls.
Among the many tables in ASHRAE 90.1 2010 is one that provides minimum R-values for walls, dependent on framing material and building type (Table 502.2(1)). That table includes a note that in most circumstances there must be “continuous insulation.”
In the past, designers of high-rise masonry buildings have been able to utilize exposed slab edges and continuous shelf angles that interrupt the insulation, as shown in these wall sections:
|Exposed Slab Edge||Metal Shelf Angle|
(The metal shelf angle provides support for the brick façade, similarly to the exposed slab, but is less apparent from the outside.) While the exposed slab edge may look like a much more significant thermal break, the high thermal conductivity of steel1 ensures that metal shelf angles have about the same poor thermal performance as the exposed slab edge.
How could these examples possibly represent continuous insulation? They don’t. And the Department of State has confirmed that.
Instead, the code requires wall sections such as this:
|Metal Brackets||Bracket Detail2|
In this example the insulation is truly continuous; the slab is fully insulated and the shelf angle is cantilevered away from the slab by brackets and does not break the continuity of the insulation. Note brackets are one example approach that’s acceptable; there may well be others that achieve the same result.
How much of a difference will this make to energy efficiency?
A lot: modeling we commissioned predicts 18% improvement in the thermal performance (R-value) of non-window area. Here’s the difference visually showing heat flow3:
Note how similar the heat flux through the shelf angle is to a completely exposed slab:
This is an important clarification to code interpretation, which will require substantial changes in how some walls are designed. As lawyers reading important cases have known for years, sometimes all the excitement can be found in footnotes!
1 Steel has conductivity of approximately 29 Btu/hr-ft2-oF, while concrete has conductivity of approximately 1.1 Btu/hr-ft2-oF. The thin shelf angle, when continuous, causes a heat loss similar to that of the exposed slab edge.
2 By attaching the cantilever brackets to the slab and resting the shelf angle in the brackets, the shelf angle itself has no contact with the slab, reducing thermal bridging dramatically and leaving space for continuous insulation behind the shelf angle. Although the brackets penetrate the insulation at discrete locations, the insulation is continuous because one can move from any starting location within the insulation to any other location without leaving the insulation. This is not the case if the shelf angle is fastened directly to the slab edge.
3 Violet corresponds to heat flow of 5 Btu/h-ft2, green to 20 Btu/h-ft2, and red 30 Btu/h-ft2. This analysis assumed 4-foot spacing for the cantilevered brackets, which is within the range identified by manufacturers of these devices. Narrower separation between the brackets would increase the U-factor.