Thermal Bridging and the Energy Code
Thermal bridging, the path heat takes through a metal anchor or a concrete slab edge past the insulation around it, sits inside Michael Stern’s field of tall building work because every facade attachment on a tower doubles as a bridge, and the energy code that governs those attachments has been tightening for half a century. The story runs in dates: an oil shock that put the sealed glass tower on trial, a physics that never changed, a Chicago tower that became a textbook case, a city code that kept absorbing stricter national standards, and a carbon law that now fines the buildings that lose the argument.
1973: the glass box on trial
In December 1973 The New York Times reported that the sealed glass box, by then as much a symbol of Manhattan as the Empire State Building, had lost its shine in the energy crisis, with architects calling it an endangered species.1 The paper’s list of faults reads like a modern envelope audit: a glass wall with poor insulating value, double glazing rarely used to hold down the initial cost, mirror glass that cut cooling loads but did nothing for heating, and towers sited without regard to the sun; one engineer told the paper he did not know of a single building in New York with double glazed windows outside a few electrically heated ones.1 The World Trade Center’s aluminum facade drew particular criticism as a high energy metal wrapped around a building whose windows could not open.1
The physics that never changed
Heat moves from warm to cold and takes the easiest route. Passive House School’s primer on thermal bridges defines a bridge as any place in the envelope where heat flows faster than through its surroundings, usually where a conductive material crosses the insulation, and gives the conductivities that make the point: aluminum at about 160 watts per meter kelvin, steel at 50, stainless steel at 17, reinforced concrete at 2.3 and wood at 0.13, against insulation near 0.04.7 The primer sorts bridges into structural, geometric and material types, measures them with a psi value for the extra heat flow along a junction, and shows that two millimeter steel studs can lift a wall’s overall heat loss by 178 percent while timber studs raise it by a third.7
How fast heat moves through what a wall is made of
W/(mK)
Where the bridges are
Armatherm’s definition, written by a maker of structural thermal breaks, lists the usual suspects: concrete slabs and steel beams that run past the insulation, metal window frames, parapets, brick shelf angles, balconies and the brackets, screws and fasteners that hold a facade on.8 It cites research putting heat loss as much as 60 percent higher in a well insulated structure with no thermal breaks than in one with them, and notes the quieter consequence: cold interior surfaces where moisture condenses and mold follows.8 Passive House School puts a number on the worst case, a conventional balcony slab with a psi value near 0.74 watts per meter kelvin, roughly 74 times what a bridge free detail would allow, and about 0.08 when a structural thermal break is inserted.7
2012: an 82 story heat exchanger
In 2012 the building scientist Joseph Lstiburek used a celebrated Chicago apartment building as his cautionary tale. His Building Science Corporation insight called the tower, with its undulating uninsulated concrete balconies, an 82 story heat exchanger, and argued that it could have been built with the same appearance using off the shelf parts: a true R 5 curtain wall between thermally broken cantilevered slabs, triple glazed units and manufactured thermal breaks whose reinforcing bars are stainless rather than carbon steel.3 He reserved his sharpest point for relieving angles, which run around a brick building at every floor and bridge more insulation than any balcony, and proposed hanging them on standoffs every second or third floor so the insulation runs behind them; windows, he added, should be pushed outboard so the thermal break in the frame lines up with the continuous insulation in the wall.3 The remedy in every case was the same: ask the structural engineer.3
Five fixes with parts off the shelf
- Break the balconyCast a manufactured thermal break into each cantilevered slab, with stainless rather than carbon steel bars crossing the foam.
- Stand off the angleHang brick relieving angles on standoffs every second or third floor so continuous insulation runs behind them.
- Push the window outSet the window so the thermal break in its frame lines up with the insulation layer of the wall.
- Upgrade the glassUse triple glazed, gas filled curtain wall units that have been available for years.
- Ask the engineerTreat relieving angles, window attachment and balcony breaks as structural details to be designed, not assumed.
2016: the city code and the national standard
New York’s code absorbed the stricter national standards as they arrived. NY Engineers’ guide to the 2016 energy code describes a city Energy Conservation Code required to be more stringent than the state’s, one that places the city in Climate Zone 4A, and routes commercial compliance through either ASHRAE 90.1 of 2013 or a prescriptive path whose envelope section covers insulation, solar heat gain, fenestration and air leakage.9 Historic buildings were exempt from the provisions for existing buildings in that edition.9 The national standard was changing beneath it: a Steven Winter Associates presentation for PHIUS records that ASHRAE 90.1 for 2016 added an addendum with methods for accounting for and mitigating thermal bridging, while leaving exterior cladding attachments and mechanical penetrations unaccounted for.4
2017: measuring the attachments
That gap is where the Steven Winter study went. Its engineers modeled how much of the nominal R value of exterior insulation survives each kind of cladding support and reported the share as an effectiveness: on a steel stud backup, aluminum clips kept about 38 percent, galvanized clips 46, stainless clips 63, fiberglass clips 64 and clips with a plastic thermal stop 67, while a continuous galvanized Z girt kept 53 percent and fiberglass or resin girts kept 91 to 96.4 Brick veneer fared better at the ties, 75 to 94 percent, and worse at the shelf angles, where a standard angle kept 58 percent and a standoff angle with insulation behind it 73.4
How much of the insulation survives the attachment
percent of nominal R value kept
2019 to 2024: Local Law 97
The argument then moved from design to operation. Urban Green Council’s summary of Local Law 97 records a law passed in 2019 that began placing carbon caps on most buildings above 25,000 square feet beginning in 2024, covering nearly 50,000 properties, about 60 percent of all floor area citywide and half its building emissions, with limits set for 60 property types that tighten over five compliance periods toward zero in 2050.5 Owners can comply by cutting energy use, electrifying heat and hot water for a credit, buying renewable certificates or offsets, or installing solar and storage, and buildings with many rent regulated units follow a prescriptive checklist instead.5 On recent benchmarking data about nine percent of properties exceed their 2024 cap and about 57 percent exceed the 2030 cap, which makes the envelope a budget line for the next decade, not a nicety of design.5
Local Law 97 from passage to zero
- 2019The City Council passes Local Law 97, setting carbon caps for most buildings over 25,000 square feet.
- 2024The caps take effect; about nine percent of covered properties exceed their limit on recent benchmarking data.
- 2030A stricter cap begins; roughly 57 percent of properties currently emit more than it will allow.
- 2035 to 2049Limits ratchet down across further compliance periods, with an electricity coefficient that falls as the grid cleans up.
- 2050All covered buildings must meet a zero emissions requirement.
2024: the structural engineer’s share
By 2024 the trade press was addressing structural engineers directly. Structure magazine’s article on structural thermal bridging ranks the four common cladding support systems, vertical girts, horizontal girts, mixed girts and intermittent clips, and finds clips best because they penetrate the insulation least, while warning that a low conductivity fiberglass clip may carry less load and so need more penetrations than an aluminum one spaced wider.2 Rails that run through the insulation act as fins drawing heat out of it, so shallow rails and horizontal runs perform better; some codes, such as the Massachusetts stretch code, now derate insulation by 27 to more than 63 percent for typical supports.2 The article puts an offset shelf angle at 59 to 80 percent more efficient than one bolted to the slab, and shows a window carried on intermittent clips over the insulation cutting heat flow at the sill by 95 percent relative to a window set conventionally.2 How those attachments also move with the frame is taken up under joint movement.
2026: the 2025 code arrives
The newest edition closes several of the old doors. NYC Buildings’ energy code page records that enforcement of the 2025 New York City Energy Conservation Code, adopted as Local Law 47 of 2026 and based on the 2024 International Energy Conservation Code and ASHRAE 90.1 of 2022, began on March 30, 2026, with new efficiency credit chapters, expanded requirements for existing buildings, new documentation and testing requirements, and historic buildings no longer automatically exempt.6 Applications filed before that date stay under the 2020 code; a sign application that touches the envelope needs an alteration application that complies.6 The glass itself, where most of the remaining heat crosses, has its own entry on glass performance.
Set against the 1973 complaint about single glazing, the slab edge of a new tower now carries a long list: a thermal break at every balcony and relieving angle, clips sized for load and heat at once, a window frame aligned with the insulation, and a report that proves it before the inspector signs.
