The coating that does most of the thermal work — and how it meets NYC energy requirements
Windows have always been the thermal weak point of a building envelope — the place where heat escapes in winter and pours in during summer. Low-emissivity coating technology is what changed that, turning glass from a transparent hole in the wall into an engineered thermal component.
A Low-E coating is a microscopically thin metallic layer, invisible to the eye, that reflects long-wave infrared radiation while letting visible light straight through. It is the single most consequential decision in a glazing specification, and it is also the one most often reduced to two words on a quotation: "Low-E included".
This page covers how the coatings work, the difference between the two manufacturing processes, why the surface it sits on matters enormously, and what New York's energy code and Local Law 33 grading actually require of a building's glazing.
Every object emits energy as long-wave infrared radiation according to its temperature. Emissivity measures how readily a surface does this, on a scale from 0 to 1. A perfect emitter rates 1.0; a perfect reflector emits nothing and rates 0.0.
Uncoated float glass has an emissivity of roughly 0.84. When a room warms it in winter, it radiates about 84% of that heat energy away — outward, toward the cold.
A Low-E coating drops that figure dramatically, in the best cases to around 0.02. The glass stops being an efficient emitter and becomes an efficient reflector of infrared. In winter it bounces interior heat back into the room. In summer it reflects long-wave heat radiating off sun-baked asphalt and neighbouring buildings.
The engineering achievement is that these coatings are effectively transparent to visible light. You get the thermal behaviour of a reflective surface without losing the daylight.
Passive Low-E, or hard coat, is applied pyrolytically — sprayed onto the glass ribbon while it is still hot during float manufacture, so the metallic oxides fuse to the surface at a molecular level.
The result is exceptionally durable. It can be cut, tempered, laminated and handled much like uncoated glass, and it can sit exposed to the atmosphere without degrading. The trade-offs are performance, with emissivity typically around 0.10–0.15, and appearance — hard coats can show a faint bluish-grey cast and a slight haze in strong raking sunlight.
Solar control Low-E, or soft coat, uses magnetron sputter vacuum deposition. Pre-cut glass goes into a vacuum chamber at room temperature, and layers of silver and metal oxides just nanometres thick are sputtered onto the surface. It is a far more precise process.
Soft coats reach the lowest emissivity available, often 0.02–0.05. But the coating is genuinely fragile: it scratches easily, and exposed to air the silver oxidises and tarnishes. It requires careful handling, delicate washing, and edge deletion — the coating is physically removed from the perimeter so the sealant bonds to bare glass rather than to a fragile layer.
Nearly all high-performance glazing uses soft coat, including the units inside Pella, Marvin and Andersen windows.
Soft coat performance is largely determined by how many silver layers sit in the coating stack.
Single silver was the original, giving good thermal performance with moderate solar control. Double silver is the current mainstream standard, balancing high visible transmittance against aggressive solar rejection. Triple and now quad silver coatings push solar control further while keeping the glass looking clear.
That last point is the significance of the whole progression. Before multi-silver coatings, the only way to control solar gain was tinting or reflective glass — a dark or mirrored building. These coatings decoupled appearance from performance, which is why modern glass towers can be both clear and efficient.
See Vitro, Guardian and AGC for the specific coating families we supply.
In a double-glazed unit the surfaces number from outside in: #1 faces the weather, #2 is the inner face of the outer pane, #3 is the outer face of the inner pane, #4 faces the room. Triple glazing adds #5 and #6.
Surface #2 — cooling priority. A solar control coating here intercepts solar energy at the outer pane and reflects it back out before it enters the cavity. The outer pane never absorbs and re-radiates that energy inward. This is the standard position where blocking summer heat is the goal.
Surface #3 — heating priority. A coating here reflects interior heat back into the room while still letting short-wave solar radiation through the cavity to help warm the space in winter. This is the northern-climate strategy.
Surface #4 — the room-side addition. A durable hard coat on the room-side face reflects interior heat back before it even warms the inner pane. Combined with a soft coat on #2, this creates a double barrier that can bring a double-glazed unit close to triple-glazing U-factors without the third pane's weight and cost. The caveat is that hard coats on #4 can introduce the faint haze mentioned earlier, and the surface gets cleaned, so durability matters.
New York sits between the heating and cooling extremes, and the honest answer often varies by elevation within one building — solar control on the west and south faces that overheat, a heating-optimised strategy on north-facing rooms. Specifying one product for a whole building is simpler to quote and frequently not the best result.
Heat crosses a sealed unit three ways, and Low-E only addresses one of them.
The coating handles radiation, reflecting the large majority of radiant heat back to its source. That leaves conduction and convection across the cavity as the dominant remaining paths — and that is what argon fill addresses, being both a poorer conductor than air and dense enough to suppress the convective loop.
Because each measure attacks what the other leaves behind, the combination gains more than either alone would suggest.
| Configuration | Centre-of-Glass U-Factor |
|---|---|
| Single pane, clear | ~1.04 |
| Double unit, clear, air fill | ~0.48 |
| Double unit, soft-coat Low-E, air fill | ~0.29 |
| Double unit, soft-coat Low-E, argon fill | ~0.24 |
Illustrative centre-of-glass figures. Whole-window values differ — frame and edge effects are included in the rated U-factor.
Adding argon to a Low-E unit typically improves performance a further 15–20%, and it costs very little. It remains the best performance-per-dollar upgrade available in a sealed unit.
Because the coating is invisible, getting it on the wrong surface is one of the more consequential errors in fabrication — and it is not always caught before installation.
Soft coat exposed to air. If a fabricator flips the glass and the soft coat ends up on surface #1 or #4, the silver oxidises. Within weeks or months the unit develops visible, irreversible tarnishing that looks like severe clouding or staining. The coating is destroyed, the performance is gone, and the unit has to be replaced. Soft coats belong on #2 or #3, sealed inside the cavity, without exception.
Right surface, wrong strategy. Less dramatic and easier to miss. A solar control coating placed on #3 rather than #2 lets the outer pane absorb solar heat and re-radiate it across the cavity inward, increasing cooling load. Conversely a coating on #2 where a heating strategy was wanted blocks useful winter solar gain. The window looks perfect and quietly underperforms for its whole life.
How it gets verified. Fabricators and installers use handheld Low-E detectors, which identify coated surfaces electronically. There is also a field check: hold a small flame near the glass and look at the reflections. A double-glazed unit shows four reflections, one per surface. On uncoated glass all four look the same yellowish-orange; where a Low-E coating is present, one reflection reads a different colour — often pinkish or bluish. Counting which reflection differs tells you which surface carries the coating.
It is a crude test, but it takes ten seconds and it has caught real mistakes before glass went into a building.
New construction and substantial alteration in the five boroughs falls under the New York City Energy Conservation Code (NYCECC), which is the state code with city-specific amendments and is updated periodically. Confirm which edition governs your project, because the required values tighten with each revision.
For glazing, compliance turns on three things.
U-factor and SHGC. The prescriptive path sets maximum values for both, and they must be documented in the window schedule on the drawings along with the manufacturer's data for the assembly. Note that these are whole-assembly figures, not centre-of-glass — the frame and edge effects are included, which is why a good IGU in a poor frame can still fail.
Window-to-wall ratio. There are limits on how much of the facade may be glazed, with higher ratios permitted where daylighting controls are provided or where the ASHRAE compliance path is used. A design with extensive glazing needs this checked early, because it can force a change in compliance path or in the glazing specification.
Compliance path. Prescriptive is the simplest. ComCheck or full energy modelling allow trade-offs — better glazing offsetting something else, or vice versa — which is often how heavily glazed designs comply.
We work from the architect's energy compliance documentation and supply glazing that meets the scheduled values. If the specified assembly will not achieve the listed figures, that is a submittal-stage conversation, not something to discover at inspection.
This is where most of our work sits, and the code treats it differently from new construction in ways that are genuinely useful to know.
Full window replacement. Where an existing window unit is replaced with a new product including sash and glazing, the replacement must meet the code's U-factor and SHGC requirements. There is no grandfathering for a full unit swap.
Glass-only replacement — the important exemption. Replacing the glazing within an existing sash and frame does not have to meet full new-construction requirements, provided the new U-factor and SHGC are equal to or lower than what was there before.
That exemption is worth understanding properly, because it opens a route that many building owners do not know exists. Replacing failed or single-glazed units with modern insulated units carrying a Low-E coating and argon fill, inside the existing frames, improves the envelope substantially at a fraction of the cost and disruption of full window replacement — and without triggering the full compliance process.
Storm windows and glazing panels. Installing a storm window or glazing panel over existing glazing is similarly exempt where the added panel carries a Low-E coating, and it can be fitted inside or outside. For landmark buildings where the existing windows cannot be altered at all, this is often the only available route to better thermal performance.
Partial curtain wall alterations. One trap worth flagging. If you replace the IGUs and some framing but leave other framing members in place, compliance is judged on the total assembly. The new components may have to compensate thermally for the old mullions that remain, which usually means the manufacturer modelling the assembly rather than quoting a catalogue figure. Plan for that, because it takes time.
Buildings over 25,000 square feet must post an energy efficiency letter grade near each public entrance, under Local Law 33 of 2018 as amended by Local Law 95 of 2019. The grade comes from the building's ENERGY STAR score, which comes from Local Law 84 benchmarking data. Labels are issued by the Department of Buildings on 1 October and must be posted within 30 days.
The scale runs A, B, C, D, F and N — there is no E grade.
| Grade | ENERGY STAR Score | Meaning |
|---|---|---|
| A | 85 or above | Top performance band |
| B | 70 to 84 | Above average |
| C | 55 to 69 | Mid range |
| D | Below 55 | Where a great many older buildings sit |
| F | — | Required benchmarking not submitted |
| N | — | Exempt or no ENERGY STAR benchmark for the type |
An F does not mean terrible performance; it means the required benchmarking was never submitted. A great many older New York buildings sit at D, which reflects a score below 55.
Now the honest part, because this matters more than a sales pitch.
Glazing improvements genuinely help. Windows are typically the weakest element of an older building's envelope, and replacing single glazing or failed sealed units with Low-E argon units measurably reduces heating and cooling load. That flows into consumption, into the benchmarking data, and into the score.
But glazing alone rarely moves a building from D to A. The ENERGY STAR score reflects total building energy use, and in most New York buildings the largest gains come from heating and cooling plant, controls and distribution. The realistic sequence for most D-rated buildings is envelope and controls work together lifting the score into C or B territory, with A generally requiring substantial systems retrofit or electrification.
Anyone promising you a specific letter grade from a window job is overselling. What we can tell you honestly is how much a given glazing upgrade reduces envelope heat loss and gain, and your energy consultant can model what that contributes to the score alongside everything else.
Local Law 97 sets carbon emissions caps on buildings over 25,000 square feet, with penalties for exceedance and limits that tighten over time.
The connection to glazing is direct: envelope heat loss and solar gain drive heating and cooling energy, which drives emissions. Reducing the load on the plant reduces emissions against the cap, and it also reduces the size of any future electrification project — heat pumps sized for a leaky envelope cost more to buy and more to run.
For owners planning capital work, the sequencing argument is worth hearing. Envelope improvements first, then plant sized to the improved load, is usually cheaper overall than replacing plant and then improving the envelope afterwards.
We are glazing contractors rather than energy consultants, and we will not model your emissions for you. What we will do is give you accurate, documented performance figures for any assembly we quote, so whoever is doing that modelling has real numbers to work with.
"Low-E" on its own tells you almost nothing. Four questions make a quotation comparable.
Which coating, and whose? A single-silver hard coat and a triple-silver soft coat are both Low-E and perform very differently.
Which surface is it on? This determines whether the unit is optimised for keeping heat out or keeping it in.
What are the whole-assembly numbers? U-factor, SHGC and visible transmittance for the complete window, not centre-of-glass. Code compliance uses assembly figures.
Is there argon, and what fill rate? Ninety percent or above is the standard threshold.
We supply all four figures on anything we quote. If a competing quotation cannot, you are not being given enough to compare.
Insulated Glass Units Argon Gas Fill Glass Types & Specifications Vitro Glass Guardian Glass Architectural Glass
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