Double IGU - Double Insulated Glass Units

What Is Actually Inside a Double Glazed Unit

A sealed unit is two panes of glass separated by a spacer around the perimeter, with the gap filled with a gas and the whole edge sealed twice over. Four things inside it determine how it performs, and three of them are invisible once it is installed.

Surface Numbering

The surfaces are numbered from outside in: surface 1 is the exterior face, surface 2 the inside of the outer lite, surface 3 the outside of the inner lite, surface 4 the interior face. Every coating decision is a surface decision, and a specification that does not name a surface is incomplete.

In a heating-dominated climate like ours, a low-e coating on surface 2 rejects solar heat before it enters the cavity — the usual choice where cooling load matters. A coating on surface 3 keeps more solar gain and reflects interior heat back inward. Most NYC work uses surface 2 because summer cooling drives the energy calculation on any building with real glazed area.

The Gas Fill

Argon is denser and less conductive than air, and it is the standard fill on any low-e unit. It typically improves the U-value by a useful margin at very little cost — it is close to free performance and should be specified as a matter of course.

Krypton performs better still and costs considerably more. Its real advantage is in narrow cavities: argon works best at around a half-inch gap, and if the frame depth forces a narrower cavity than that, argon loses efficiency while krypton does not. That is when krypton is worth paying for — not as a general upgrade.

A word on gas loss. Units lose a small percentage of fill per year through the seal. A properly made unit stays effective for decades; a badly made one does not. This is one of several reasons the edge seal matters more than the specification sheet suggests.

The Cavity Width

Wider is not automatically better. Below about half an inch, conduction across the gap dominates and performance drops. Much above it, the gas starts to circulate — convection carries heat across the cavity and undoes the benefit. There is an optimum, and it is roughly the half-inch that standard one-inch units use.

Acoustics pull the other way: a wider cavity and unequal glass thicknesses genuinely help against noise. Thermal and acoustic optimum are not the same unit, which is why an acoustic specification is a deliberate choice rather than an upgrade.

Why Units Fail

Seal failure is what people mean when they say a window has "blown". The seal admits moist air, the desiccant in the spacer eventually saturates, and condensation appears inside the cavity where nobody can clean it. Once that happens the unit is replaced, not repaired.

The causes are consistent: poor quality sealing at manufacture, units sitting in standing water because the frame drainage is blocked, and glazing units under stress from being forced into a frame that is too tight. Which is to say two of the three are installation and maintenance rather than the glass.

Double, or Something Else?

Double glazing is the right default for most of our work. It meets the energy code, fits standard frame depths, and the weight is manageable on ordinary hardware.

Move to triple when the thermal target cannot be met otherwise and the frames can take the depth and the weight. Not for noise — a well-specified asymmetric laminated double will usually beat a standard triple against traffic.

Move to vacuum glazing when you need very low heat loss and the frame depth is fixed — a historic sash, a landmark approval, an opening that cannot be enlarged.

Stay with double and change the glass when the problem is noise or solar gain rather than conducted heat. An acoustic laminate or a better coating inside a standard unit solves more problems than adding a third pane does.

Double Glazing Across the Region

These describe the kind of work each area brings rather than any specific client project.

Volume residential — East New York, Canarsie, the Bronx, Staten Island. Code-driven work across many similar openings, where the sensible route is the cheapest compliant specification rather than a premium coating. Argon-filled low-e on surface 2 covers most of it.

Arterial frontage — Ocean Parkway, Woodhaven Boulevard, Third Avenue. Noise is the complaint, so the answer is an asymmetric build with an acoustic laminate inside a standard unit, not a thicker unit.

Exposed and waterfront — the Rockaways, Manhattan Beach, the Hudson and the Shore. Frame drainage is critical here, because a unit standing in water will fail at the seal regardless of how well it was made.

Historic districts — Park Slope, Carroll Gardens, Brooklyn Heights. A one-inch unit frequently will not fit an original sash rebate. Where the approval requires the original joinery to stay, vacuum glazing is usually the only route to real performance.

Tell us the frame depth available, which elevation the opening is on, and whether the problem is heat, noise or both. We supply and install across Manhattan, Brooklyn, Queens, the Bronx and Staten Island, and throughout Northern New Jersey, the Jersey Shore, Philadelphia and the Lehigh Valley.