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Cross-market term

Insulating glazing unit

Two or more panes sealed around a spacer with a gas-filled gap between them. The gap does the insulating; the panes mostly carry the coatings that decide how much solar heat and light get through.
  • 4Markets
  • NoSame thing everywhere
  • 6Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Same word, different thing

    double glazing

    also sealed unit, IGU, g-value, warm edge spacer, low-E, argon filled

    G-VALUE is the solar transmittance, quoted 0 to 1 and measured to a European method. It is close to but not identical with the American SHGC, and the two are not interchangeable in a calculation.

  • Australia

    Close, not identical

    double glazing

    also IGU, SHGC, insulated glass unit, toned glass, WERS rating

    Uses SHGC rather than g-value, and solar control usually matters more than the U-value — the reverse of the British priority, which is why an imported specification often optimises the wrong number.

  • United Statesyours

    Same word, different thing

    insulated glass unit

    also IGU, double pane, thermal pane, SHGC, U-factor, low-E, NFRC label

    SHGC to the NFRC method, alongside a U-FACTOR that is not the British U-value either — different reference conditions and, historically, different units.

  • Canada

    Same word, different thing

    insulated glass unit

    also IGU, sealed unit, ER rating, energy rating, SHGC

    The ENERGY RATING combines U-factor, solar gain and air leakage into one number that can be NEGATIVE, so it cannot be compared with any single-property figure at all.

The governing standard, by market

  • United Kingdom

    BS EN 1279 / BS EN 410

    Insulating glass units including durability of the edge seal, and determination of luminous and solar characteristics of glazing.

  • United States

    NFRC 100 / NFRC 200 / ASTM E2190

    Procedures for determining fenestration U-factors and solar heat gain coefficient, and insulating glass unit performance evaluation.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

SHGC Effective Shading Fraction Calculator →Window Heat Loss Savings Calculator →Glazing Gasket/Wedge Linear Footage Calculator →Acoustic Laminated Glass STC Rating Estimator →Structural Glass Panel Weight Calculator →Blast-Resistant Fenestration Rating Checker →

Frequently asked questions

Is a g-value the same as an SHGC?
Close enough to be confusing and not close enough to substitute. Both express the fraction of incident solar energy that ends up inside — direct transmission plus the share of what the glass absorbs and then re-radiates inward — on a scale from zero to one. They differ in the reference conditions the test uses, and, more importantly in practice, in what the number is attached to: a g-value is usually quoted for the GLASS, while an NFRC SHGC is usually quoted for the whole WINDOW including the frame, which lowers it because the frame transmits no sunlight. So taking a glass g-value from a European datasheet and entering it as a whole-window SHGC overstates the solar gain, and doing the reverse understates it. The rule that avoids both is to establish whether a figure describes the glass or the assembly before using it, in either direction.
Why does the gas in the gap matter, and does it leak out?
It matters because the gap conducts and convects, and argon does both less well than air — the same reason the gap has an optimum width rather than being as wide as possible. Too narrow and the panes conduct across it; too wide and the gas circulates, carrying heat from the warm pane to the cold one, so performance stops improving and then gets worse. Argon's lower conductivity and higher viscosity move that optimum and improve the figure by a useful margin for almost no cost. It does leak, slowly, through the edge seal — a small percentage a year is normal, and the unit's insulation degrades gradually rather than failing. The failure people actually see is different: the desiccant in the spacer saturates, moisture gets in, and the unit MISTS internally. That is an edge-seal failure, it cannot be cleaned, and it is why the durability standard for units is about the seal rather than about the glass.
Why is the spacer called a warm edge, and does it change the rating?
Because a traditional aluminium spacer is a thermal bridge running around the entire perimeter of every unit, and the edge is where condensation and mould appear first. Aluminium conducts hundreds of times better than the gas it is holding apart, so it short-circuits the gap at the edge: the glass there runs several degrees colder than the centre, and in a humid room that band is where water forms. A warm edge spacer is made of stainless steel, a structural foam or a composite — materials chosen to hold the panes apart while conducting as little as possible. It improves the whole-window U-value by a modest amount, which is the part that appears on the label, and it raises the edge surface TEMPERATURE by rather more, which is the part occupants notice. It is the same distinction the thermal bridge concept draws generally: a small energy saving and a large comfort and condensation difference.