There are so many specialist terms used in the glazing industry that sometimes it’s difficult to remember the difference between your U-Values, K-Values and R-Values!

We’ve put together an ABC guide to some of these terms in our glazing glossary...

A is for Annealing

Cooling hot glass slowly to relieve internal stresses from forming process to increase its durability. The annealing process is carried out in a kiln known as a lehr.

B is for Bead

A wooden or metal strip which holds glass in place on windows and doors.

C is for Cavity

The insulating argon gas filled gap created by the spacer bar in between the two glass panes in double glazed units.

D is for Desiccant

A silica gel or molecular sieve in the cavity of an insulated unit which absorbs any moisture  trapped between the glass panes.

E is for Emissivity

The emissivity of a material is how effective it is at emitting energy as thermal radiation. Low-E or low-emissivity glass is energy efficient as it is covered with a coating that reduces the emissivity of glass, keeping the heat inside a building.

F is for Fin

A glass fin or a glass mullion is a vertical support for joints between glass panels.

G is for Gasket

Secures the glass in the frame and provides a seal between the window frame and double glazed unit. GGR Glass stock a range of gasket rollers for pushing gaskets into place on a window frame without damaging the seal.

H is for Heat Treated

Heat treated glass has had heat applied to it to strengthen the material.

I is for Inside Glazed

Where external glazing has been installed from within the building, like with one our GGR’s glazing robots.

J is for Jamb

The main vertical sides of a window frame.

K is for K-Value

Measurement of the amount of heat that passes through glass, or more specifically the number of BTUs (British Thermal Units) that pass through one inch (1”) thick by one foot (1’) square section of material in a hour creating a 1°F temperature difference between the surfaces.

L is for LT

LT, an abbreviation of light transmittance, how much visible light is transmitted through the glass.

M is for Mullion

A vertical or horizontal division between the units in a window.

N is for Notch

Where a section of glass is cut out from the panel, for example to make room for a handle or hinge on a frameless glass door.

O is for Oriel

An Oriel window is a protruding bay window that does not touch the ground.

P is for PVB (Polyvinyl butyral)

Polyvinyl butyral is a resin which is used to create a protective interlayer between panels to make laminated glass.

Q is for Quarter Glass

The backside glass windows in a car.

R is for R-Value

The R-Value of a piece of glass is how resistant it is to heat flow or transfer of thermal energy. The higher the R-Value the more insulating the glass is.

S is for Spandrel panel

An opaque glass cladding panel often used on a building’s facade to hide structural elements that the architect doesn’t want to be seen.

T is for Transom

The horizontal bar over the top of a window or door from a smaller window or fanlight above it.

U is for U-Value

The U-Value is how fast heat flows through a piece of glass to the outside air. This is usually measured in watts per square metre, per degree Kelvin – W/m2K. The lower the U-Value the more insulating the glass unit is.

V is for Vacuum Power                 

Vacuum power can be used to safely lift glass, either manually with suction cups for small panels or using a below-the-hook glass vacuum lifter for larger and heavier units.

W is for Wind Load

Measured in N/m2, wind load is the pressure acting on a building from the wind.

X is for Extra Large Panels           

The architectural trend for using larger glass panels means that specialist equipment is needed to safely lift these heavier units, such as GGR’s 2.6 tonne capacity Hydraulica 2600 glass lifter and Extra Long Liftable A-Frame.

Y is for Yorkshire Light

A Yorkshire Light window is a style of sliding window with one fixed sash and one sliding sash.

Z is for Zero Marks

GGR stock a range of low marking hand cups, suction mounts and vacuum lifting pads which leave no residue on glass panels after use.


  • Glassolutions glossary
  • National Glass Association glossary

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