The technical detail behind double glazing, made clear
Sealed units, glass coatings, gas fills and warm-edge spacers explained in plain terms — so you can read a quote, compare like for like and know what actually drives thermal performance.
- IGU construction
- Warm-edge spacers
- Argon fill
- Low-E coatings
- U-value & G-value
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Inside a sealed double glazing unit
A modern double glazing unit — the insulated glass unit, or IGU — is a sealed sandwich of two glass panes separated by a spacer bar around the edge. The cavity between them is what does the insulating work, and every part is chosen to slow heat loss.
| Outer pane | Faces the weather; toughened or laminated where safety glass is required. |
|---|---|
| Sealed cavity | Typically 16–20 mm, filled with dry air or an inert gas such as argon. |
| Spacer bar | Separates the panes and holds desiccant; warm-edge types cut heat loss at the frame edge. |
| Desiccant | Drying agent inside the spacer that keeps the cavity dry and prevents internal misting. |
| Edge seals | A primary and secondary seal keep moisture out and gas in for the life of the unit. |
| Inner pane | Often coated with a microscopically thin low-E layer to reflect radiant heat. |
Units, glass and spacers — the parts that matter
Click a term to expand a plain-English definition, then follow through to the full reference page.
UnitsSealed insulated glass units (IGUs)
The sealed pane-cavity-pane assembly that forms a double glazed window. Performance depends on cavity width, gas fill, spacer type and coatings — not just “double” versus “single”. Read about double glazing units.
SpacersWarm-edge spacer bars
The bar around the unit’s edge. Traditional aluminium spacers conduct heat; warm-edge spacers use low-conductivity materials to reduce edge heat loss and cold-spot condensation. Read about warm-edge spacers.
CavityArgon gas fill
An inert gas denser than air, used to fill the cavity and slow convection heat loss. A common, low-cost way to improve a unit’s U-value. Read about argon-filled units.
GlassLow-emissivity (Low-E) coatings
A microscopically thin metal-oxide layer on the glass that reflects radiant heat back into the room while letting light through — the single biggest factor in a modern unit’s thermal rating. Read about Low-E glass.
GlassGlass thickness & safety glass
Pane thickness affects strength, sound reduction and where toughened or laminated safety glass is required by the building regulations. Read the glass thickness guide or compare toughened versus laminated glass.
RatingsU-value & G-value
U-value measures heat loss (lower is better); G-value measures solar heat gain. Together they describe how a unit balances keeping warmth in against letting free solar heat through. Read about U-value and G-value.
Getting the spec right
Knowing the spec is half the job; the other half is a fitter who confirms it on a proper survey. We match your enquiry with vetted, accredited local installers so you can compare free, no-obligation quotes — and check the glass specification is written down before anything is agreed.
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Technical guides
Nine plain-English references covering the units, glass and spacers behind every double glazing quote. Start at the glazing technology hub.
Double glazing units (IGUs)
How a sealed unit is built and what drives its performance.
Warm-edge spacer bars
Why the spacer material matters for edge heat loss.
Argon-filled double glazing
What the gas fill does and how much it helps.
Low-E glass explained
The coating that reflects heat back into your rooms.
Window glass thickness
How pane thickness affects strength and sound.
Toughened vs laminated glass
The two safety glasses and where each is required.
U-value & G-value
The numbers that describe thermal performance.
Misted / blown units
Why units fog up and what your options are.
Replacing glass units
Replacing just the glass and getting quotes.
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Double glazing, answered
What is the difference between a double glazing unit and a window?
The unit, or IGU, is the sealed glass assembly — two panes, a spacer and a sealed cavity. The window is the whole product: the unit plus the frame, hardware and seals that hold it in the wall.
Does argon gas really make a difference?
Argon is denser than air and slows convection inside the cavity, which typically lowers a unit’s U-value at very little extra cost. It works alongside Low-E coatings and warm-edge spacers rather than on its own.
Why has my double glazing misted up inside?
Internal misting means the edge seal has failed and moisture has entered the cavity — often called a “blown” unit. The glass itself can usually be replaced without changing the whole window. See our misted units guide.
What U-value should I look for?
Lower U-values mean less heat loss. According to typical industry guidance, modern replacement double glazing should comfortably beat older units; an installer confirms the figures for your chosen spec on survey. See U-value and G-value explained.
Are quotes free and is there any obligation?
Yes — the quotes are free and there is no obligation. Funding and contribution options may be available for those who qualify, subject to eligibility and a home survey.
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