Your Complete A–Z Window Terminology Guide
Feeling overwhelmed by window jargon when talking to installers or reading quotes? You’re not alone. In my 38 years in the UK window industry, I’ve seen countless homeowners make poor decisions simply because they didn’t understand the terminology being used.
I’m Oliver Greene, and today I’m sharing my complete A–Z glossary of window terms – everything you need to confidently discuss your window project with installers, understand quotes, and make informed decisions.
This isn’t just a dictionary – it’s your toolkit for becoming an informed window buyer. Each term includes practical context so you understand not just what it means, but why it matters for your project and budget.
For comprehensive information on window installation processes, see our detailed window installation guide. To explore different window styles mentioned in this glossary, visit our complete window types guide. For comparison of specific terms and technologies, check our comparison section.
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A
Argon Gas
Inert gas used between glass panes in double and triple glazing. Argon is denser than air, reducing heat transfer and improving insulation. Standard in quality double glazing, adding approximately £20-30 per window but improving U-values by 0.1-0.2 W/m²K.
Astragal Bars
Decorative glazing bars that sit on the glass surface, creating the appearance of individual panes without affecting the sealed unit. More cost-effective than Georgian bars but less authentic in appearance.
Awning Window
A window hinged at the top that opens outward from the bottom. Popular in modern architecture, awning windows provide ventilation even during light rain and offer good security when partially open.
B
Bay Window
A window that projects outward from the main wall, creating additional interior space and enhanced views. Available in various angles (30°, 45°, 90°), bay windows typically cost 30-50% more than equivalent flat windows due to complex installation requirements.
BFRC (British Fenestration Rating Council)
Independent organisation that provides energy efficiency ratings for windows. BFRC labels (similar to appliance energy labels) show overall energy rating, U-value, and annual energy costs, making window comparison straightforward.
Bow Window
Similar to bay windows but with a curved projection rather than angled sections. Bow windows create elegant curved interior spaces but require specialist installation and typically cost more than bay windows.
C
Casement Window
The most common UK window type, hinged on one side and opening outward like a door. Casement windows offer excellent ventilation, easy cleaning from inside, and good security when fitted with quality locks.
Oliver’s Note: “Casement windows are my go-to recommendation for most homes – they’re versatile, efficient, and cost-effective.”
Cavity Wall
Standard UK construction method with two walls separated by an insulating gap. Window installation in cavity walls requires careful consideration of thermal bridging and insulation continuity around the frame.
Cill (Window Sill)
The horizontal ledge at the bottom of a window opening, both inside and outside. External cills must slope away from the building to shed water, while internal cills provide decorative finishing and additional display space.
D
Double Glazing
Two panes of glass separated by an insulating gap, typically 12-20mm wide. Standard in UK construction since the 1990s, double glazing reduces heat loss by 50-70% compared to single glazing and significantly improves noise reduction.
Draught Proofing
Sealing systems that prevent air leakage around window frames. Quality draught proofing can reduce heat loss by 10-15% and eliminate uncomfortable draughts. Look for brush seals, compression seals, or magnetic strips depending on window type.
DGU (Double Glazed Unit)
The complete glass assembly in double glazing, including both panes, spacer bar, desiccant, and edge sealing. DGUs are manufactured in specialist facilities and cannot be repaired if the seal fails – replacement is the only option.
E
Espagnolette
Multi-point locking system common in casement windows, where turning the handle engages locks at multiple points around the frame. Provides excellent security and weather sealing but requires quality mechanisms to prevent failure.
Energy Rating
BFRC classification from A++ (most efficient) to G (least efficient), considering U-value, solar gain, and air leakage. A-rated windows typically reduce heating bills by £15-25 per square metre annually compared to C-rated alternatives.
F
Fanlight
Small window above a door or larger window, originally designed for ventilation. Modern fanlights are often fixed (non-opening) and used for additional natural light in hallways and entrances.
FENSA
Fenestration Self-Assessment Scheme – the UK’s leading competent person scheme for window installation. FENSA-certified installers can self-certify Building Regulations compliance, saving homeowners time and money on separate approvals.
Fixed Light
Non-opening window panel, typically used alongside opening windows for maximum light with reduced cost. Fixed lights are 20-30% cheaper than equivalent opening windows and offer better security and weather sealing.
G
Georgian Bars
Glazing bars that divide glass into smaller panes, either within the sealed unit (authentic appearance) or applied to surfaces (cost-effective option). True Georgian bars affect thermal performance but provide authentic period character.
G-Value (Solar Factor)
Measure of solar heat gain through glass, from 0 (no solar gain) to 1 (maximum solar gain). High G-values (0.6-0.7) benefit south-facing windows for free winter heating, while low G-values (0.3-0.5) prevent overheating in west-facing rooms.
Glazing Bead
Removable strip that holds glass in the window frame. External glazing beads should be avoided for security – quality windows have internal beads that cannot be removed from outside.
H
Head
The top horizontal member of a window frame. The head must support the weight of the glass and provide weather sealing at the junction with the wall above.
Hinges
Hardware that allows windows to open and close. Quality hinges are crucial for smooth operation and longevity – look for stainless steel or galvanised options with 10+ year warranties.
I
IGU (Insulated Glass Unit)
Another term for sealed double or triple glazed units. IGUs contain desiccant to absorb moisture and inert gases (argon or krypton) for improved insulation.
Ironmongery
Collective term for window hardware including handles, hinges, locks, and stays. Quality ironmongery can cost £50-150 per window but significantly affects operation, security, and longevity.
J
Jamb
The vertical sides of a window frame or opening. Jambs must provide structural support and weather sealing, and accommodate hinges or sliding mechanisms depending on window type.
Joist
Horizontal structural timber supporting floors or ceilings. Window installation may require joist modification, particularly for large openings or structural glazing.
K
Krypton Gas
Premium inert gas used in high-performance glazing, offering better insulation than argon but at higher cost. Krypton allows thinner cavities while maintaining performance, beneficial for heritage applications requiring slim profiles.
L
Laminated Glass
Safety glass with plastic interlayer that holds fragments together if broken. Laminated glass provides security, noise reduction, and UV protection but costs 30-50% more than standard glass.
Lintel
Structural beam above window openings that supports the wall above. Lintels must be adequate for the window size and wall loading – undersized lintels cause cracking and structural problems.
Low-E Glass
Low-emissivity glass with microscopic metallic coating that reflects heat while allowing light through. Low-E coatings improve U-values by 0.3-0.5 W/m²K and are standard in quality double glazing.
M
Mullion
Vertical structural element dividing windows into separate sections. Unlike glazing bars, mullions are load-bearing and create distinct window openings within larger frames.
Meeting Rail
Horizontal members where sash windows meet when closed. Meeting rails must provide weather sealing and security, often incorporating locks and draught seals.
N
Newel
Structural post in bay or bow windows that supports the window weight and transfers loads to foundations. Newels require careful design and installation to prevent settlement and cracking.
Noise Reduction
Sound insulation provided by windows, measured in decibels (dB). Standard double glazing provides 25-35dB reduction, while acoustic glazing achieves 40-50dB reduction for traffic noise control.
O
Opening Light
Part of a window that opens, as opposed to fixed lights. Opening lights cost more than fixed equivalents but provide ventilation, emergency egress, and cleaning access.
Oriel Window
Bay window supported by brackets or corbels rather than extending to ground level. Oriel windows are common in Tudor and Victorian architecture and require specialist structural support.
P
PAS 24
Enhanced security standard for windows and doors, tested against burglary attempts. PAS 24 certification indicates resistance to common attack methods and is often required by insurance companies for high-risk areas.
Passive House
Ultra-low energy building standard requiring exceptional window performance (typically U-values under 0.8 W/m²K). Passive House windows cost 50-100% more than standard options but eliminate heating requirements.
Q
Quadruple Glazing
Four panes of glass with three insulating cavities, achieving U-values under 0.5 W/m²K. Quadruple glazing is expensive and heavy, requiring substantial frames, but offers ultimate thermal performance.
R
Rebate
Stepped recess in window frames that accommodates glass units and provides weather sealing. Rebate design affects thermal performance, water drainage, and glass retention.
Reveal
The internal sides of window openings in thick walls. Deep reveals can cause thermal bridging and require careful insulation detailing during installation.
S
Sash Window
Traditional window with vertically sliding panels, typically double-hung (both panels move) or single-hung (only bottom panel moves). Sash windows require counterweights or spring balances for operation.
Sealed Unit
Complete double or triple glazed assembly with permanent edge sealing. Sealed unit failure (indicated by condensation between panes) requires complete unit replacement.
SHGC (Solar Heat Gain Coefficient)
American equivalent of G-value, measuring solar heat transmission through glass. SHGC and G-value are interchangeable for UK applications.
T
Thermal Break
Insulating barrier in aluminium frames preventing heat transfer between inner and outer sections. Thermal breaks are essential for aluminium window efficiency – without them, U-values can exceed 3.0 W/m²K.
Tilt and Turn
European window style that tilts inward from the top for ventilation or turns inward like a door for cleaning and emergency egress. Popular in modern architecture but requires substantial internal clearance.
Toughened Glass
Safety glass heat-treated to increase strength and break into small, relatively harmless pieces. Required by Building Regulations in certain locations including low-level glazing and doors.
Triple Glazing
Three panes of glass with two insulating cavities, achieving U-values of 0.8-1.2 W/m²K. Triple glazing costs 20-40% more than double glazing but offers superior insulation and noise reduction.
U
U-Value
Measure of heat loss through windows in watts per square metre per degree Kelvin (W/m²K). Lower U-values indicate better insulation – aim for under 1.2 W/m²K for good performance.
uPVC
Unplasticised polyvinyl chloride – the most common UK window frame material. uPVC offers good insulation, low maintenance, and cost-effectiveness but limited colour options and bulky profiles.
V
Vent
Opening mechanism for controlled ventilation, including trickle vents (small permanent openings) and night vents (secure ventilation positions). Building Regulations often require ventilation provisions in new windows.
Venetian Window
Three-part window with larger central section flanked by narrower side panels. Venetian windows create impressive facades but require careful proportioning and structural support.
W
Warm Edge Spacer
Insulated spacer bar between glass panes that reduces thermal bridging compared to standard aluminium spacers. Warm edge spacers improve U-values by 0.1-0.3 W/m²K and reduce condensation risk.
Weather Bar
Horizontal barrier at the bottom of door openings that prevents water ingress. Weather bars must be carefully detailed to avoid creating trip hazards while maintaining weather protection.
Window Board
Internal window sill, typically timber, uPVC, or stone. Window boards provide finishing and display space but must accommodate thermal movement and potential condensation.
X, Y, Z
Zone Toughening
Selective toughening of glass areas that require safety glazing while leaving other areas as standard glass for cost efficiency. Common in large glazed areas where only portions need safety treatment.
Putting Your Knowledge to Work
Now that you understand window terminology, you can:
- Ask Informed Questions: Use correct terminology when discussing your project
- Understand Quotes: Decode technical specifications and identify quality differences
- Avoid Confusion: Prevent miscommunication that leads to wrong products or installation errors
- Negotiate Effectively: Understanding what you’re buying puts you in a stronger position
Ready to use your new window knowledge? Contact our expert team for a consultation where you can put these terms into practice and get professional advice tailored to your specific project needs.
Understanding window terminology transforms you from confused buyer to informed customer. Armed with this knowledge, you can confidently navigate the window replacement process, ask the right questions, and make decisions that deliver optimal performance and value for decades.
Want to expand your window knowledge further? Download our technical specification guide for detailed explanations of performance standards and installation requirements, or subscribe to our monthly newsletter for ongoing window industry insights and homeowner tips.