Windows are a fundamental part of the building envelope. On high-end residential and architectural projects, their specification affects far more than the appearance of a façade. Frame design, glass composition, thermal performance, solar gain, weather resistance, ventilation, security and the way each window interfaces with the surrounding construction all need to be considered together.
This becomes particularly important as architects pursue larger areas of glass and slimmer sightlines. A window that looks minimal still needs to perform as part of the wider building envelope.
At Sky House Design Centre in Amersham, a wide range of architectural glazing systems can be viewed at full scale, including specialist window and structural glazing solutions from IQ Glass. Seeing these systems installed can make the differences between frame profiles, glass specifications and opening mechanisms much easier to understand than comparing technical drawings alone.
Start with the architectural requirement
Window specification should begin with the role the window needs to perform.
A fixed window designed primarily to frame a landscape view has very different requirements from an opening bedroom window, a large sliding window, a window within an exposed coastal façade or a window forming part of a Passive House envelope.
The first questions should therefore be architectural rather than product-led. Does the window need to open? How large is the opening? Is the objective to minimise visible framing? Does the window need to provide purge ventilation? Is it in an exposed location? What thermal performance is required? Does the glass need additional solar, acoustic or security performance?
IQ Glass provides a useful overview of these different approaches in its guide to choosing the right window system.
Once those requirements are understood, the appropriate frame and glazing specification can be developed around them.
Choosing the right window material
The frame material has a major influence on sightlines, structural capability, maintenance requirements and architectural character.
For contemporary architecture, thermally broken aluminium is frequently specified because its strength allows relatively slender profiles to support large areas of glass. Aluminium can also be powder coated or anodised in a wide range of finishes, allowing the window system to become either an intentionally visible architectural feature or a quiet part of the façade.
IQ Glass offers a range of slim aluminium window systems for projects requiring opening casements with minimal framing.
Where still slimmer opening sections are required, a sliding mechanism can provide another solution. IQ Glass's slim framed sliding windows use a 21 mm vertical sightline, with surrounding framing capable of being concealed within the building finishes. Double or triple glazing can be specified depending on the performance requirements of the project. (iqglassuk.com)
Steel can provide a different architectural language, particularly on heritage projects or schemes where narrow traditional profiles are important. Timber and aluminium-clad timber systems can offer high thermal performance while introducing a warmer internal material finish.
There is therefore no single "best" window material. The correct choice depends on the architecture, dimensions, performance targets and required finish.
When should a frameless window be specified?
Where a window does not need to open, removing the visible frame altogether can be possible.
Structural glazing allows the perimeter fixing to be concealed within the surrounding construction, creating what is often referred to as a picture window. The finished opening can appear almost entirely frameless.
IQ Glass uses its thermally broken Invisio system for frameless structural glass windows, including large picture windows, corner glazing and bespoke shaped windows. Triple glazing can also be incorporated using the deeper Invisio+ system. (iqglassuk.com)
This is particularly useful where the objective is to frame a significant view or create an uninterrupted relationship between the internal space and landscape.
However, a frameless appearance does not remove the need for careful technical design. The hidden connection between glass, structure, insulation and waterproofing becomes even more important precisely because the visible frame has disappeared.
Sky House has a more detailed guide to Invisio+ and triple-glazed structural glazing for projects where both minimal detailing and demanding thermal performance are required.
Understand the difference between Ug and Uw values
Thermal performance is one of the most important areas of window specification, but headline U-values can also be easily misunderstood.
The Ug value describes the thermal performance of the glass itself. The Uf value relates to the frame, while the finished Uw value describes the performance of the complete window installation.
This distinction matters. A very high-performance glass unit does not automatically mean the finished window will achieve the same figure.
IQ Glass's technical guide to U-values in architectural glazing explains how the glass, framing, spacer, dimensions and configuration all contribute to the overall result. (iqglassuk.com)
The thermal target should therefore be established at an early stage and confirmed for the actual window size and configuration proposed, rather than relying on the best centre-pane glass figure shown on a datasheet.
For projects with particularly stringent energy requirements, there are dedicated high-performance systems available. IQ Glass's Passive House window range includes certified timber and aluminium-clad timber windows with published Uw values between 0.68 and 0.79 W/m²K when triple glazed. (iqglassuk.com)
Double or triple glazing?
Triple glazing can significantly improve thermal performance, but it should form part of a considered specification rather than being added automatically.
Additional panes increase the overall thickness and weight of the glass unit. That can affect frame design, maximum panel sizes, hardware and opening mechanisms. Solar performance also needs to be considered alongside insulation.
On demanding low-energy projects, however, triple glazing can allow extensive areas of glazing to be incorporated while maintaining much stronger thermal performance.
Hellidon Passive House demonstrates this at building scale. IQ Glass designed and installed large triple-glazed sliding systems around the home's courtyard, including an approximately 21 metre wide glazed elevation. The principal system achieved a reported Uw value of approximately 0.76 W/m²K while maintaining the minimal architectural language of the house. View the Hellidon Passive House project (iqglassuk.com)
The lesson is not simply that triple glazing is "better". The glass, frame, dimensions and wider building envelope need to be designed as one system.
Consider orientation and solar gain
Large areas of glass can provide exceptional daylight and views, but orientation must be considered carefully.
Highly glazed south and west-facing elevations can be susceptible to excessive solar gain. On these elevations, the g-value of the glass becomes an important part of the specification.
Solar control coatings can reduce the proportion of solar energy passing through the glass while retaining high levels of natural light. IQ Glass's solar control glass range includes neutral coatings suitable for use within double and triple glazed units.
For example, IQ's published 70/35 solar control specification provides approximately 70% light transmission with a g-value of 35%, significantly reducing solar energy entering through the glazing while retaining a relatively neutral appearance. (iqglassuk.com)
Solar control should still be considered alongside the whole building design. External shading, roof overhangs, ventilation and the orientation of individual elevations can all contribute to controlling overheating.
IQ Glass has a dedicated technical guide to controlling solar gain and overheating in highly glazed spaces.
Site exposure can change the entire specification
The same window should not automatically be specified for a sheltered suburban site and an exposed coastal property.
Wind pressure, driving rain, salt-laden air and building height can all affect the required performance of the glazing system. Frame finishes, hardware, drainage and glass thickness may need to change accordingly.
This is particularly important for projects close to the coast. IQ Glass has extensive experience with architectural glazing in marine environments, where both the glass system and exposed metal components require greater consideration.
The Orestone End project in Salcombe provides a useful example. The five-storey coastal house uses large areas of slim framed and frameless glazing overlooking the sea. The specification included solar control low-iron glass, anodised aluminium framing and glazing engineered for project wind loads of 1.81 kN/m². See the Orestone End coastal glazing project (iqglassuk.com)
This is why site-specific engineering matters. Maximum glass dimensions quoted on a product page should never be treated as a guarantee that the same dimensions are appropriate on every building.
Safety and security glazing
Glass composition also needs to respond to where the window is positioned.
Low-level glazing, full-height glass and other critical locations may require safety glass. Depending on the application, this can involve toughened or laminated constructions.
Laminated glass can also be useful where additional security or acoustic performance is required. The correct glass build-up should therefore be determined as part of the wider window specification rather than treating the glazed panel as a standard component.
Security requirements should also extend beyond the glass. Locking mechanisms, hardware, opening configuration and the strength of the complete system all contribute to the finished window's performance.
Do not ignore the window-to-building junction
Many problems attributed to windows are actually problems at the interface between the window and the surrounding construction.
A high-performance thermally broken frame will achieve little if the connection to the wall creates a significant thermal bridge. Likewise, a tested window system still relies on correct drainage, waterproofing, air sealing and installation.
The window needs to be coordinated with the wall insulation, internal and external finishes, waterproofing strategy and structural opening.
This is especially important with minimal and frameless glazing because much of the supporting frame may be deliberately concealed.
For architects, early coordination between the glazing specialist, architect, structural engineer and contractor allows these junctions to be designed before the surrounding construction is finalised.
Opening type should be considered alongside ventilation
A window's opening mechanism influences both its appearance and how the room can be ventilated.
Conventional side-hung and top-hung casements remain appropriate for many applications. Larger architectural openings, however, may benefit from sliding, pivoting or automated systems.
Sliding windows are particularly useful where an architect wants a large opening element without requiring the internal or external swing space of an oversized casement.
IQ Glass's slim framed sliding windows can also incorporate a slot aeration setting, allowing the panel to be locked in a partially open position for ventilation. (iqglassuk.com)
The appropriate opening strategy should be considered alongside the project's wider ventilation design rather than selected purely for appearance.
Look at completed projects, not just product data
Technical data is essential, but completed projects provide a different type of evidence.
They demonstrate how a system has been resolved around real structures, actual site conditions and architectural constraints.
At Hidden House in Clerkenwell, for example, IQ Glass installed slim framed sliding windows alongside minimal sliding doors and thermally broken rooflights within a highly constrained conservation-area site. The glazing was used to introduce light and visual connection into a single-storey dwelling surrounded by existing walls and historic buildings. View the Hidden House project (iqglassuk.com)
For specifiers, examining relevant case studies can reveal more than simply looking at the manufacturer's most visually dramatic project. Look for projects with similar dimensions, exposure, architectural conditions and performance requirements to the scheme being designed.
Compare architectural windows at full scale
A technical drawing can show that a window has a 21 mm sightline. Standing in front of the actual system shows what that sightline means across an entire elevation.
The same applies to handles, opening forces, frame finishes, thresholds, glass coatings and the visual difference between framed and frameless glazing.
The IQ Glass showroom at Sky House Design Centre contains full-scale installations of architectural glazing systems, including slim framed windows, structural glazing, glass façades and other specialist glazing technologies. (skyhousedesigncentre.com)
For architects and homeowners who are still developing the specification, viewing the systems in person provides an opportunity to compare the physical details before committing them to the building.
Window specification checklist
Before finalising an external window specification, confirm:
- the required fixed or opening configuration
- the desired sightline and visible frame
- maximum glass and opening dimensions
- overall Uw performance, not simply the centre-pane Ug value
- whether double or triple glazing is appropriate
- solar orientation and required g-value
- safety and security glass requirements
- wind and weather exposure
- frame material and external finish
- drainage and waterproofing strategy
- interface with the building's insulation and airtightness layer
- hardware and ventilation requirements
- maintenance and access for future glass replacement
- relevant testing and certification
- who is responsible for designing the final glass-to-building junction
Resolving these points early allows the window to become part of the architectural design rather than a component that has to be made to fit later.
Discuss your window specification at Sky House
Sky House Design Centre brings specialist architectural and interior brands together in one showroom in Amersham, Buckinghamshire.
Within the centre, IQ Glass operates a major architectural glazing showroom where architects, designers and homeowners can view window systems, structural glazing and specialist glass technologies at full scale and discuss the technical requirements of a project with the glazing team. (skyhousedesigncentre.com)
For projects involving large glazing areas, minimal windows, frameless structural glass or demanding performance requirements, early specialist input can help ensure the architecture, engineering and glass specification develop together.
