The Complete Guide to Glass Specification

Glass specification is described using a small number of figures. Once you understand what each one means, reading a specification sheet becomes straightforward rather than intimidating.

U-value

Rate of heat loss through the glass, measured in W/m²K. Lower is better insulated.

g-value

Proportion of the sun's energy that passes through as heat. Lower means less solar gain.

Light transmittance

Percentage of visible light that passes through. Higher keeps a room feeling bright.

Acoustic rating

Rw, measured in decibels. Higher figures mean more external noise is blocked.

Privacy rating

A 1–5 scale used for obscure glass, from lightly textured to fully private.

These figures often pull in different directions. A coating that reduces solar gain will usually also reduce light transmittance slightly. A laminated pane that improves acoustics and security adds a small amount of weight and cost. Specification is about finding the right balance for a particular room and orientation, not chasing the single best number on any one measure.

Almost all modern windows and doors use a sealed double-glazed unit, or IGU (insulated glazing unit) — two or three panes of glass separated by a spacer bar and hermetically sealed, with the cavity filled with argon gas rather than ordinary air. The argon is denser than air and slows heat transfer across the cavity, which is why it improves the U-value at essentially no visual cost.

• Outer pane: usually the pane exposed to weather, sometimes toughened for safety or given a self-cleaning coating.
• Cavity and spacer: gas-filled gap, typically 16–20mm, with a warm-edge spacer bar to reduce cold bridging at the perimeter.
• Inner pane: carries the low-e (low emissivity) coating on the inward-facing cavity surface in most modern units, reflecting heat back into the room.
• Optional third pane: added for triple glazing, with two gas-filled cavities instead of one.

Every specialist glass type is a variation on the standard double-glazed unit, adding a coating, an interlayer or a texture to solve a specific problem. It helps to see them side by side before deciding which apply to a given project.

The single biggest driver of glass specification on a Lake District project is orientation. A north-facing window rarely needs solar control glass — the priority there is thermal performance, since the room gains little useful heat from the sun in any case. A south or west-facing wall of glass, particularly bifolds or a large sliding door opening onto a garden, is a different matter: without solar control, rooms with a lot of glazing on these elevations can become uncomfortably hot on clear days even in a cool climate, because the effect is driven by direct sun rather than air temperature.

Solar control coatings work by reflecting a proportion of the sun's infrared energy before it enters the room, while allowing most of the visible light through. A well-specified solar control unit on a west-facing bifold door can meaningfully change how usable a kitchen-diner is on a bright afternoon, without making the glass look tinted or reducing the sense of connection to the garden.

• South and west elevations with large areas of glass: solar control worth serious consideration.
• North and shaded east elevations: standard low-e glass is usually sufficient.
• Rooflights, which see much higher solar exposure than vertical glass: solar control is strongly recommended.
• Conservatories and garden rooms: often benefit from solar control on the roof and south/west walls together.

Privacy is usually solved in one of three ways: obscure textured glass, a reflective or tinted coating that reduces view-through in daylight, or switchable privacy glass that changes from clear to opaque electronically. Each suits a different situation. A bathroom window needs permanent, reliable privacy and is well served by an obscure pattern. A ground-floor front window that is overlooked from the street but should still let in a clear view during the day is often better served by privacy or reflective glass, since fully obscure glass there can feel like living behind frosted panels around the clock.

Switchable privacy glass, which uses an electrically charged interlayer to switch between clear and frosted at the flick of a switch, is the most flexible option and increasingly popular for internal screens, home offices and en-suites that open directly off a bedroom. It carries a noticeably higher cost than fixed glass, so it tends to be specified selectively rather than throughout a house.

Noise reduction in glazing comes from mass, from an asymmetric build-up (panes of different thickness either side of the cavity), and most effectively from a laminated pane with a PVB interlayer, which damps vibration passing through the glass. For a typical suburban or rural home a standard double-glazed unit already blocks most everyday noise. Acoustic glass earns its keep on busy roads, near flight paths, or where a bedroom window faces a noisy neighbour, and it is discussed in full in our acoustic glass guide.

Building Regulations Part K sets out critical locations — low-level glazing, glazing near doors, and glazing in doors themselves — where safety glass is a legal requirement, not an optional upgrade. Two types satisfy this: toughened glass, which is heat-treated to around five times the strength of standard glass and breaks into small, blunt granules rather than sharp shards, and laminated glass, which sandwiches a tough plastic interlayer between two panes so that if it breaks, it stays in one piece rather than falling out of the frame.

Laminated glass has the added benefit of being far harder to breach quickly, which is why it is often specified for ground-floor windows and doors even where it is not a strict regulatory requirement, and why it also delivers a useful acoustic improvement. The two are compared in detail in our laminated versus toughened guide.

Standard float glass has a faint green tint caused by iron oxide impurities, visible when you look through several panes edge-on or compare a window to a fully open aperture next to it. Low-iron glass uses a purer sand with less iron content, giving a noticeably clearer, more neutral view. It makes no difference to thermal or acoustic performance — it is a purely visual upgrade — but on a large sliding door or a picture window framing a Lake District view, where the glass is the main event, it is a popular and worthwhile specification.

Triple glazing adds a third pane and a second gas-filled cavity, typically improving the whole-window U-value from around 1.2–1.4 W/m²K to around 0.9–1.1 W/m²K. That is a genuine, measurable improvement, and it also tends to improve internal surface temperatures, reducing the cold-radiating feeling you can get standing near a large double-glazed pane on a frosty night. The Residence Collection, for example, achieves around 1.2–1.4 W/m²K double glazed and around 0.9 W/m²K in a triple-glazed specification.

The trade-offs are cost — typically 10–20% more than an equivalent high-performance double-glazed specification — and weight, which occasionally requires a heavier frame section or additional hardware on large opening sashes. For most homes achieving current Part L standards, well-specified double glazing already performs comfortably. Triple glazing earns its premium fastest on north-facing elevations, exposed rural sites, and self-build or Passivhaus-influenced projects where every U-value point is being pursued deliberately.

One of the most useful things to understand about glass specification is that it does not need to be uniform across a house. It is entirely normal, and often the right approach, to specify different glass in different rooms according to orientation, privacy needs and use.

1. 01Start with orientationMap which elevations face south and west, since these drive solar control decisions.
2. 02Flag overlooked roomsBathrooms, en-suites and any ground-floor room close to a pavement or neighbour need a privacy decision.
3. 03Identify noise sourcesNote any bedroom or study near a road, railway or other consistent noise source.
4. 04Mark critical locationsLow-level glazing and glazing near doors will need safety glass regardless of other choices.
5. 05Set the baselineChoose the default double or triple glazed low-e specification for everywhere else.

Glass specification is usually priced as an uplift per window or door rather than as a separate line item, and the figure depends on the size of the opening and the system it sits in. As a guide, moving from standard clear low-e glass to solar control, acoustic laminated, or low-iron glass typically adds a modest percentage to the unit cost of an individual window; triple glazing typically adds 10–20% across a whole specification. Obscure and privacy glass in a bathroom-sized window is usually one of the smaller upgrades in cash terms, since the window itself tends to be smaller.

The only way to get a reliable figure for a specific project is a survey, since glass cost scales with pane size and the frame system it needs to sit within. As a whole-house figure, glazing upgrades are usually a modest proportion of an overall replacement project costing £20,000–£70,000 for a typical three or four bedroom home.

During a survey we walk each elevation with you, noting orientation, overlooking and any known noise issues, and recommend glass room by room rather than applying one specification to the whole house. Samples of the main glass types are available to view and compare side by side, since photographs struggle to convey the difference between, say, a reflective and a privacy specification, or the subtlety of low-iron clarity.

“The frame lasts decades and rarely changes character. The glass is what most people are actually looking at, and it is worth the extra ten minutes of conversation to get right.”