The glass matters as much as the frame. A plain-English guide to what you are buying, what the regulations actually require, and when to pay more for upgraded glazing.
Every window we supply is a sealed double-glazed unit (sometimes called an IGU — Insulating Glass Unit). The construction is consistent:
The combined unit is 28mm overall. This is the industry-standard sealed unit for UK domestic A+ rated uPVC windows.
The three main glass choices in our configurator cover 99% of domestic specifications.
Low-iron, low-emissivity float glass. Maximum light transmission, maximum view. Used everywhere except where privacy or safety requirements override.
Patterned or sandblasted glass that transmits light but obscures view. We supply a standard satin-etch pattern (often called "Cotswold" or "Pelerine" in the trade) which is the most forgiving choice for bathrooms and W/Cs — it does not dominate the room the way heavily textured glass can.
Building Regulations do not require obscure glass in bathrooms. It is a privacy choice. If the window is overlooked by a neighbour or onto a public pavement, obscure makes sense. If it is on a secluded garden elevation, clear glass with a blind usually feels more pleasant.
Standard float glass is thermally treated to around 650°C then rapidly cooled, which puts the surface under compression and the core under tension. The result is around 4 to 5 times stronger than ordinary glass. When it does break, it fractures into small cuboid fragments with blunt edges rather than sharp shards — the same principle as a car windscreen.
Toughened glass is a legal requirement in certain locations under Approved Document Part N of the Building Regulations — covered in detail below.
Can you combine them? Yes. Bathroom windows within 800mm of floor level typically need obscure and toughened. Our configurator combines both options when selected, with the uplift calculated off the base price.
Approved Document Part N of the Building Regulations for England and Wales sets out "critical locations" where ordinary glass must be replaced with a safety-rated equivalent (toughened or laminated). The rules are straightforward:
| Location | Toughened required? |
|---|---|
| Any glazed pane within 800mm of finished floor level | Yes |
| Any glazed pane within 300mm of a door edge, up to 1500mm from floor level | Yes |
| Glazed doors (not relevant to our range) | Yes |
| Glass with a maximum dimension over 250mm in a critical location | Yes |
| Windows above first floor in habitable rooms | Usually no |
In practice the most common scenarios requiring toughened glass are:
Building Control will check this. If you are replacing windows under a building notice or as part of a renovation, the inspector will verify toughened glass in critical locations. Getting this wrong delays sign-off. If in doubt, specify toughened — the £15-30 per window uplift is trivial compared to a Building Control rejection.
Approved Document Part L covers energy efficiency. For replacement windows in England and Wales the headline rule is simple:
Whole-window U-value must not exceed 1.4 W/m²K.
All our windows ship with a whole-window U-value of approximately 1.2 to 1.3 W/m²K, depending on size (smaller windows have a worse ratio because the frame-to-glass ratio is higher). This comfortably clears Part L. A certificate is available on request.
WER is an easier-to-read single letter ranking that bundles U-value, solar gain, and air leakage into one rating from A++ down to E.
| Rating | Typical U-value | Notes |
|---|---|---|
| A++ | < 0.8 | Passivhaus / triple glazing territory. |
| A+ | < 1.2 | Modern benchmark. Our standard specification. |
| A | < 1.4 | Meets Part L minimum for replacement. |
| B | < 1.6 | Does not meet current regs as a replacement. |
| C or lower | > 1.6 | Should not be fitted. Walk away. |
The soft-coat low-emissivity layer on the inner pane serves two purposes:
For the vast majority of UK domestic applications, the standard low-E Pilkington K or Planitherm Total+ specification we ship handles both requirements adequately.
If the window overlooks a busy road, railway or flight path, acoustic glass is available on custom quote. The construction uses a laminated inner pane with a PVB (polyvinyl butyral) interlayer that damps sound transmission. A standard 4-20-4 argon unit delivers around 32 dB reduction; an acoustic unit can deliver 38 dB or higher. Price uplift is typically 30-45%.
For normal suburban locations the standard unit is already very effective, and most customers who think they need acoustic glass find that standard glazing plus a heavy curtain is enough.
Triple glazing reduces U-values from around 1.2 to around 0.7 W/m²K, which is a genuine improvement on paper. In a UK climate the practical payback is long — typically 20-30 years on energy bill savings alone — because the marginal heat loss difference between a well-specified A+ double-glazed unit and a triple-glazed unit is small compared to the heat lost through walls, floors, roofs and air leakage.
Triple glazing makes most sense if you are building or renovating to Passivhaus or near-Passivhaus standards, if the room has a serious thermal bridge issue around the window opening, or if the acoustic reduction is a priority.
We do not offer triple glazing in our online range because for the vast majority of our retrofit customers it would not be the highest-impact investment. If you have a specific requirement, call us.
When in doubt, specify toughened. The uplift is small; the consequences of under-specifying (Building Control rejection, or worse, a broken pane that injures someone) are substantial.
Every configurator page includes glass and safety upgrades. Pricing updates instantly as you select.
Open the Configurator →This page is intended as plain-English guidance. It is not a substitute for professional advice from a qualified glazier or Building Control officer. Always verify requirements for your specific installation with your Local Authority Building Control.