The frame is only half the window
Most of the surface area of a window is glass, so most of what a window does for heat, noise and safety comes down to the glass specification. A good uPVC frame with basic glass will underperform a modest frame with well chosen glass on almost every measure that matters day to day.
Single versus double glazing
Single glazing is one pane. Double glazing, often called an insulated glass unit, is two panes separated by a sealed gap, usually filled with dry air or an inert gas and held apart by a spacer that also absorbs moisture.
The insulating work is done by the trapped layer, not by the extra glass. That is why the width of the gap matters as much as the panes themselves. A sealed unit resists heat transfer considerably better than a single pane, which is relevant both for keeping heat out in summer and for reducing condensation in cooler conditions.
Double glazing also reduces noise, though less dramatically than people often expect. Two panes of equal thickness with a narrow gap resonate together and let a surprising amount of sound through.
Toughened glass
Toughened, or tempered, glass is heat treated so that it is substantially stronger than ordinary glass of the same thickness. Its defining property is how it fails. Instead of long sharp shards it breaks into small blunt granules, which is why it is used where a person could fall against the glass.
Reasonable places to specify it include low level glazing, full height panels, doors, and anywhere children are likely to be. Note that toughened glass cannot be cut or drilled after treatment, so every dimension has to be final before it is made.
Laminated glass
Laminated glass bonds two panes to a plastic interlayer. When it breaks, the fragments stay stuck to the interlayer rather than leaving the opening. That gives it three useful properties at once: it holds together after impact, it is harder to break through quickly, and the interlayer damps sound noticeably better than an equivalent thickness of plain glass.
For a window facing a busy road, laminated glass is usually a more effective acoustic choice than simply adding a second plain pane.
Low-E coatings
A low emissivity coating is a microscopically thin metallic layer applied to one face inside a sealed unit. It reflects a portion of radiant heat while still letting visible light through. In a hot climate the practical effect is that less solar heat enters the room without the window going dark.
Low-E only makes sense inside a sealed double glazed unit, because the coating has to sit on a protected internal surface.
Choosing room by room
- Bedrooms facing a road: laminated glass, ideally in a sealed unit with unequal pane thicknesses
- West facing living rooms: sealed double glazing with a low-E coating to cut afternoon heat gain
- Doors, low level panels and anything full height: toughened, for safety rather than performance
- Bathrooms: obscured or frosted, toughened if the panel is low
- Internal or sheltered openings: single glazing is often perfectly adequate
One practical point
Glass specification is fixed at the time of manufacture. Changing your mind later means a new sealed unit, not an adjustment. It is worth deciding this carefully before the order goes in, room by room rather than as a single blanket choice for the whole house.