The thermal envelope your heating and cooling either fight or work with.

A window's U-value is the rate at which heat leaks through it (watts per m² per degree of temperature difference). Lower is better. The numbers worth knowing for UK housing:

• Single glazing: U-value ~5.0 W/m²K. Basically a hole in the wall.
• 1990s double glazing: ~2.8 W/m²K. The 'normal' UK window most people have.
• Modern A-rated double glazing: ~1.2–1.4 W/m²K. The current sensible standard.
• Triple glazing: ~0.8–1.0 W/m²K. Excellent; but only relevant for very low-energy homes.

Triple glazing is genuinely better than double. It's also 25–50% more expensive, heavier (more strain on frames and hinges), and the marginal improvement only pays back in heat-loss savings if your home is already very well insulated.

The maths: replacing 2.8 W/m²K windows with 1.2 W/m²K double glazing saves about 70 kWh/m² of glass per year in a UK climate. Replacing those with triple glazing (~0.9 W/m²K) saves a further 14 kWh/m²; about a fifth of the original gain for half again the cost.

If you're heading for Passivhaus, EnerPHit, or a fully retrofitted low-energy home, triple is the right call. For everyone else, A-rated double is the right call, and the saved money goes further on insulation or air-tightness work.

The glazing is only half the window. PVCu frames are the UK default; cheap, fine performance, mid-life cosmetics. Aluminium-clad timber and modern aluminium frames offer dramatically better aesthetics and slimmer sightlines, at 2–3x the cost.

Composite frames (timber inside, aluminium outside) are the premium choice for renovations where appearance matters as much as performance; typically £1,200–£2,500 per window vs £400–£800 for PVCu.

Whatever frame material, specify warm-edge spacer bars between the glass panes. The standard aluminium spacer is a thermal bridge that causes condensation around the glass edges; warm-edge spacers eliminate it. Costs nothing extra at order, makes a real difference.

Modern double glazing has a 'g-value' (solar transmittance) of 0.5–0.7; meaning 50–70% of the sun's energy hitting the glass enters the room. South-facing windows in particular contribute meaningful free heating in spring and autumn.

Higher g-value = more solar gain = better in winter but worse in summer. Lower g-value (solar-control glass, g ~0.3) reduces summer overheating but loses you free winter heat.

For south-facing rooms in UK homes, the right answer is usually standard glazing plus external shading; get the winter solar gain AND control the summer overheating. Solar-control glass is the compromise that loses both.

Once solar energy is through the glass, internal blinds simply re-radiate it into the room. Stopping the heat before it enters the glass is fundamentally different physics; and the only effective way to control summer overheating.

External roller shutters (continental-standard, £800–£2,000 per window) are best-in-class. External awnings (£600–£2,200 each) work brilliantly for living-room patio doors. Even strategic deciduous planting (oak, beech, hornbeam) shading west-facing windows by year 5 can cut peak room temperatures by 3–5°C at zero running cost.

Get the shading right and the AC you thought you needed shrinks by half, or disappears entirely.