Performance comparison
Double glazing 4-16-4 argon-filled with low-e coating: U-value 1.1–1.4 W/m²K, light transmission 70–80%, sound reduction 32–35 dB Rw. Triple glazing 4-12-4-12-4 argon or krypton-filled: U-value 0.6–0.8, light transmission 60–70%, sound reduction 38–42 dB Rw. Weight: triple glazing 25–35 kg/m² vs double glazing 18–22 kg/m² — frames need to be sized for the load. Solar gain: triple glazing reduces solar gain by 10–15% versus double, which matters on south-facing windows where you want winter sun for heating. Frame compatibility: most modern aluminium, timber and uPVC frames accept triple glazing units up to 44mm thick; older frames may need replacement.
When triple glazing is worth it
Passivhaus or EnerPHit targets: triple glazing is essential to achieve the whole-house thermal performance target. North-facing elevations: no solar gain benefit to lose, full thermal benefit gained. Noisy roads (Bayswater Road, A40, A3, North Circular): the 6–8 dB acoustic improvement is significant. Very large glazed openings (>3m wide): the cumulative heat loss matters more on big glass walls, and the frame structure already supports the weight. Bedrooms facing east or north: comfort improvement from cold radiation. Listed building secondary glazing: secondary glazing achieves similar overall U-value at lower cost — often the preferred route on heritage sash windows.
When double glazing is enough
South-facing windows in standard renovations: solar gain matters more than U-value; double glazing with high g-value coating is better. Small windows under 1m²: the marginal benefit is too small to justify the extra cost. Budget renovations: prioritise other fabric improvements (wall and floor insulation, airtightness) before stepping up to triple glazing. Conservation areas requiring slim-profile sashes: triple-glazed slim sash units cost £1,500–£3,500 per window vs £800–£1,800 double — often outside the budget. Use double glazing plus secondary glazing in heritage contexts.
