What is a U-value?
A U-value measures how much heat passes through a building element (a roof, wall or window) for every degree of temperature difference between inside and outside. The unit is W/m²K — watts per square metre per kelvin. The lower the number, the less heat you lose. Building Regulations Part L sets minimum standards; modern roofs are expected to be well below 0.20 W/m²K.
Why 0.15 W/m²K matters
A U-value of 0.15 means the roof loses around 10× less heat than a typical polycarbonate roof at 1.8, and around 20× less than old single glazing. In practical terms it's the difference between a conservatory that feels cold within minutes of the heating going off, and one that holds warmth like a normal living room.
How conservatory roof types compare
- Polycarbonate: 1.8 – 3.0 W/m²K. Fails current Building Regs by a wide margin.
- Single-glazed glass: ~5.0 W/m²K. Found in older conservatories — wildly inefficient.
- Modern self-cleaning double-glazed glass: 1.0 – 1.6 W/m²K. A clear improvement on polycarbonate but still 7–10× more heat loss than a solid roof.
- Solid (\n) lightweight tiled system: 0.15 – 0.18 W/m²K. Meets and exceeds Building Regs Part L.
What it means for your heating bill
Heat loss through the roof is the dominant energy drain in any conservatory. Cutting the U-value from ~2.0 down to 0.15 typically reduces conservatory-related heating costs by 30–50% in real homes. Combined with eliminating draughts and condensation, the room becomes cheap to keep warm and genuinely usable in winter.