How to keep a conservatory warm in winter without huge bills

Where the heat goes

A conservatory loses heat four ways: through the roof, through the glazed walls, through the floor and through gaps. Warm air rises, so it collects under the roof, which is also the thinnest and largest surface in most conservatories. That is why the room can feel fine at head height near the heater and freezing everywhere else.

The sum for any surface is simple. Watts lost = U-value x area x the temperature difference. A U-value is how many watts escape per square metre for each degree of difference, so lower is better.

A worked example: a 12 m² roof on a freezing night

Say it is 0°C outside and you want 20°C inside, a difference of 20 degrees.

The yearly figures use U x area x 2,150 heating degree days (the England average) x 24 / 1,000, and electricity at 26.32p per kWh, the Ofgem cap for 1 October to 31 December 2026. They assume the room is kept warm all winter. If you heat it only in the evenings, your figure will be lower, but the proportions stay the same.

How big a heater do you need?

Add the walls to the roof. A typical 3 m by 4 m conservatory has around 12 m² of glazing in its walls. With older double glazing at about 2.8, that is another 672 W at a 20 degree difference. Floor losses and draughts come on top.

• Polycarbonate roof plus older glazed walls: 552 + 672 = 1,224 W before the floor and draughts. You need a 2 kW heater, and it will run hard.
• Panel roof plus the same walls: 103 + 672 = 775 W. A 1 to 1.5 kW heater copes, cycling on and off on its thermostat.

Run those for 4 hours on a cold evening. With the polycarbonate roof the room needs about 4.9 kWh, around £1.29. With the panel roof it needs about 3.1 kWh, around £0.82. Heating from cold takes extra on top, and a poly roof cools the room faster once the heater goes off.

Does the type of electric heater matter?

Every plug-in electric heater turns 1 kWh of electricity into 1 kWh of heat. Oil filled radiators, convector heaters, fan heaters and infrared panels all cost the same per unit of heat. What differs is how they feel. Fan heaters warm the air fast but it rises to the roof. Oil filled radiators are slow and steady. Infrared panels warm people and surfaces rather than the air, which suits a room with a cold roof.

The exception is an air to air heat pump (a wall mounted split unit), which can deliver roughly 3 kWh of heat for each kWh of electricity. It needs a proper installation, and it works best in a room that holds its heat.

Can I put a radiator in off the central heating?

Not while it is an exempt conservatory. One of the conditions for the building regulations exemption is that the room has no fixed heating from the house system. Once the roof is replaced with insulated panels, the room loses its exemption anyway and building control approval is needed. If you want a radiator, tell us at the survey so it can be discussed with building control in the same application. More on this in our building regulations guide.

Cheap fixes that help

• Draught-proof the doors. Worn seals on French or bi-fold doors can leak more heat than you would expect. New brush strips and gaskets cost little.
• Check where the roof meets the house. A gap along the wall plate or a failed lead flashing lets cold air and water in.
• Cover the floor. A thick rug over a stone, laminate or concrete floor makes the room feel several degrees warmer at foot level.
• Thermal blinds and curtains. Closing them at dusk traps a layer of still air against the glass. They help more on the walls than under the roof.
• Use a thermostat and a timer. Heat the room for when you use it. A heater left on full all day is the fastest way to a big bill.
• Keep the door to the house shut. Otherwise the house heating tries to warm the conservatory too.

Why the room still feels cold at 20°C

A thermometer on the wall can read 20°C and the room still feel chilly. Your body loses heat by radiation to cold surfaces as well as to the air. Sit under a roof whose inner face is at 15°C and you feel it on your head and shoulders, even with warm air around you. That is also why cold air seems to fall from a glass roof: air cooled against it sinks. A roof whose inner surface sits at around 19°C removes most of that effect.

Middle options

Internal insulation fitted under the old roof costs about £1,000 to £3,200, averaging around £2,850. It keeps the polycarbonate on top and adds insulation and a ceiling inside. It can work, but the quality of the vapour control matters, because a badly detailed job can trap condensation between the insulation and the cold sheet above. Our page on conservatory roof insulation compares the approaches.

Upgrading to thicker polycarbonate costs £2,000 to £3,500 for a 3 m by 3 m roof. Going to 35 mm brings the U-value to about 1.4, still more than three times the loss of a 75 mm panel.

The roof that holds heat

An insulated panel roof has two aluminium skins bonded to a 75 mm solid foam core, with a U-value of about 0.43 W/m²K. That cuts roof heat loss by about 80% against 16 mm polycarbonate. The inner skin stays close to room temperature, so the cold down-draught you feel from a glass or polycarbonate roof largely goes, and so does most condensation. See why your conservatory roof drips condensation.

Prices start at £3,000 for roofs up to 12 m², £4,000 for 12 to 25 m² and £5,000 for 25 m² and over, fitted, with the building control application and a 10 year insurance backed guarantee included. The same roof keeps the room cooler in summer too.

Next step

For a rough price for your size and style, try the conservatory roof cost calculator. For a fixed figure, book a free survey. If you want to go deeper on running costs, read how much it costs to heat a conservatory.