The short answer
A conservatory is cold in winter because it is largely glass or plastic, which lets heat out far faster than a brick wall, and warm air rises to the roof where most of the loss happens. Draughty frames, thin polycarbonate, an uninsulated floor and an open doorway to the house add to it. A better-insulated roof makes the biggest single difference to a cold conservatory.
Where the heat actually goes
Heat leaves any building in every direction, but in a conservatory it leaves quickly and the roof is a major route. Warm air is lighter than cold air and drifts upward, so the warmest air in the room sits against the roof, which is also the surface most exposed to the night sky. A single skin of glass or a thin plastic sheet does very little to slow that loss. The air cools against it, drops down the glazing, and you feel it as a chill at head height or as a cold draught around the ankles.
Walls matter too. A conservatory with a high proportion of glazing around its sides loses heat through them as well, and the low walls, if any, are often thinner than those of the house. But on most conservatories the roof is the largest single area of thin material, and it faces straight up. That makes it the first place to look.
What each part of the roof contributes
The roof type decides how much heat stays in. Here is how the common types behave.
- Polycarbonate. Multiwall sheets trap a little air between their skins, which helps, but they remain thin and light. They are the type most associated with cold winters and hot summers.
- Glass. Double-glazed units perform better than a single pane, and modern coatings help further. They are still glazing, however, and the roof still loses heat faster than a solid construction would. Our page on a glass conservatory roof compares the options.
- Solid insulated roof. A solid roof carries real insulation, plasterboard below and tiles or another covering above. It behaves like the roof of a house. That is why people who want to use the room all year often choose this option.
- Lightweight tiled roof. A lighter system built to suit an existing frame gives a similar effect with less structure.
For comparison, the energy rules in Approved Document L expect 0.15 W/m²K from a new roof, a standard a thin glazed roof is nowhere near.
Other causes of a cold room
The roof is the main factor, but it is seldom the only one. Before spending on a new roof, check these.
- Gaps and draughts. Perished seals, loose opening vents, worn door and window gaskets and gaps at the wall junction all let cold air in. A feeble draught across the floor often means a gap, not a cold roof.
- The floor. Many older conservatories sit on a slab with little or no insulation under it. Cold comes up through the floor, particularly through tiles.
- The dwarf walls. Cavity and insulation in the low walls vary a lot between conservatories.
- The door to the house. If the conservatory is open to the house without a door, warm air from the house flows into it and out through the roof. Closing the door keeps the house warm and stops the conservatory from being an extra heat sink.
- Moisture. Damp air feels colder. A conservatory with running condensation on its glazing usually has a ventilation or moisture fault to solve.
Why a leaking roof feels colder too
A roof that lets in rain also lets in cold air, and damp insulation or a wet frame loses heat much faster than dry material. If the conservatory has become colder over a few winters, while the leaks have grown, the two problems may share a cause: ageing seals and sheets. We look at the common causes in our answer on why a conservatory roof leaks.
Fixing the leak will not turn a glass room into a warm one, but it stops the loss that comes from gaps and wetness. It also lets you judge fairly whether a more substantial roof change is worth it.
What changes actually help
Some of the improvements are cheap, others are structural. In rough order of effort, they run like this.
- Seal the draughts. Renew seals and gaskets, adjust vents and doors, and close gaps at the edges.
- Use the room sensibly. Heavy curtains or insulated blinds help in the evening, and a closed door to the house stops heat from the house draining away.
- Insulate the floor and low walls. This helps comfort at ground level and is independent of the roof.
- Upgrade the roof. A better-glazed roof or a fully insulated solid one brings the largest gain. Our guide to how to insulate a conservatory roof goes through the routes.
Going solid brings in a Building Regulations application, which we explain on the main conservatory roof replacement page. The same page lists the types if you are comparing them.
Cold in winter, hot in summer
The same thin roof that loses heat in January overheats the room in July. A conservatory that is a fridge in winter and an oven in summer is a classic sign of an under-performing glazed roof, and a single roof upgrade tends to help with both. We look at the summer half in our answer on how to stop a conservatory too hot in the warmer months.
If you want to find out which of these factors apply to your own room, ask for a survey. It costs nothing, we photograph the roof, the seals and the junctions on your conservatory, and any work you choose is priced firmly in writing afterwards. The phone number is 01225 285181, or you can write to us on WhatsApp.
