Where this starts
A conservatory is a box with a very high ratio of transparent envelope to floor area, and glass is both a poor insulator and an excellent solar collector. Those two facts together produce the whole complaint: sunshine pours in and cannot get out, and at night the same envelope loses heat far faster than a wall would.
THE ROOF IS THE DOMINANT SURFACE and it is the one people think about last. A conservatory roof faces the sky, which means it sees the sun at its most direct in summer and radiates to a cold sky at night in winter. On most conservatories the roof is also the WORST-performing part of the envelope — polycarbonate or early double glazing — so it is simultaneously the biggest gain in summer and the biggest loss in winter. Any serious improvement starts there.
ORIENTATION TELLS YOU WHICH HALF IS WORSE. A south or west-facing conservatory is a summer problem first: it takes the afternoon sun when the outside air is already warm. A north-facing one is a winter problem: little solar gain to offset the losses, so it is cold whenever the sun is not on it. East-facing warms early and cools by afternoon. That is worth establishing because it decides whether shading or insulation is the better spend.
VENTILATION IS THE CHEAPEST SUMMER FIX and it is usually inadequate by design. Hot air rises and collects at the top of a glazed roof, and unless there are opening vents up there it stays. Roof vents and a low-level opening together produce a stack effect that removes far more heat than a fan; side windows alone do very little because they let the hottest air sit above them.
WHY HEATING IT IS THE WRONG FIRST MOVE, and worth being direct: a conservatory added to a house is normally required to be thermally separated from it — external-quality doors kept closed — precisely because it cannot be heated economically. Removing that separation and extending the central heating into it turns a seasonal room into a permanent heat loss, and in many cases changes what the building regulations require of the whole assembly.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A poorly performing roof
Most commonPolycarbonate or early sealed-unit glazing overhead, unbearable in summer sun and cold immediately after dusk, with the roof the coldest surface to touch on a winter evening.
How to check: What the roof is made of. It is the largest and usually the weakest part of the envelope in both directions.
No effective high-level ventilation
Most commonHeat stratifying so that it is markedly hotter at head height than at floor level, with only side windows and no roof vents.
How to check: Whether anything opens at the TOP. Hot air collects above any opening that is not at the highest point.
Orientation and no shading
CommonUnusable for specific hours that track the sun, on a south or west elevation, with no blinds, no external shading and no overhang.
How to check: Which way it faces and when it is worst. A problem that follows the sun round is solar gain rather than insulation.
Thermal separation removed
CommonDoors to the house taken out or left permanently open, the house harder to heat since, and the conservatory still cold.
How to check: Whether there is still a closable, insulated separation. Without it the conservatory is heating the outside through the house.
A solid floor with no insulation beneath
CommonA floor cold underfoot all winter, often a slab laid directly on the ground as part of a budget build, with the room slow to warm even when heated.
How to check: Whether the floor is cold to touch in winter. An uninsulated slab is a large surface losing heat to the ground.
Glazing without a solar control coating
Less commonClear glass throughout, very high summer gain, in an installation of some age before solar control became common.
How to check: The age and the specification of the glass. Modern solar control glass cuts a substantial share of the gain without darkening the room.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the largest surface, it faces the sky, and on most conservatories it is the worst-performing part of the envelope in both seasons. Any real improvement starts there.
- 2
Hot air collects above any opening that is not at the top, so a room with only side windows cannot ventilate its own heat however many of them are open.
- 3
A problem that tracks the sun is solar gain and wants shading; one that is worst at night and in dull weather is fabric loss and wants insulation.
- 4
A conservatory is normally required to be thermally separated because it cannot be heated economically, and removing that makes the whole house harder to heat.
- 5
An uninsulated slab on the ground is a large continuous loss and explains a room that will not warm up even with heating in it.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where replacing the roof is being considered, since a solid or tiled replacement changes the loads on the existing frame and base and is usually a building control matter.
- Where the separation between conservatory and house has been removed, because reinstating it or regularising the removal has regulatory consequences for the whole assembly.
- Where heating is being extended into it, which in most cases requires the separation question to be settled first and may not be permitted as a simple addition.
- Where the frame or the base shows movement, cracking or water ingress, which is a structural question about a lightly built addition rather than a comfort one.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.