A conservatory is the only room in the house built to let the weather in, and most of the time that is exactly the trouble. In July the air under the glass can feel like a greenhouse, and in January you can see your breath beside a radiator that is working flat out. The cause is rarely a mystery once someone looks up. The roof is the largest surface exposed to the sun and the night sky, so it decides most of the heat that arrives and most of the heat that leaves. This guide sets out how we diagnose a room that swings between extremes, what each fault points to, and which cures are worth paying for.
Why a conservatory swings so far in both directions
Think about what a normal room has that a conservatory lacks. A brick and block cavity wall holds heat and releases it slowly, so the temperature indoors moves gently through the day. A conservatory has very little of that thermal mass. Its walls are mostly glazing and a low dwarf wall, and its roof is a thin skin of glass or plastic. Sun comes straight through and warms the floor, the furniture and the air, and as soon as the sun drops or a cloud passes, the heat leaves through the same thin skin.
The roof is where this matters most. Warm air rises, so in summer it collects under the ridge and has nowhere to go, and in winter that same air is the first to be lost through a roof with no resistance to heat flow. A glazed or polycarbonate roof installed years ago may also pre-date modern coatings and multi-chamber sheets, so it can let in more sunshine and let out more warmth than a roof fitted today. That is why the same room can be unbearable at three in the afternoon and bitter by eight in the evening.
Too hot: where the heat is coming from
Overheating comes from three things working together: sunshine entering, warm air trapped, and nothing to take it out again. The first job is to find which of the three is the biggest on your room.
Sunshine entering is mostly about the roof covering and which way the room faces. A south or west facing conservatory gets the longest, strongest sun, and a clear glass or clear polycarbonate roof passes most of it straight through. Glass with a solar control coating, applied as a thin layer in the glazing unit, reflects a share of the sun's heat while still letting daylight in. Tinted or opal polycarbonate does something similar, though usually at the cost of a dimmer room. If your roof is plain clear sheet and the room faces the afternoon sun, that is the first place to look.
Trapped air is the second cause. Many older conservatories have a couple of small opening windows in the side frames and nothing at the top. Hot air rises and stays under the ridge, so the lower windows only move the cooler air around. A roof vent or opening roof light at the high point lets the heat escape and pulls cooler air in at the sides, which is the same stack effect that ventilates a house loft. Check whether the vents you have actually open. Hinges and stays on old uPVC frames seize, and a vent that cannot open does nothing for you.
The third cause is heat soaking into the room itself. A dark tiled floor and heavy furniture absorb the sun and then hand it back after the sun has gone. That is not a roof fault, but it explains why a room can stay stuffy into the evening, and it is worth knowing before you spend money on the roof.
Too cold: where the heat is going
A cold conservatory is a heat loss problem, and the roof is the biggest loss. Single glazed roofs are the worst. A twin-wall or multi-wall polycarbonate sheet holds some air in its channels and does better, and a sealed double glazed unit with a low emissivity coating (a very thin metallic layer that reflects heat back into the room) does better again. Even so, no glazed roof comes close to the resistance of a properly insulated solid roof, which is why a conservatory with a glass or plastic roof feels cold at night however much heating you put into it.
The second loss is through the junctions. The conservatory meets the house wall along a flashing (a strip of lead or membrane that laps over the roof edge and is tucked into the brickwork) and a head detail, meets the frame at the eaves, and meets the ridge where the roof sheets are capped. Each of those lines can leak air. You can often find them on a windy day with your hand or a lit incense stick. Gaps at the wall plate, tired seals around the glazing bars and worn rubber gaskets on polycarbonate sheets all let cold air in and warm air out.
The third is the frame. Older aluminium frames without a thermal break act as a bridge, carrying cold straight through to the inside face. This is where condensation appears first in winter, and it can fool people into thinking the roof is leaking. If the dripping is on the cold surfaces of the frame and the glass, it is probably condensation. If it comes through at a joint after rain, it is a different problem, and our guide to leaking conservatory roofs covers that.
A quick way to tell which fault you have
Before you buy anything, spend a week noting when the room is uncomfortable. The pattern tells you more than any brochure. This table is the one we use as a first pass at a survey.
| What you notice | Most likely cause | First thing to try |
|---|---|---|
| Unbearable on sunny afternoons, fine at night | Clear roof sheet and no high-level ventilation | Roof vent, external shading, or a solar control upgrade |
| Cold the moment the sun goes in | Thin roof covering with little resistance to heat loss | Insulated panels, or a new roof with proper insulation |
| Draughts on a windy day | Failed seals at the wall junction, eaves or ridge | Reseal, replace gaskets, check the flashing |
| Streaming glass and frames in winter | Condensation on cold surfaces, poor ventilation | Trickle vents, extraction, warmer glazing |
| Hot in summer and cold in winter | The roof covering itself is the wrong build-up | Survey, then consider replacement |
The cheaper fixes, and what they really do
Many people reach for blinds first, and they are a fair start. Internal blinds stop the sun hitting the floor and the people, but the heat has already come through the roof into the room, so they only reduce glare and some radiant heat. Blinds fitted outside the glazing, or shading sails above the roof, stop more heat before it enters. Both are modest in cost compared with roof work, and neither helps in winter. We compare this approach in more detail in our short answer on conservatory roof blinds or a new roof.
Window film is the next step. A reflective solar film applied to the inside of the glass cuts some of the sun's heat. It can be a sensible answer for a glass roof in good condition, though it darkens the room, can void a glazing warranty, and is not a cure for winter cold. Roof vents and opening roof lights cost more but do something blinds cannot, which is to let the hot air out. If your conservatory has no high-level opening at all, adding one is often the most effective single step for summer.
For winter, the usual answer on a polycarbonate roof is to improve the covering. Thicker or multi-chamber polycarbonate improves on a thin sheet, and insulation panels can be fitted over or under the existing roof structure. We set out how those options compare, and where they fall short, in insulating a conservatory roof. Panels can also cause condensation if they are fitted without thought for ventilation, which our answer on insulation panels and condensation explains.
When only a new roof will fix it
There comes a point where patching a roof stops being sensible. If the glazing is clear single sheet, the bars are corroded or sagging, the sheets have gone cloudy or brittle, and the room is both too hot and too cold, you are paying twice to fight the same fault. A replacement roof built for the job can deal with both extremes at once: insulation to keep heat in, a ceiling or liner that stops it radiating down, and a covering that does not pass the full strength of the sun.
The choice is between a new glazed roof with better glass, and a solid roof, typically a lightweight tiled system with insulation and a plastered or boarded ceiling. They suit different rooms, and our comparison of glass, polycarbonate or solid conservatory roofs sets out the trade-offs. A solid roof is the most effective cure for both heat and cold, though it makes the room darker and costs more. Typical UK ranges for a solid tiled conservatory roof run from £6,100 to £15,000, which works out at about £650 to £1,200 for every square metre. Treat that as a guide only: the number we hold you to is the fixed quote written up once we have been on the roof.
A solid roof is also heavier than glass, and going from glazing to a solid covering brings in Building Regulations, since the frame and foundations have to prove they can carry the added weight. We cover that check in the frame and foundation checks for a solid roof, and the paperwork in conservatory roofs and Building Regulations. We handle that side where the work is notifiable.
How we check it on your conservatory
We start with a free survey, and for this problem we look at more than the roof. We check which way the room faces, what covering is on it, whether any vents open, how the roof meets the house wall, the state of the seals and gaskets, and whether the frame has a thermal break. We photograph what we find so you see your own conservatory, not a diagram.
Then we tell you plainly which fix fits. Sometimes a vent and an external blind will do the job and we will say so, and sometimes the honest answer is that the roof has reached the end of what repair can do. Either way the price arrives as a fixed written quote, and anything we build is covered by a written guarantee on our workmanship, ten years long. To book a survey, the number is 01256 213956, and WhatsApp reaches the same team, or read about our full conservatory roof replacement in Basingstoke service.
