Why does my conservatory get so hot, and can I make it usable?
A glass box collects far more solar energy than it can lose, and the roof does most of it. What genuinely works, roughly in order of effect, and what is a waste of money.
- Difficulty
- beginner
- Time
- 25 min
- Read
- 6 min
- Safety
- caution
Short answer
The roof is the problem, not the walls — it faces the sun for most of the day and glass or polycarbonate lets solar radiation in and then traps it. Shade or replace the roof first, ventilate high and low second, and use internal blinds only as a last resort, because by the time light hits an internal blind the heat is already inside.
Conservatories are the worst-performing space in most houses in both directions — freezing in January, unusable in July. The July problem is straightforward physics: short-wave solar radiation passes through glazing easily, is absorbed by the floor and furniture, and is re-radiated as long-wave heat that the same glazing does not let back out. Add a roof aimed at the sky and you have built a solar collector. Almost everything that works is about stopping the light before it gets in.
Safety
Step by step
- Accept that the roof is doing most of it.A conservatory roof presents a large area to the sun for most of the day, at an angle that receives strong radiation from mid-morning to late afternoon. Side glazing only faces the sun for part of the day and is often partly shaded by the house, fences or planting. Any intervention on the roof outperforms the same money spent on the walls.
- Shade the roof from the outside if you can.External shading — sails, awnings, mesh fixed above the glazing, or a pergola planted over it — stops the energy before it enters. External is dramatically more effective than internal for the same reason external greenhouse shading beats internal blinds. The practical obstacles are fixing and wind loading, so anything fitted outside must be genuinely secure.
- Consider a solar-control or replacement roof as the real fix.Replacing polycarbonate with a solar-control glass unit, or with an insulated solid roof, is the change that turns a conservatory into a room you can use in both seasons. It is the expensive option and it is the one that works. A solid or partly solid roof usually brings building regulations into play and can affect whether the structure still counts as an exempt conservatory, so check before committing.
- Get high-level ventilation working.Hot air collects at the top of the roof and stays there. Roof vents, or opening top-hung windows at high level, let it out; doors and low windows then let cooler air in underneath. Ventilation only at floor level cannot shift a hot layer above it.
- Create a genuine through-path.Open something low on the shaded side and something high on the sunny side, and the temperature difference does the work for you. Opening one door achieves far less than two openings at different heights.
- Fit internal blinds knowing their limit.Internal blinds intercept light after it has entered the glazing, so the heat is already in the room; the blind itself gets hot and re-radiates. Reflective-backed roof blinds do meaningfully better than fabric ones. Useful for glare and for privacy, worth having, but not the answer on their own.
- Reduce what the room stores.A tiled or stone floor in direct sun is a heat store that keeps the room warm into the evening. A rug over the sunlit strip, or blinds that keep the sun off the floor specifically, cuts the amount of heat the room holds overnight.
- Separate it from the rest of the house.Keep the door between the conservatory and the house shut on hot days. A conservatory left open to the hallway heats the whole ground floor. This is also why conservatories should have doors between them and the house in the first place.
- Ventilate it at night as well.If the structure is secure enough to leave a high vent open overnight, the room starts the next day much cooler. If it is not secure, opening it first thing before the sun is on it is the next best thing.
Tips
- A conservatory that is unusable in summer and unusable in winter is not really a room, and the arithmetic on a replacement roof often looks better than people expect once they count the months they cannot use the space.
- Extending the house heating into a conservatory is normally not permitted without the required separation, and it does nothing for the summer problem anyway.
- White or reflective external shading beats dark external shading — dark mesh absorbs the energy and then radiates some of it inwards.
- Plants left in a conservatory in a heatwave will cook. Move them out, or treat the space as a greenhouse and shade and ventilate it like one.
Common mistakes
- Buying internal roof blinds as the first and only measure — It is the most commonly sold fix and the least effective per pound, because it acts after the energy has already passed through the glazing.
- Fitting a portable air conditioner and leaving the shading undone — A single-hose unit in an unshaded glass room is fighting a solar gain that keeps arriving. The running cost is high and the result is disappointing.
- Leaving the door to the house open on a hot day — It turns the conservatory into a heater for the whole ground floor rather than a room you are choosing not to use.
- Leaving a pet shut in with the doors closed — The temperature rises fast and far above outdoor air. It is the same hazard as a parked car and it kills animals every summer.
If it doesn't work
Unusable from late morning onwards
Cause: East or south-facing glazing with no external shading — Fix: Shade externally before mid-morning, and get high-level ventilation open early rather than waiting until it is already hot.
Hot until late at night
Cause: A solid floor storing heat and re-radiating it — Fix: Keep direct sun off the floor during the day, and ventilate high overnight where it is secure to do so.
Cooler outside than in but the room will not shift
Cause: Only low-level openings, so the hot layer at the top is trapped — Fix: Open something at high level — a roof vent or top-hung window — to let the hot layer out and draw cooler air in below.
Alternatives
- External shade sail or awning: The cheapest genuinely effective measure. Stops radiation before the glazing. Needs secure fixing and must cope with wind.
- Solar-control glazing replacement: Replacing polycarbonate with a modern solar-control unit cuts the gain substantially while keeping the light. Expensive, and the most durable answer.
- Insulated solid roof: Turns the space into a usable year-round room. Usually engages building regulations and may change the structure's exempt status — check first.
- Reflective-backed internal roof blinds: Materially better than plain fabric and much cheaper than reglazing, but still acting after the heat is inside.
Questions people ask
Do internal blinds do anything at all?
They help with glare, privacy and some of the radiant heat, and reflective-backed ones are noticeably better than fabric. But they cannot match external shading, because the energy has already passed through the glass by the time it reaches them. Treat them as a useful addition rather than the solution.
Is a portable air conditioner worth it for a conservatory?
It is the hardest room in the house to cool mechanically, because the solar gain keeps arriving all day and the fabric is poorly insulated. If you do use one, do the shading first, use a dual-hose model if you can, and expect a high running cost for a modest result.
Will a solid roof need building regulations approval?
Usually yes, and it can also affect whether the structure still qualifies as an exempt conservatory, which has implications for how it must be separated from the house and heated. Ask building control before ordering anything.
Why is it still hot hours after sunset?
The floor, walls and furniture absorbed energy all day and release it slowly. Keeping the sun off the floor specifically, and ventilating at high level in the evening, is what breaks that cycle.