An electric rooflight and a trickle vent do different jobs and you generally need both. The rooflight provides purge ventilation, rapid clearing of moisture and pollutants when you open it. A trickle vent provides background ventilation, a continuous low-rate trickle of fresh air that runs all the time. Under Building Regulations Part F these are separate duties, and one cannot substitute for the other. A trickle vent cannot clear a steamy kitchen, and a rooflight left shut does nothing for background air. This page explains the difference and where each matters.
Purge against background: the core difference
| Aspect | Electric rooflight | Trickle vent |
|---|---|---|
| Part F duty | Purge ventilation | Background ventilation |
| Rate | High, when open | Low, continuous |
| Job | Clear moisture and pollutant spikes fast | Keep air fresh between purges |
| Measured as | Openable area, at least 5% of floor area | Equivalent area in mm² |
| Effect on a steamy kitchen | Clears it in minutes | Negligible on a spike |
| Effect on overheating | Vents heat directly at ceiling level | Little, rate is too low |
| Runs when | You open it, or the schedule does | Always |
What purge ventilation does
Purge ventilation is the big, fast air exchange that clears a spike: the steam off a pan of pasta, the moisture from a shower, the stale air that has built up overnight. It needs a large openable area, and Part F sets that at a minimum of 5 per cent of the room’s floor area. An opening rooflight is purge ventilation, and because it sits at ceiling level where warm moist air collects, it is the most effective openable area you can have for the job.
What background ventilation does
Background ventilation is the slow, constant trickle that stops the air going stale between purges. It runs day and night at a low rate, measured as an equivalent area in square millimetres, and it comes from trickle vents in window and door frames or from a mechanical ventilation system. It keeps humidity from creeping up over hours and days, which is a different problem from clearing a spike.
Why one does not replace the other
People sometimes hope a good opening rooflight lets them skip trickle vents, or that trickle vents mean they do not need an opening rooflight. Neither works.
- A trickle vent cannot clear a moisture spike. Its rate is far too low to shift the steam from cooking or a shower before it condenses on the coldest surface. You would be leaving the moisture to settle.
- A rooflight left shut does nothing for background air. Overnight, or when you are out, a closed rooflight provides no ventilation at all, so humidity builds up with nothing to carry it away. That is what the trickle vent is for.
- They are separately required. Part F asks for both duties in a habitable room, which is why a compliant extension has trickle provision and adequate openable area. The full requirement is on the ventilation Building Regulations answer.
The damp-extension trap. A common cause of condensation in a new extension is relying on bi-fold doors for purge, which nobody opens in winter, and having too little background trickle. The result is humidity that never clears. The fix is an opening rooflight for the purge duty at the high point of the room, plus adequate background ventilation. We cover the sealed-extension problem on the ventilation in sealed extensions page.
Overheating: where the rooflight wins outright
On overheating, there is no contest. A trickle vent’s rate is far too low to remove solar and casual heat gain. An opening rooflight at the highest point of the room vents hot air directly, because heat stratifies at ceiling level, and paired with a low inlet it drives the stack effect with no fan. If overheating is your concern, the rooflight is doing the work, backed by good glazing that stops the heat arriving, which the energy efficient skylight installation page covers. The wider ventilation logic is on the electric skylight installation page.
Common questions
Can an opening rooflight replace trickle vents?
No. A rooflight provides purge ventilation, the rapid clearing of moisture and pollutants when it is open. Trickle vents provide background ventilation, a continuous low-rate fresh-air trickle that runs all the time. They are separate duties under Part F. A rooflight left shut overnight does nothing for background air, so it cannot replace the trickle vents. A compliant room needs both.
Do trickle vents make an opening rooflight unnecessary?
No. A trickle vent’s rate is far too low to clear a moisture spike from cooking or a shower, so it cannot do the purge job. It also does little for overheating. An opening rooflight clears a steamy kitchen in minutes and vents heat where it collects at ceiling level. The trickle vent keeps the air fresh between those purges. The two solve different problems and Part F asks for both.
Which matters more for condensation?
Both, at different timescales. The opening rooflight clears the moisture spikes from cooking and showering before they condense, and the trickle vent stops humidity creeping up over hours and days between those purges. A new extension that runs damp usually lacks one or the other, most often relying on bi-fold doors nobody opens in winter for purge, and too little background trickle. Fixing it means adding the missing duty.
Get a fixed price for proper ventilation
Tell us the room, its layout and where the moisture problems are. We advise where an opening rooflight best provides the purge duty and how it works with your background ventilation, and give you one written price covering the unit, the controls and the electrical connection, fixed after survey with a 10-year workmanship guarantee.
Request a fixed quote for an opening rooflight across Ipswich, Suffolk and north Essex.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.