Guide · Electric and Opening Rooflights
Opening rooflights for the stack effect
Warm air rises, and an opening at the highest point of a room lets it leave. That is the whole mechanism. The stack effect is buoyancy doing the work of a fan, and in a room with enough height above head level it moves a genuinely useful volume of air with no power, no noise and nothing to run.
This page is about designing for it rather than hoping for it: how much height the effect needs, where the incoming air has to come from, what an opening angle does to the free area, and the days of the year when the stack quietly stops working. It is written for the rear extensions, loft conversions and double height voids going up across Chelmsford and the thirty miles of Essex around it, where ceilings have got taller and the glass has moved into the roof.
Density difference, and the pressure that drives the flow
Air at 21 degrees is lighter than air at 8 degrees. The difference is small, roughly four per cent by density, but it acts over the full height of a column of air and it produces a real pressure difference between the top of a room and the bottom.
Warm indoor air is pushed upward by the heavier cold air outside trying to get underneath it. Open something at the top and that warm air leaves. Open something at the bottom and cold air is drawn in to replace it. Neither opening does much on its own, and that is the point most people miss when they specify a single roof opener and expect a through draught.
Neutral pressure level, and where the room changes its mind
Somewhere between the two openings there is a horizontal plane where inside and outside pressure are equal. Above it, air flows out. Below it, air flows in. That plane sits closer to whichever opening is larger.
The practical consequence is that if the roof opening is much larger than everything below it, the neutral plane rises towards the ceiling and the driving height shrinks. A big rooflight with the windows shut is not a ventilation strategy. It is a chimney with the bottom blocked.
Height between openings, and why it is the whole equation
Flow rate scales with the square root of the vertical distance between the inlet and the outlet. Doubling that distance does not double the flow, it multiplies it by about 1.4, but it is the one variable that is fixed at design stage and can never be changed afterwards.
In a standard 2.4 metre room with a rooflight in a flat ceiling and a window head at 2.1 metres, the useful separation is a few hundred millimetres and the stack contributes very little. Raise that ceiling to 3.2 metres in a vaulted extension and the separation becomes well over a metre. The same rooflight, the same window, and a different result entirely.
The stack effect is bought at the design stage in ceiling height, not at the order stage in product specification.
Where the incoming air has to come from
Every cubic metre that leaves through the roof has to be replaced through something lower down, and the inlet is the part nobody draws. A door to a hallway, a window on the tilt setting, a set of bifolds cracked open by 50mm, a trickle vent: any of these will do, and their combined free area needs to be at least as large as the roof opening or the roof opening becomes the limiting factor.
Where the inlet is genuinely small, the flow through the rooflight drops to whatever the inlet allows and the extra glass area is doing nothing. On a still, cold morning this shows up as a rooflight that is open and a room that feels unchanged.
Free area against opening angle
Brochure figures quote the sash size. What matters is the free area, which is the gap the air actually passes through, and on a top hung unit that is roughly the perimeter of the opening multiplied by the perpendicular gap, not the area of the glass.
| Sash 1000 x 1000mm, top hung | Approximate gap at the bottom edge | Effective free area |
|---|---|---|
| Opened 10 degrees | About 175mm | Roughly 0.25 square metres |
| Opened 20 degrees | About 345mm | Roughly 0.45 square metres |
| Opened 30 degrees | About 500mm | Roughly 0.6 square metres |
The numbers stop climbing in proportion once the sash is well open, because the sides of the opening start to shade each other and the geometry flattens off. The useful reading is that the first fifteen degrees buy most of the ventilation, and that a unit which only manages ten degrees is doing about half the work of one that manages twenty.
Temperature difference, and the days the stack does nothing
The driving force is the gap between inside and outside temperature. On a January evening with 20 degrees inside and 3 degrees outside, the stack is strong and a small opening shifts a lot of air. On a July afternoon with 26 degrees inside and 28 outside, the difference reverses and the flow can run backwards down through the rooflight.
This is why an opening rooflight is not a substitute for solar control glass on a south facing plane. The hours you most want the heat gone are the hours the stack is at its weakest. Getting the glazing specification right comes first, and the opener manages what is left.
Night cooling, and the hours that do the real work
The stack effect earns its place overnight in summer. By two in the morning the outside air in Essex has usually dropped to 13 or 14 degrees even after a hot day, and a room sitting at 25 has a substantial difference to work with.
Left open from eleven until six, a roof opener will pull the thermal mass of a room down by several degrees, so the following day starts from a lower base. Doing this reliably means the unit has to be motorised and scheduled, because nobody gets up to open a rooflight at midnight. That is one of the strongest arguments for an electric rooflight in a room that overheats.
Vaulted ceilings, and the volume above head height
A vaulted or raised ceiling does two things for ventilation. It increases the separation between inlet and outlet, and it creates a reservoir of warm air above the level people occupy.
That reservoir is genuinely useful. Cooking steam, body heat and the warm layer that builds through an afternoon all collect up there rather than at seated level, so the air being expelled through the rooflight is the hottest and most humid air in the room. A flat 2.4 metre ceiling mixes everything together and the opener removes an average rather than the worst of it.
Positioning the opener relative to the ridge
On a pitched slope, the higher up the slope an opening sits, the more of the room’s warm layer it can reach. A roof window low on a slope, close to the eaves, sits below the warmest air and vents the middle of the room instead of the top.
There are limits. Roof windows want a reasonable distance below the ridge for the flashing and the head detail to work, and the rafter layout usually settles the position anyway. On a flat roof the equivalent question is where the ceiling is highest, which on a monopitch ceiling means the opener belongs at the tall end and not centred for symmetry.
Kitchens, cooking moisture and where the air actually goes
A hob produces steam, aerosolised fat and combustion products, all of which rise fast in a plume. An extract hood over the hob catches that plume at source and is the right tool for it. A roof opener catches what escapes the hood, which in an open plan room is a great deal.
The sensible arrangement puts the rooflight away from directly above the hob, so the two are not competing for the same air, and uses the opener for the general clearing job: the warm, damp, faintly greasy layer that fills a kitchen extension after an hour of cooking. Sited over an island or a dining end rather than the cooking end, it clears the room without pulling the hood’s plume sideways.
Bathrooms, utility rooms and humidity as the trigger
Wet rooms are the clearest case for stack ventilation because the moisture is buoyant. Steam from a shower is warmer and lighter than the surrounding air and heads straight for the ceiling.
An opening rooflight above a shower clears that plume with no fan noise, provided the unit is rated for a humid environment with stainless fixings and a gasket that will take constant wetting. Pairing it with a humidity sensor so it opens when the moisture rises and shuts when it has gone removes the human element entirely, which matters because the person in the shower is the last person who will reach for a switch.
Loft rooms, and the stack that runs up a staircase
In a converted loft the stack does not stop at the room. The staircase is a vertical shaft connecting three floors, and a roof window at the top of it is venting the whole house rather than the loft alone.
That is powerful and it needs thinking about. In winter it means a roof window left ajar at the head of an open staircase pulls warm air out of the ground floor continuously. In summer, with the front door area open, the same arrangement gives a house-wide flush that no individual window can match. The Victorian terraces and interwar semis across Chelmsford and Colchester with loft conversions are precisely this geometry.
Wind, and how it helps or fights the stack
Wind pressure is usually stronger than buoyancy. Air passing over a roof creates suction on the leeward side, which pulls air out of an opening there, and positive pressure on the windward face, which pushes it in.
Most of the time this reinforces the stack: the roof is the low pressure zone and the opening is at the top, so both effects push the same way. Where it fights, typically with a rooflight on a windward slope in a strong wind, the flow can reverse and cold air is driven down into the room. On exposed sites out towards Maldon, Burnham and the estuary this is worth allowing for by keeping the stroke moderate rather than specifying the widest opening available.
Insect mesh, and the free area it takes back
Anything across the opening reduces the flow. A standard insect mesh has a free area of roughly 60 to 70 per cent of the aperture it covers, so a unit opened to 0.45 square metres of gap delivers closer to 0.3 with mesh in place.
That is not an argument against mesh, which is genuinely worth having on a unit that will sit open on summer evenings with lights on inside. It is an argument for accounting for it when sizing, and for keeping it clean, because a mesh clogged with dust and pollen from an Essex summer loses a great deal more than the manufacturer’s figure suggests.
Background ventilation, and the other half of Part F
Approved Document F asks for two different things: purge ventilation, which is the fast clearing job the opener does, and background ventilation, which is the small continuous exchange that keeps humidity down when everything is shut.
Background air comes from trickle vents in windows and rooflights, sized in equivalent area per room. A rooflight ordered with a ventilation flap in the frame provides it without the sash moving at all, which is worth specifying on a unit high in a vaulted ceiling: the flap deals with the everyday, the motor deals with the occasional. Building Control looks at both figures, and the notification is handled as part of the job.
Specifying an opener for stack performance
Four numbers decide whether an opening rooflight actually ventilates. The vertical separation between it and the lowest inlet. The free area at full stroke, in square metres, not the sash size. The stroke in millimetres and the angle it produces. And the combined free area of everything at low level that will feed it.
Ask for all four before ordering. A supplier who quotes only the sash dimensions is describing a hole in a roof, not a ventilation strategy, and the difference shows up in August. Where the unit is out of arm’s reach, add a fifth question: how it will be told to open, since an unreachable opener that relies on somebody remembering is an ornament. Rain sensing and control options are covered on their own pages, and the wider set of decisions sits on the electric and opening rooflights hub.
Essex rooms where the stack genuinely works
The best candidates in the county share a shape. Rear extensions in Chelmsford, Braintree and Colchester with a vaulted or raised ceiling and bifolds along the garden wall have both the height and the inlet, and a single roof opener transforms them. Loft conversions in the interwar semis along the commuter belt have the staircase shaft and benefit at whole-house scale.
The harder cases are the flat roofed single storey additions across Harlow and Basildon with a 2.4 metre ceiling throughout, where separation is minimal and the opener is doing purge work rather than stack work. That is still worth having, but it should be specified for opening area rather than for height. Weatherboarded and timber framed properties in the Blackwater villages and around Thaxted often want a conservation rooflight profile, which caps the opening angle, so the sizing conversation starts from what the frame allows. Coverage across the county is set out on the areas page, and the broader daylight and ventilation thinking in the guides.
Keep reading
More on electric and opening rooflights
Rain sensors and what they actually do
Read the guide →Wiring an electric rooflight at first fix
Read the guide →Solar powered rooflights without a mains run
Read the guide →Remote controls, wall switches and smart home links
Read the guide →Electric blinds fitted inside the rooflight
Read the guide →
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