Opening rooflights for the stack effect

Buoyancy, and the pressure a warm room generates

Air at 21 degrees weighs about 1.20 kilograms per cubic metre. The same air at 11 degrees weighs about 1.24. That difference of four per cent sounds trivial and it is the entire engine.

Stack a column of the lighter air inside a building against a column of the heavier air outside it, and at the top of the column the inside air is pushing outwards while at the bottom the outside air is pushing inwards. Nothing is being driven by anything other than weight. The taller the column and the bigger the temperature difference across it, the harder both ends push. A single storey room with a rooflight two metres above the door has a short column and generates very little. A Regency stairwell in Montpellier with a rooflight ten metres above the hall floor generates five times as much from exactly the same temperature difference.

The two figures that set the flow, and the rule of thumb behind them

The pressure a stack develops works out at roughly 0.04 pascals for every metre of height and every degree of temperature difference. That figure is worth memorising because it turns a vague idea into something you can check.

Take a loft bedroom in a Severn Vale semi with the rooflight 4.5 metres above an open ground floor window, on a July night when the room is at 24 degrees and the outside air has fallen to 16. Eight degrees across 4.5 metres gives about 1.4 pascals. That is a small number by the standards of a fan, and it is a large number by the standards of a house, because it acts across the full area of both openings continuously and costs nothing to sustain.

Both figures are multipliers, so height cannot be traded against temperature. Doubling the height difference doubles the pressure. Waiting two hours for the outside air to drop another four degrees does the same thing for free.

The neutral pressure level, and where it sits in a house

Somewhere between the highest opening and the lowest there is a horizontal line where inside and outside pressures are equal and no air crosses the envelope at all. Above that line air leaves. Below it air enters. That line is the neutral pressure level, and its position is decided entirely by the relative sizes of the openings.

Make the rooflight large and the ground floor openings small, and the neutral level rises towards the rooflight. The room under the glass then sits below the line for most of its height, which means it is drawing air in rather than pushing it out, and the strategy quietly stops working. Make the low openings generous and the neutral level drops, the whole upper floor sits in the outgoing zone, and the rooflight does the job it was specified for.

One opening at the top, and why it achieves so little on its own

A rooflight opened on its own in a closed house is the single most common version of this, and it disappoints people.

With no deliberate inlet, the only air available is whatever leaks through the fabric: gaps at skirtings, an unsealed loft hatch, a letterbox, a chimney if one is still open. In a modern airtight house at Quedgeley or Bishops Cleeve those paths add up to very little, so the stack pressure builds and no flow follows. The sash sits open, the room stays warm, and the owner concludes the window is too small. The window is fine. There is nowhere for the replacement air to come from.

The same rooflight with a ground floor window on the latch will often move ten times the air.

Sizing the low level inlet against the rooflight above it

Two openings in series behave like two resistances in series, and the smaller one dominates. The combined effective area is found from the reciprocal of the squares: one over the effective area squared equals one over the inlet area squared plus one over the outlet area squared.

The practical consequence is worth spelling out. Two openings of 0.3 square metres each give a combined effective area of about 0.21 square metres. Enlarge the rooflight to 0.6 while leaving the inlet at 0.3 and the combined figure rises only to about 0.27. Nearly all of the gain from doubling the glass is thrown away by the inlet.

Equal areas top and bottom extract the most flow from a given total. Where they cannot be equal, the low level opening is the one worth being generous with, because it is cheaper than glass and easier to provide.

Opening angle, and the free area a sash actually delivers

Free area is not frame area, and on an opening rooflight it is not sash area either. It is the smallest cross section the air has to squeeze through on its way out.

On a top hung sash the aperture is a wedge: widest at the bottom edge, closing to nothing at the hinge. A 1180 by 780 sash opened to 200mm at the free edge presents rather less than the 0.2 square metres the arithmetic first suggests, because the two triangular side gaps contribute little. On a centre pivot the sash divides the frame into an upper slot and a lower one, and only the upper slot is doing stack work. Opening a centre pivot wider eventually starts to reduce the useful top slot rather than increase it.

Chain stroke is what sets the angle on a powered unit, and it is chosen at order stage rather than adjusted later. The actuator page covers the strokes available.

Cheltenham townhouse stairwells, four storeys of usable height

Central Cheltenham gives the best stack geometry in the county. A Lansdown or Pittville townhouse runs basement or lower ground through to a second or third floor, with a single open stairwell connecting every level and a shallow lead flat at the top hidden behind the parapet.

A kerb mounted opening rooflight on that flat sits eight to eleven metres above the hall. The stairwell is already a chimney; the rooflight simply gives it a top. On a hot evening the difference is immediate: open the front door lights or a lower ground window, open the rooflight, and the whole house begins to move air within a minute or two.

Two constraints on those roofs. The unit is usually invisible from the street, which changes the consent conversation, and it is unreachable from any floor, which decides the operation type before anything else does.

Severn Vale loft conversions, and the heat that collects at the gable end

A loft conversion in Longlevens, Hucclecote or Tuffley has a specific problem. The rooms are shallow, the ceiling follows the rafters, and the highest air in the room collects in the triangular volume at the gable end or against the ridge, which is exactly where nobody has put an opening.

Warm air stratifies. On a still August afternoon there can be four or five degrees between knee height and the ridge in a converted loft, and the top layer is the layer you want to lose. A rooflight placed high on the slope reaches into that layer. A rooflight placed low, where it gives a better view and a lower cill, sits underneath it and vents air that was already comfortable.

Where the room has a gable window, the pairing writes itself: gable window low as the inlet, rooflight high on the slope as the outlet, and the diagonal between them crosses the whole room.

Stroud and Chalford houses cut into the valley side

The valley houses through Chalford, Brimscombe and Rodborough are three storeys at the back and one at the front, built into the bank with a retaining wall tight behind them. That gives an unusual amount of internal height and very few elevations to put openings in.

It also gives something the vale does not: cool air. Air drains downhill at night, and a house on the valley side sits in that flow from about an hour after sunset. A low opening on the downhill face and a rooflight at the top of the house make use of it, and the temperature difference across the stack in these valleys is routinely two or three degrees better than the same house would see on the flat.

The catch is access, which affects installation rather than performance and is priced from the survey.

Wind, and what it does to a stack that was working

Wind and buoyancy are separate drivers and they do not always agree. On a calm night the stack has the field to itself. In a breeze, wind pressure on the roof usually helps, because air passing over a pitched slope creates suction at the surface and pulls the sash aperture outwards.

The exception is a rooflight on a slope facing directly into the wind, on the windward side, where positive pressure can push against the outgoing flow and stall it. On an exposed Forest of Dean slope facing the prevailing south west, that happens often enough to be worth planning for. Where a roof has openings on two slopes, one of them is always on the sheltered side.

Wind also introduces the rain problem, which is what a sensor exists to solve. Those are covered on the rain sensor page.

The summer night cycle, and the hours that do the cooling

Night purge is the most valuable thing a stack does in a British summer, and the timing is narrower than people assume.

Outside air in Gloucester typically falls below internal temperature somewhere between eight and nine in the evening, and reaches its minimum around four in the morning. The stack pressure therefore rises through the night and peaks in the two hours before dawn. A rooflight opened at bedtime and closed at breakfast catches nearly the whole of it. A rooflight opened at six in the evening, when outside air is still warmer than the room, is importing heat.

The purpose is not to cool the air. It is to cool the mass: plaster, screed, blockwork and the timbers of the roof. Fabric taken down four degrees overnight absorbs heat all through the following day, which is why the second and third nights of a heatwave matter more than the first.

Winter, and running the stack in short bursts

In January the temperature difference is twenty degrees or more, so the same openings generate four or five times the pressure and move air very fast indeed. Left open, a rooflight in that condition empties the heat out of a house in minutes.

Used deliberately, that is exactly what you want. Five minutes of a fully open rooflight and an open window downstairs will change the air in a house completely and take almost no heat out of the fabric, because plaster and masonry give up their warmth far more slowly than air does. It is the correct way to clear a bathroom, a kitchen or a bedroom in winter, and it is far better than leaving a small gap open all day.

Doors, and the path the air has to take through the house

A stack needs a continuous route from the low opening to the high one, and internal doors are the most common thing blocking it.

A closed bedroom door with a 10mm gap under it presents an effective area of around 0.01 square metres, which is two per cent of the rooflight above it and reduces the whole arrangement to that figure. Undercuts, transfer grilles or simply leaving the door on the latch are what make the difference. On a loft conversion the door at the bottom of the new stairs is usually a fire door with a self closing device and cold smoke seals, and it is a legal requirement rather than a preference, so the path has to be planned around it rather than through it. In that case the inlet belongs on the same floor as the rooflight.