Opening rooflights for the stack effect

The physics in one paragraph, without the arm-waving

Air inside a heated house is warmer than air outside, so it is less dense. A column of it therefore weighs less than a column of outside air of the same height. That difference produces a small pressure difference between the top of the house and the bottom: slightly negative at the bottom, slightly positive at the top. Open a hole at the top and air leaves through it. Open a hole at the bottom and air is drawn in to replace it.

The numbers are small. A 5 degree difference over 5 metres of height produces roughly two pascals of driving pressure. Two pascals is about a fifth of what a gentle breeze does to the windward face of a building. That is why the arrangement matters so much more than the size of the unit. You are working with a very small budget of pressure and you cannot afford to waste any of it on restriction.

Height difference does most of the work

Driving pressure scales directly with the vertical distance between the inlet and the outlet. Double the height and you double the flow potential. That single fact should shape where the rooflight goes.

In a two-storey Oxford terrace with a converted loft, a rooflight in the loft slope sits perhaps 7 or 8 metres above the ground-floor front door. That is a genuinely useful column. A rooflight over a single-storey rear extension with the inlet in the same room sits maybe 2.5 metres above it, and the effect is weak enough that wind will overwhelm it most days.

The practical reading is that a rooflight high in a tall volume is doing real ventilation work, while a rooflight in a low flat-roof extension is mostly providing a hole for a fan or a breeze to use. Both are worth having. Only one of them is a stack.

Temperature difference is the seasonal half

The other multiplier is the difference between inside and outside air temperature, and it swings enormously through the year. On a January night with 20 degrees inside and 2 outside, the stack is strong and you want it working in short controlled bursts. On a July afternoon with 22 inside and 28 outside, the difference reverses and air will try to come down the rooflight rather than out of it.

This is why summer ventilation strategy is usually a night-time strategy. Between about eleven at night and six in the morning the outside air drops below the inside temperature, the stack re-establishes in the right direction, and a rooflight left open shifts the heat that built up in the fabric during the day.

It also explains a complaint we hear about. A rooflight opened at two in the afternoon in August does very little, and people conclude the unit is useless. It is not. It is being asked to work against the temperature gradient.

Where the neutral pressure level sits

Somewhere between the lowest opening and the highest one there is a level where inside and outside pressure are equal. Below it, air comes in. Above it, air goes out. That level moves depending on where the openings are and how big each one is.

If the only opening in a house is the rooflight, the neutral level sits roughly in the middle of that opening, and the top half exhausts while the bottom half draws in. Very little net air moves. That is the single most common reason a homeowner reports that opening the rooflight “does nothing”.

Open a window downstairs and the neutral level drops towards it, putting the whole rooflight above it and turning the entire opening into an outlet. Nothing about the rooflight changed. The arrangement did.

Sizing the low-level inlet against the roof opening

Flow through the system is limited by whichever opening is smaller. A large rooflight paired with a 100mm trickle vent downstairs performs like a 100mm trickle vent.

A reasonable working rule is to make the total low-level inlet free area at least equal to the rooflight’s free area, and preferably somewhat larger. Enlarging the inlet beyond the outlet keeps improving things slightly, because it pushes the neutral level further down and puts more of the outlet in exhaust. Enlarging the outlet beyond the inlet does almost nothing.

In practice the inlet is usually an existing window on a lower floor, a door to a cooler hallway, or in newer work a purpose-made low-level vent. It does not need to be architectural. It needs to be open at the same time as the rooflight, which is the part that fails.

Free area is not the same as sash size

Manufacturers publish a free area figure, sometimes called equivalent area, and it is far smaller than the pane. A centre-pivot roof window opened to its normal stop presents a gap top and bottom, restricted by the sash, the frame profile and the friction hinges. A 780 x 1180 unit might give somewhere around 0.1 square metres of free area, not the 0.9 square metres of glass.

Top-hung units generally give more free area at the same nominal size, because the whole bottom edge lifts clear rather than splitting the opening in two. Flat-roof vents that lift on parallel arms give a slot round three or four sides, which sounds small and adds up better than expected.

The stairwell is the best chimney in an ordinary house

If there is one place a rooflight earns its keep as a ventilation device, it is over a stairwell. The stair is already a continuous vertical shaft connecting every floor, it is normally open at both ends, and it is the one route air can take through a house without passing a closed door.

Put an opening rooflight at the head of that shaft and you have the full height of the building as your column. Open a window on the ground floor and the whole house flushes rather than one room.

The complication is that a rooflight over a stairwell is almost always out of reach, which makes an electric unit the practical choice. We cover the reach question in detail on electric rooflights out of reach and why they suit, and the control side on remote controls, wall switches and smart home links.

Night purge, and how to run it in an Oxford summer

Night purge means opening the roof outlet and a low-level inlet after dark, letting cool air run through the house until early morning, and closing everything before the outside temperature climbs past the inside. The thermal mass in the floors and walls holds that coolness into the afternoon.

Done properly it is the single most effective thing an opening rooflight does. Done badly, by leaving everything open all day, it does the opposite: it imports hot afternoon air into a structure you have spent the night cooling.

The discipline required is why powered units with a timer or a schedule suit this job. A vent that has to be opened by hand at eleven at night and shut at six in the morning does not get operated. One that runs on a schedule does. Nothing about the physics needs a motor. Human behaviour does.

Winter moisture wants a short, sharp opening

In winter the stack is at its strongest and heat is expensive, so the strategy inverts. You want a brief, high-volume flush that swaps the moist indoor air for dry outdoor air and then stops, rather than a small opening trickling all day.

Warm indoor air holds far more water vapour than cold outdoor air. Ten minutes with the rooflight fully open and a downstairs window open will exchange the air in a stairwell several times over, and because the surfaces stay warm, the room recovers its temperature quickly. An opening left at a hair’s crack for six hours loses far more heat and shifts far less moisture.

Bathrooms and shower rooms behave the same way. Full opening, short duration, immediately after use, with the door shut so the moisture goes up and out rather than into the landing.

Closed doors are the thing that stops it working

A stack needs a continuous path from inlet to outlet. Every closed door on that path is a near-total blockage, and internal doors in modern work are often better sealed than people assume, particularly where they have been fitted for fire separation.

This is worth thinking through room by room. A rooflight in a rear bedroom with the door shut and the window shut is ventilating nothing. The same rooflight with the door open onto a landing with a window open at the far end is part of a working circuit.

Open-plan layouts help, which is one reason a rooflight over an open kitchen and dining space performs better than the same unit in a cellular plan. Where doors must stay shut, you are back to single-sided ventilation and the height column shrinks to the height of the room.

Wind will either help you or wipe you out

On any day with real wind, wind pressure exceeds stack pressure by a wide margin, and the ventilation you get is whatever the wind decides. That is not necessarily bad. A rooflight on the leeward slope sits in suction and exhausts strongly. One on the windward slope can be pushed into acting as an inlet, driving cold air down into the room.

Oxford’s prevailing wind is south-westerly, so a rooflight on a north-east facing slope is more often in suction than not. That is worth knowing when there is a genuine choice of slope, though daylight, room layout and the planning position usually decide first.

The design conclusion is not to chase wind direction. It is to make sure the unit and its fixings are specified for an open sash in gusty conditions, and to accept that on windy days the stack calculation is irrelevant.

Small opening angles do more than you would think

Because the driving pressure is so low, the flow through a stack opening is roughly proportional to free area rather than rising steeply with it. A powered unit opened to 15 or 20 percent of its travel gives a surprising fraction of the ventilation of one opened fully, while presenting far less exposure to rain and far less load on the mechanism.

This is the practical argument for a motor over a pole in a ventilation role. A motorised sash can be held at a repeatable small opening indefinitely. A pole-operated one goes to whatever position the hook happened to reach.

It also reduces wear. A sash held at a modest angle in a breeze puts much less strain on the chain and the hinges than one at full stroke, which matters over the years the mechanism has to keep working.

What a stack will not do for you

It will not cool a room below outside air temperature. On a 30 degree afternoon, ventilation brings in 30 degree air. If a room under a roof is overheating in daytime, the answer is shading, glazing specification and insulation, not a bigger opening.

It will not remove the need for extract ventilation where the Building Regulations require it. A shower room or a kitchen has a mechanical extract requirement that an opening rooflight does not satisfy on its own, whatever the airflow you can demonstrate.

It will not work reliably in a shallow single-storey volume with no low-level inlet, and it will not work at all in a sealed room. If those describe your situation, say so early and specify accordingly rather than buying an opening unit and hoping.

Where the unit sits on the roof, and why it matters

Within a pitched slope, higher up the slope is better: it adds height to the column and puts the opening closer to the ridge where the warmest air pools. On a flat roof, the entire deck is at one level, so position is decided by the room below rather than by the physics.

On a vaulted or partially vaulted ceiling there is often a real choice, and a unit set towards the ridge end of the vault will outperform one set low near the eaves by a useful margin in the same room.

Structure usually has the final say. Rafter spacing, purlins, valleys and the position of any steel decide what is buildable, and those constraints are settled on the roof rather than on a drawing. Where two positions are both buildable, the higher one is the better ventilator.

What the perimeter has to survive on a hard-working vent

A rooflight that opens twice a day for fifteen years is a moving assembly in a wet environment, and the perimeter around it takes more punishment than one round a fixed light. Water runs down the slope onto the head of the unit, wind drives it sideways, and the sash pumps air past the seals every cycle.

Four things decide whether that stays dry over the long run. The upstand height on a flat roof, which is where the covering turns up and terminates. The falls, so water is moving away from the unit rather than standing at it. The flashing laps, each one shedding onto the one below in the right order. And the point where the roof covering finishes against the frame, which is the detail nobody sees once the job is finished.

Those are the four invisible details, and they are the reason our workmanship carries a ten-year guarantee. The unit itself is the small half of the job.

Deciding the ventilation strategy before the ceiling closes

The sequence that works is to settle the ventilation intent first, then the opening type, then the control, then the cable if there is one. Doing it the other way round produces a fixed light in the only high-level position in the house, or a powered unit with no supply route left.

At survey we measure the height difference between the proposed rooflight and the most likely low-level inlet, look at which doors on that path realistically stay open, check the free area the shortlisted units actually publish, and say plainly if the arrangement will not produce a working stack. Sometimes the honest answer is that a fixed light plus a decent extract fan will serve the room better and cost less.

Where a powered unit is right, the cable route is set out with the ceiling still open and a qualified electrician makes the connection to the fixed wiring and certifies that part of the work. The roof is not left open overnight at any stage. You get a range by email once we understand the room, and a fixed price after the survey, when the position, the unit and the perimeter detail are all settled.