What is stack ventilation?

Air expands as it warms. A given volume of warm air weighs less than the same volume of cool air, and lighter air floats on top of heavier air the way oil floats on water. Heat a room and the warmest air collects against the ceiling. Give that warm air a way out at the highest point and it will leave under its own weight, because the cooler air arriving to replace it is heavier and sinks to the floor.

That is the whole engine. It runs on the temperature difference between inside and outside, and on the vertical distance between your low inlet and your high outlet. The bigger either of those gets, the harder the air moves. A tall stairwell with a rooflight at the top and a door open at the bottom shifts a surprising amount of air on a still day, purely because the outlet sits several metres above the inlet.

People also call it the chimney effect, and a chimney is exactly the right picture. A fire does not need a fan to draw smoke up the flue. The hot gas rises because it is light, and clean air is dragged in at the hearth to take its place. A room with a high opening and a low one behaves the same way, just at gentler temperatures.

Stack ventilation only works if the outlet is genuinely high, and the highest point of most rooms is the ceiling. A window in a wall, however large, lets air out somewhere around head height, so the warmest layer sitting above it never gets a route out. It stays trapped against the ceiling and keeps radiating heat back down into the room all evening.

A rooflight or lantern sits above that trapped layer. Open it and the hottest air in the room, the air doing the most to make the space uncomfortable, is the first to leave. This is why an opening skylight paired with an open window or door lower in the building outperforms two wall windows of the same total area. The two wall windows give you cross ventilation, which relies on a breeze. The skylight gives you stack ventilation, which works when the air outside is dead still, which is precisely when you need it most.

An electric opening unit makes this practical at ceiling height, where nobody can reach a manual handle. Many are controlled by a wall switch or a remote, some carry a rain sensor that closes the glass automatically when a shower starts, and some can be set to open on a room thermostat so the room begins to vent before anyone notices it has warmed up. Our electric skylight installation work is built around exactly this: putting the exhaust for the stack where the warm air actually collects.

Stack ventilation needs two openings, not one. The rooflight is the outlet. You also need an inlet lower down for the replacement air to enter, otherwise the warm air has nowhere to escape to and the flow stalls. A door to a cooler hallway, a window on a lower storey, or trickle vents at the bottom of a window frame will all serve as the inlet.

A few things make the effect stronger:

• Vertical separation. The greater the height between the low inlet and the high outlet, the stronger the draw. A rooflight over a double-height space or at the head of a stairwell has far more to work with than one in a low single-storey room.
• Two openings that are open at the same time. One opening on its own mostly lets a little air trickle in and out across its own width. The pull only builds when air can enter low and leave high in a continuous path.
• A clear route between them. Warm air needs to travel from the inlet, across the room and up to the rooflight. A closed internal door in the middle of that path breaks the flow.
• Free opening area, not just glass size. A rooflight that lifts open a few centimetres offers far less exhaust than one that opens to a wide angle. If ventilation matters to you, ask what the free area is when the unit is fully open, not just the size of the pane.

Night purging is where this earns its keep in summer. On a hot day you keep the house shut against the heat. After dark, when the outside air has cooled, you open the low inlet and the high rooflight and let the stack flush the day’s warmth out of the building overnight, so the room starts the next day cooler. An electric unit with a timer or a thermostat does this without anyone staying up to open and close the glass.

Stack ventilation and solar gain are two sides of the same problem, and around here they meet on south-facing roofs. Southend-on-Sea faces south across the Thames Estuary, so a great many rear extensions and loft rooms collect sun from mid-morning until the sun swings west, with reflected light off the water adding to the total. That is heat pouring into the room through the glass. Stack ventilation is one of the ways you get it back out.

The order matters, though. Ventilation moves heat that is already in the room. It does far less about heat that has not arrived yet. The first job on a south-facing roof is to stop as much of the sun’s energy at the glass as you can, which is a question of the G-value of the glazing, the proportion of solar energy the glass lets through. Clear double glazing sits around 0.6 and lets most of it in. Solar-control glazing nearer 0.3 turns much of it away before it becomes heat you then have to ventilate. Get the glass right first and stack ventilation has a much smaller job to do. You can read how we approach that on our energy-efficient skylight installation page.

The pairing we often reach for on a south-facing room is a solar-control fixed unit for the bulk of the daylight, with one opening rooflight at the highest point to drive the stack. Low G-value glass keeps the summer heat down, and the opening unit clears whatever does build up, along with cooking heat, steam and stale air the rest of the year.

If a room is running hot and stuffy, look at it as two separate questions. First, how much heat is coming in through the glass, which is about orientation and G-value. Second, how the warm air gets out, which is where an opening rooflight and a low inlet give you stack ventilation. A room with a south-facing roof usually needs both answered together.

If you would like us to look at yours, tell us which way the roof faces, roughly how big the room is and where a low inlet could go, and we will come back with a glazing specification and a sensible position for an opening unit. Ring 01702 898232 or request a quote, and we will look at the whole room rather than just the hole in the roof.