The short answer
Yes, an opening rooflight can help cool a house. Warm air rises and leaves through the open sash, and cooler air is pulled in through a window or door low down to replace it. This is called stack ventilation. It works best when the rooflight is at the top of the house, a low opening is also open, and the glass has some shading.
Why warm air leaves by the roof
Stack ventilation needs no fan and no power. It relies on one fact: air that has been warmed is lighter than the air around it, so it climbs.
On a hot afternoon the upper floors of a house collect that warm air. The kitchen has been cooking, the sun has been on the roof, and every person in the house is a small heater. The warm layer sits against the ceiling, and a loft bedroom or a stairwell landing gets the worst of it. Open a rooflight at the highest point and that layer has a way out. As it escapes, the pressure below drops slightly, and air is drawn in to fill the gap.
The effect grows with the height between the low opening and the high one. A rooflight over a single-storey kitchen extension has a modest vertical gap to work with. The same unit at the top of a stairwell, with a window open at ground level, has two floors of height and draws a good deal more air. That is why stairwells have been used as ventilation shafts in houses for centuries, long before anyone fitted a motor to a sash.
The fuller guide to opening units is on our page about ventilating a room with an opening rooflight. This page deals with the cooling trick specifically.
Giving the air somewhere to come from
A high opening on its own does very little. The cooler air has to arrive from somewhere, and its route decides how well the house cools.
A sensible summer sequence looks like this.
- Open a window or door low down on the shady side of the house, ideally on the ground floor.
- Open the rooflight at the top, so the warm air has an exit.
- Leave the internal doors between the two open, or at least wedged, so the air can travel.
- Close the rooflight and the low windows again once the house has cooled and the sun is on the roof.
Two openings on the same level with nothing above them give some cross-ventilation but no stack effect. One opening on its own stirs the air and no more. The vertical separation is the whole mechanism, so the lower the inlet and the higher the outlet, the better it works. A flat rooflight at the top of a tall room does the job as well as a sloping one, provided the air can reach it.
Where a rooflight works best
Some rooms make the most of the physics and some make very little of it.
A stairwell or a landing is the natural place, because it connects floors and gives air a clear vertical run. A loft bedroom under a roof window keeps the warmest air in the house, so letting it out matters there. A tall kitchen extension with a lantern benefits when a bifold or side door is open at the floor. A low, small room under a rooflight that has no other opening gains much less. Our page on electric rooflights for stairwells and landings covers the case where the rooflight is too high to reach, and electric or manual opening rooflights helps decide how to operate it.
Part O of the Building Regulations sets the overheating rules for new homes. An extension to an existing house is treated differently, so the reason to plan for cooling is simply that the room will be nicer to use. Part F, which governs ventilation, and Part L, which governs energy, still apply to the work. The glass itself should still be specified to keep out heat, not just let air through.
What the open sash lets in besides air
A sash that is open to the sky is open to everything the sky carries, and in this part of Sussex that includes noise.
Ventilating at night is when stack cooling does its best work, because the outside air is coolest. It is also when a loft bedroom is most exposed to aircraft on the Gatwick routes, to traffic on the A23 and M23, or to the trains on the Brighton main line, depending on the address. An open sash has no acoustic glass in the way. We listen for that at the survey, and the quiet spec records the compromise: where a room hears too much to sleep with the sash wide open, we set out a slightly open position, a night ventilation approach, or a design that takes the opening to a quieter side. Our answer on night ventilation through an opening rooflight without the noise goes further into it.
Rain is the other risk. A rooflight left open in a summer storm wets the room quickly, which is why a sensor that closes the sash suits a stack ventilation plan so well. Our page on whether electric rooflights close by themselves when it rains explains it.
Shade and glass still do most of the cooling
Stack ventilation removes heat that is already in the house. Shading and glass stop a good part of it arriving.
A south-facing rooflight in an unshaded roof can collect heat faster than ventilation removes it. A solar control coating on the glass, an external blind or an internal blind fitted as part of the unit lowers what gets in. A rooflight specified with a low g-value, which is the measure of how much of the sun's energy passes through the glass, stays far calmer than a clear one. Our hub on energy efficient rooflights explains how that glass is chosen. The combination of a coated unit, a blind and an open sash at the right moment is what cools a room, rather than any one of them alone.
Small questions about cooling with a rooflight
Will an open rooflight cool a house on its own?
Not if no other opening is available for cooler air to enter. The low opening is as important as the high one, and the two need an open route between them.
Is stack ventilation safe at night?
It can be, with restrictors on the sash and windows you can secure. Security and noise both shape how far you open it while you sleep.
Does an electric rooflight cool better than a manual one?
The physics is the same. A motor makes it more likely that the sash is open at the right time, particularly when it is out of reach.
Does opening the rooflight change the noise in the room?
Yes, because the glazing no longer stands between you and the outside. The comparison of opening and fixed rooflights for noise explains what changes and what does not.
If you want a room that can breathe out through the roof, get a quote and we will look at the stairs, the windows and the sash together. The specification goes into the quiet spec, and the full service is on the electric and opening rooflight installation page.