Skylights

Warm air weighs less than cool air, so it gathers at the top of a room. Give it an opening up there and it leaves, pulling fresh air in through any opening lower down as it goes. That slow exchange is called the stack effect, and a skylight is the top half of it: an opening cut into the roof, glazed so it also lets daylight in and made to open so the heat has a way out.

Two things govern how well it works, and neither is the size of the glass. The first is the opening below it: without a low inlet nothing moves, so this is half a system rather than a device. The second is the height between the two, because lifting hot air well clear of the people underneath gives the effect something to work with. Size is still what gets underestimated: guidance for buildings ventilated this way matches the area at the ridge to the area at the walls and puts each between 15 and 20 percent of the floor below, far beyond a single roof window, which is why a skylight is better read as a relief opening than a ventilation system.

The catch is that the effect is weakest on exactly the day it is wanted: on a hot afternoon the inside and outside air may differ by only 3 to 6 C (5 to 10 F), leaving buoyancy almost nothing to work on, while a breeze of 3 to 5 km/h (2 to 3 mph) can supply most of the exchange instead - so facing the opening into the prevailing warm-season wind matters more than enlarging it. A hole in a roof is the least forgiving one on a building, and it is the flashing, the layered sheet that steers water out over the covering below, rather than the glazing that sets how long it lasts. Unshaded glass in a roof takes high-angle sun at the hours it is meant to be shedding heat, so shade or an insulated cover belongs in the design, and it only works when somebody can open it, which makes a reachable handle or an automatic opener part of the component. Upkeep is resealing the flashing on the roof's own cycle and working the mechanism before each warm season.