Without a cross section, floorplans and whatnot it's difficult to go into great detail, but in general:
- Any ventilation provides cooling, AS LONG AS the temperature outside is lower than inside.
- The stack effect (using thermal rising to passively increase air displacement) might work, however be prepared that this will mean any upper floor room you use as the exhaust will become a lot hotter. If this is unacceptable (for example because it's your bedroom), then you should probably stick to other options:
- Opening up windows on opposite sides of the building has fewer thermal side effects. Any slight wind will be amplified by the fact that your house as a whole is an obstacle, so the air will mostly pass through the path of least resistance, being your opened windows.
- Keep in mind you will have to secure inner open doors, opened windows (by means of latches), this also applies to anything that the wind can pick up (magazines, bank cards, small children etc).
- You can amplify this a bit (especially in a room that's only has its own windows open) with a table fan on the window sill. The colder air outside will be pushed in at that level, pushing out the hot air at the ceiling level.
- This has the most effect at night (biggest temperature difference), so keep an eye on the outside and inside temperatures and turn it on when the temperature has dropped. If you keep this up every night, it will remove some heat that's stored in the walls and make things less unpleasant over a longer period. Assuming a 40W table fan at max power, that would be 0.04 kWh and about £0.16 for an 8 hour night's sleep worth of free cooling (based on a july 2022 high end UK electricity price of £0.51/kWh).
- Repeating that first bit for emphasis: ventilation (passive OR active) only works if the outside temperature is LOWER than the inside temperature. So if your room reaches 40C and outside is 30C then go ahead and open up, but if your room is 28C then you should keep the windows closed.
- And to step outside of your frame a little: external sunshading removes direct sunlight heating up your rooms and can have a significant effect. I would go for a combination of external sunshading and ventilation.
EDIT: In case anyone's suffering from the high heat right now: a quick fix that helps a lot is to duct tape some cardboard to the outside of the windows. Make sure they fully cover the glass, and if you tape them over the frame itself (rather than onto the glass), the air gap provides a little bit of extra insulation from the heat. It pays off to have leftover flatpacks from a certain Scandinavian home goods store in the basement.
Note that both duct tape and cardboard will of course deteriorate over time and fail at the next rain storm (apart from also just looking ugly), so it's not a permanent solution. But it will help eliminate heat from the sun during a heat wave. I'm using this right now and it's keeping the bedroom and living room quite pleasant despite temperatures reaching 30C today.