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I live in a typical 1960s UK home, built of brick with two floors. My home has big windows (especially facing south) and has a really large solar gain causing it to be very warm in the summer.

I'm trying to do what I can to cool the home without any kind of mechanical or electrical intervention. My first step is to replace some of the windows to make better use of the stack effect, as many windows are divided into sub panels of which only a small proportion opens.

I'm mindful that airflow is complicated, but should I attempt to measure and balance out the size of openings upstairs and downstairs to optimise the effect or does any ventilation in any location help? Are there any other gotchas or tips when optimising the effect? It's not currently practical to ventilate directly upwards and out of the roof unfortunately.

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    Reducing the amount of sunlight coming into the house probably will give the best bang. Using an awning outside to shade the windows is usually the best, but window shades/drapes inside can help a lot also.
    – crip659
    Jul 12 at 10:40
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    There are so many variables here that you can't control. This isn't an HVAC system you're installing. Like Mig said in his answer, summarizing: this will require a lot of manual control based on indoor and outdoor temps. I've done it (doing it now, in fact) by placing a box fan on a window sill and blowing nice cool night air into the house. But later in the day, I'll have to turn off the fan, close the windows, pull the shades, etc. BTW? Why are you opposed to any mechanical ventilation? Fans typically take tiny amounts of power. Jul 12 at 11:41
  • @GeorgeAnderson I do have some fans that I move around the house, but installing some kind of larger system for a couple of weeks a year during which I'm home all day feels cost prohibitive. Maybe I'll have to do it anyway eventually, but for now I'd like to avoid it. Jul 12 at 13:02
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    @ChrisFletcher I obviously also don't have a HRV system. So fans in the windows it is. And like you....just a few weeks out of the year. Jul 12 at 13:28

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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.

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    Secure small children! Also, consider fans in the window upstairs blowing air out to assist with the natural flow of hot air upwards.
    – FreeMan
    Jul 12 at 12:51
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    Thanks for your comprehensive answer! You referenced upstairs bedrooms getting hotter. Is it not possible to also cool those through the stack effect? Or is it just a case that downstairs rooms will cool faster than an upstairs room would with the additional ventilation? Thanks Jul 12 at 16:13
  • You mentioned big windows mostly facing south, so that's a lot of hot air driving the stack effect. This will result in that hot air leaving the living room, but that also means there's a continuous stream of hotter air going through an upstairs room. It's moving alright, but the air upstairs will always be warmer (and therefore heating up said room). If the living room's the most important you can consider it, but using opposite windows (on both floors, and keeping the stairwell closed) might be more pleasant. I would try both of these scenarios.
    – MiG
    Jul 12 at 16:26
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    Specifically for the bedroom I can recommend night cooling (table fan, start around or after sunset, depending on outside and inside temperature), a ceiling fan and if it gets really hot, also a standing fan pointed roughly between you and your significant other's heads from the feet end of the bed. I'm doing all these things during summer, and we're able to sleep.
    – MiG
    Jul 12 at 20:54
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    And if the bedroom's hotter than outside during the day, do it during the day as well. You can let the fan run at max speed then, without bothering anyone.
    – MiG
    Jul 12 at 20:56

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