@Thayer I have a question for your expertise in coolness: would you also keep curtains closed of a north facing window? It’s always in the shadow of the house, but the air outside of it (if window is opened) is still hot. I keep the window closed during daylight hours but the curtains open and wondering if that’s best, but also appreciate having some light in the room.
@Thayer (for added context) it’s all triple-glazed and so the windows are great at maximising solar gain. Wonderful in winter. Bad in summer.
@pauljrobinson totally valid and yes they basically become storage heaters! Grab some reflective window film and apply to the window (it removes easy at the end of summer) about £25 on Amazon, and then also if you can hang anything outside the window like aluminet, this adds another huge layer of reflection. Then, ALSO close the blinds if you can, this keeps the heat in that space, not in your room. You can also upgrade to thermal fitted inside blinds if not outside, really helps.
@pauljrobinson not pretty, but this was my window in the room that faces the sun in the pm - reduces *any* heat on the pane at all, vs it being almost too hot to touch before. I'm fixing it to the soffit today with command strip hooks (just curing now) so I can put it up and remove easy (2nd floor). It ain't pretty, but boy does it work in lieu of external blinds/shutters. The room is 10deg cooler than it was, easy.
@pauljrobinson here we go I just hung it out properly now ready for today's blast. So it's just on command hooks on the soffit - but that 1ft away from the window allows the sun to be both reflected, and any heat energy in the aluminet to be distributed and away before getting anywhere near to the window. Total cost - window film, £25, aluminet shade £30, command hooks for outdoors, £9.
@pauljrobinson oh and I just realised I didn't answer your OG question: dead easy - if it's hotter outside than inside, keep windows shut. If it's just blinds, and that's on the shade side of your house, opening blinds is perfectly fine if the glass pane itself hasn't heated up to be a storage heater. Touch it, you'll know! :) If it's hot, keep blinds/curtains drawn to keep the heat isolated in that area.
@Thayer awesome thanking you kindly!

@Thayer @pauljrobinson I have taken the cheap option and ripped up an old sheet, opened the window - hung it over the frame and closed the window (trapping it)

Does a great job of keeping the heat out while allowing some light in

It's still too hot - but less bad

On a utility room I actually painted the windows white (not the frames - the glass) - paint will wash off later in the year.

@Thayer @pauljrobinson just to be clear, these changes really help.

But they are neither magic nor air conditioning

With such warm, short nights the house doesn't cool down enough and we start today with indoors at 25C downstairs, 29C upstairs - it didn't really cool off overnight at all.

There is a reason Mediterranean houses are all whitewashed - it reflects the heat. The bricks my home is built of are toasty

@Thayer wow this looks cool in every sense - love it