@judith Standard UK style plastic frame double glazing modules can be supplied with vent holes and caps, you just have to ask a glazier or similar. They are used to it for kitchen fans and the like but aircon will be a new one to many.

For a fixed installation it depends hugely on

- Whether you are doing a mini-split or multi-split. Mini split (one room unit to one external) is cheaper but you generally only get one "permitted development" per house - unless detached (one rule for the rich ..  8) ) so fitting a mini-split might limit choices later if you target one room then change your mind.

- Whether the installer can run a single hole through your wall and slap one half on each side directly. That makes pipework and the like much much easier

- Whether it's all below "working at heights" regulations. If you have a unit a floor up then someone has to run the pipes down and stuff which will (or should!) require a scaffold tower in many cases.

@etchedpixels I did know about custom cut-outs in double glazing panels we had one years ago for a cat flap(!)

Our house is detached and my tentative thoughts were for just our bedroom but reading your reply perhaps the room below would be worth doing too. I doubt we would need scaffolding because the house is on a hill and the ground is highly stepped. You obviously have knowledge of the costs, what is the order of magnitude for a single unit or a split to two rooms?

Will depend on local demand, how difficult the piping is to run etc but probably a couple of grand in summer for  a mini split, two way mini split maybe 3 grand, 3 way 4. Will be more if you have to run piping in awkward ways or you need scaffold towers and so on.

If you want an upper bound then heatable will give you a fixed price quote online so anyone local should be cheaper 8)

Based on southern Europe experience:

1. Insulation works wonders (no surprises here)

2. If you do not have fan coils or central air ducts, do not bother with central cooling. A2A units do the job. The portable ones are usually not powerful enough against upper 30’s+ (unless you are talking about a small space - of course proper heat loss sizing always matters). If internal temperature stays below upper 20’s, fans can also be of help. 

3. Window shutters (they're not an aesthetic choice in the south, they are a necessity). And green foliage shadow is the best natural defense against a blazing sun

4. If you are not surrounded by buildings, chances are the night breeze will help (large, uninsulated buildings and paved roads/yards emit at night all the heat they absorbed during the day - just touching them is sheer despair…). Perforated shutters are not enough to make a difference unless the Δt is substantial. You are better off with screens that allow air circulation and keep the mosquitos out. To keep burglars out you may install removable bars….

(example attached)

We have a particularly extreme (by UK standards) variation on this problem. Our home is basically a 1930s bungalow but it had been significantly extended before we bought it. That means about a quarter of the whole house is a single room that is a rectangle, and the wall facing the garden - one of the long edges of the rectangle is both facing almost exactly due East and is glass from floor to roof. Below is a picture of one half of it.

That means it's a serious sun trap and in the past we've just opened both sets of French doors and the top half of the front door (a barn door, facing due west) to get a through draught through the whole house. Even so, being in sun until about midday, that back room is a significant sun trap and so the temperature is always high.

My wife read about the French idea of using Blanc de Meudon to "paint" some shading onto windows and we looked into it. The story plus the French uptake of the technique guaranteed that anything called "Blanc de Meudon" was suddenly horrendously expensive (around £80 for 1kg on Amazon when I last looked), but I took a punt on the same quantity of chalk powder at less than a quarter of the price. Mixed by volume two parts chalk powder to one part water, it creates a smooth liquid paste that you can paint onto the windows, and when the water evaporates it leaves the chalk coating. Removing it is simply a matter of water and a cloth and it comes of instantly.

I wasn't convinced until we tried it. Pretty it is not (think of boarded up shops with windows painted white) but at the moment I have a six degree difference between the half of the room that's not shaded and the half that is. For less than a fiver for the chalk involved, I'm very happy.

1. @majordennisbloodnok Any gardening goods supplier worth their reputation will supply paint (or spray) on white window wash intended for greenhouse use. Can’t remember the maker now but once applied, it seemed to stand any amount of rain. I had to use a wet cloth and scrub it off! Sorry but my 78 year old grey cells have forgottened the name, make and description of the stuff now! I think I last bought some from Woolies (so not that recent😉) and it was quite inexpensive. Regards, Toodles.

Posted by: @toodles

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@majordennisbloodnok Any gardening goods supplier worth their reputation will supply paint (or spray) on white window wash intended for greenhouse use. Can’t remember the maker now but once applied, it seemed to stand any amount of rain. I had to use a wet cloth and scrub it off! Sorry but my 78 year old grey cells have forgottened the name, make and description of the stuff now! I think I last bought some from Woolies (so not that recent😉) and it was quite inexpensive. Regards, Toodles.

That, of course, is the problem. We didn't want something that would be difficult to get off.

As it happens, Blanc de Meudon (or plain old chalk and water as we've done) is used precisely for that purpose as one of its many functions. Painting it onto the inside of greenhouse glass is a great way of maintaining plenty of light but keeping it cooler, and when it gets taken off for the winter, the runoff is inert and harmless.

Interesting approach although in Wales it would last about a week at best in our weather. I guess in that position I'd add hooks or magnets and clip something over them for summer if I couldn't simply extend the roof outwards a bit so it shaded under high sun but not the low winter sun.

Our conservatory has the same problem big time but it's grade II listed so it's difficult to go messing with, instead we use it as a greenhouse so it's full of tomatoes.

Posted by: @etchedpixels

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Interesting approach although in Wales it would last about a week at best in our weather.

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Only if you paint it on the outside.

That is exactly what I did since I didn't know how much mess it would involve, and I'll probably continue in that vein given the ladder need for reaching the top of the windows. However, for normal windows I'd be far more inclined to paint it on the inside and that would, of course, be immune to the Great British Rain.

In the long term, I'm looking around for somewhere that'll make cellular blinds that are wide enough to deal with that shape of window, with a cord in the middle for hoisting the whole lot up. Great theory but so far no-one I've found makes them anywhere near wide enough.

On the inside its far less efficient and you can crack the glass if it reflects it all back and forth in the glazing.

Ideal is certainly shutters or shade on the outside. I'd go with extending the roof to cause the shading as I'm intrinsically lazy and that means I don't have to mess with stuff a few times a year 8)

Posted by: @etchedpixels

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On the inside its far less efficient and you can crack the glass if it reflects it all back and forth in the glazing.

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Do you have any articles you could point me to that discuss that as a valid concern? I'd be interested, especially since window whiting is by no means a new idea. I'd've expected that if putting window whiting on glass could carry the risk of cracking it wouldn't've become so well used for vacant shops and for greenhouses, so I'll admit I'm sceptical but happy to be swayed by anything other than anecdote.

@majordennisbloodnok When I read that, I assumed it related to double or triple glazing and the heat being trapped between the layers of glass. Regards, Toodles.