Solar reflective film?

[tim...]

If I'm going to go for that solution I won't be bothering with reflective glass.

It will be chunks of chipboard :-)

[The Natural Philosopher]

It is a mistake to assume that the outside is cooler than the inside!

In the mid afternoon on a sunny day, it is likely that the house interior will, if it has any appreciable thermal mass, be likely cooler than the outside.

Hence my advice to block all inbound radiation and put an insulating barrier in the windows

One of the reasons modern studwork houses are insufferably hot upstairs is that they have cast concrete ground floors, but the upper storeys have virtually no thermal mass at all...and hot air stays high.

Under these circumstances you have to block all incoming light with an insulating barrier, perhaps use fans to push hot air down to the cooler ground floor

[The Natural Philosopher]

What about 'test' did you not understand?

The point was that if that works, then consider thermally insulated curtains.

Everybody things that double glazing means you don't need insulating curtains.

BUT if you haven't got shutters, thermal curtains will do nearly as good a job

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[The Natural Philosopher]

Because mostly it is Or was. These days there are a lot more materials available.

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[Richard]

Coming soon, "How to rid my property of antipodean vermin?"

[The Natural Philosopher]

Well just add insulation to your blackout grade curtains. In fact I do NOT have blackout grade curtains. They let a small amount of light through the three layers that comprise them, but what they do so, is put an effective 1cm air gap of still air (because it is full of 'bump' interlining) between the window void and the room.

That limits transfer of heat *into* the room when they are in bright sunlight, and closed.

At night, you then *open* the curtains, and the windows, to let heat

*out*, when outside air temperatures are *below* the room. That works if you have internal concrete floors and/or internal blockwork.

If you haven't, you have almost no option except active cooling for that room

Here I do have a lot of concrete in the floor and a brick/block chimney stack, and on excessively hot days (> +30°C) I can keep the interior down to around 25°C using this technique. Because night time temperatures are usually below 20°C

[The Natural Philosopher]

Look the whole point is that reflective film is

(a) permanent (b) not very effective.

Venetian blinds are better at keeping direct sunlight out but letting light in, but if they are *inside* the room they simply get hot themselves. Unless they are INSIDE the DG or outside the window, they wont really work.

Thermally insulated curtains that are drawn at least on the sunny side of the room, are by far the best, even if you need to switch the lights on!

[Theo]

OK. So how hot does that blackout stuff get if you leave it up in the daytime? Does it help with the temperature?

Another thought came to me - I have one of these things in the back of my car (which I've never used):

They're cheap and you can fold them away when not in use. Perhaps taping something like that to the inside of the window would do for the few days you need it? Or velcroing or some other removable fixing.

Theo

[1] Bonus points for comedy photoshop:

[newshound]

Decades ago I had some metrology kit in a (then) modern lab building, Large alloy single glazed windows, south facing, brick over block construction.

I was mapping a surface with a stylus instrument, taking a couple of hours for a raster type scan. Looking at a "3d" image I found it seemed to have a flat bit, then a sloping bit, then another flat bit. This turned out to be the effect of the sun going in and out of cloud, because of thermal expansion between the sample and the support for the transducer.

Reflective foil on the window *completely* eliminated the effect. We were a bit more careful about temperature control after that. We had air-con because of the thermal output from the PDP-8 that was doing the control, logging, and plotting, but it was bloody noisy so we only used it in hot weather.

[tim...]

no idea

quite warm

again can't be sure It didn't stop the room getting really warm last year

Good idea

They are presumably more tear resistant that any of the obvious alternatives

Thanks

[Martin Brown]

That would be best, but glass is quite transparent to most incident sunlight. It is fairly opaque in the near band thermal infra red.

Not true. Solar film will still work pretty well on the inside of the window (although it will end up slightly warm). There is a slight advantage to having the outside surface of the glazing able to behave as a black body for outgoing long wave thermal radiation.

Try it out with some cheap aluminised wrapping paper and blue tack in the true spirit of quick and dirty DIY solutions.

Perhaps as global warming becomes more of an issue we will have to adopt some of these Mediterranean solutions in the UK.

[Vir Campestris]

We had the patio doors at our old house replaced by ones meeting modern standards with the special layer in them to stop heat escaping.

The main thing I noticed was that it cut down thermal gain; presumably film would do the same.

I don't think that's necessarily a good thing - there are more sunny days in cold weather than there are too hot days in summer.

Andy

[Theo]

To follow up to this, I did an experiment.

Two very similar rooms at the top of the house which suffer from overheating due to a ribbon of east-facing glazing.

In Room 1 I taped an foil/plastic/paper emergency blanket to the window, trying to seal as tightly as possible. The normal roller blind was then down behind the foil. I closed the door to keep it sealed off. I cracked a window behind the foil to ventilate the gap.

In Room 2 had no foil, but I pulled the blind down. The door was open (which I think didn't make much difference). The windows were shut.

A sunny morning, but not especially warm outside (~20C)

The temperature plots of rooms 1 and 2 respectively are here:

1 at the top)

Ignore the first data at 26C, that was the sensors having lost communication some days before and needing to be reset. Also be aware the Y scales aren't exactly corresponding.

Room 2 heated up rapidly as the sun hit it. Room 1 didn't, and stayed about

3C cooler throughout the morning. In the afternoon the sun had moved around and room 1 warmed up a bit as heat leaked in, but by evening it was still cooler 25C to 27C (although it was also much colder outside by this point and the window might have helped cool).

So the foil did make a big difference.

Having done some digging around, I've come across 'honeycomb' or 'cellular' blinds, some of which have foil lining inside:

This is a nicer version of my foil blanket. The handy thing is they fit to the window panes individually, rather than covering the whole opening. In theory this means there's no gap behind the pane and they reflect all the heat straight back outside. Some of them have a track and some fit by hooking underneath the window gasket.

I've ordered some samples and will see how they look.

Theo

[tim...]

well you're the only person saying this

even the people selling the stuff (and presumably wanting to make sale are telling me it has inferior performance stuck on the inside)

well yes,

but that will take 15-20 years before block management companies take note and propose solutions at a block level

by that time I will be past caring

[Owain Lastname]

Yes

It definately helps reduce solar gain and there is also some pyschological effect in that it reduces glare and that 'baking' effect so you don't feel like you're sitting in full sun while indoors.

It's just a more sophisticated version of greenhouse whitewash - which, if things are desperately hot, you might try on the outside of the glass for a few weeks, applied by brush or spray on long pole, and then washed off when the rain comes.

If you have 25% reflective film you can assume (very roughly) that's like having a 25% smaller window for the sun to shine into.

But you still have a full size window you can open at night to let the cooler night air in. A small fan on the windowsill helps considerably in this too.

I assume you're an upstairs flat?

If you can fit one, an awning or external roller shutter, operated from inside, would also probably be effective.

You have my sympathy, having melted as a small child in a south-facing flat during the 1976? heatwave in the south of England.

Owain

[tim...]

I'm currently tasked with making something out of cardboard/foil to try this myself

but the warm weather has gone away for now so testing it will have to wait.

I refer the honourable gentleman back to the point where I (tried to) explain that the room doesn't start off in the morning at a cool 16 degrees but at whatever temperature it reached the day before (because, on the days in question, there is no overnight cooling)

So the aim is to find a way to stop the room ever getting to 30 degrees, not for it to get there slower.

I've been looking to fit something like this for ages

unfortunately the depth of the fittings I have doesn't come close to the require depth for this supplier

[Martin Brown]

What makes you think a salesman knows what he is talking about?

If the film is on the outside surface it will reflect very slightly more incoming radiation away but it will also prevent the outer glass surface from radiating heat away like a black body as a direct result. That slows heat loss from the window and from the room. Similar semi mirror coatings are used to decrease heat loss by the likes of Pilkington K low emissivity glass. It is a consequence of Kirchoff's law of radiation which is pretty much a restatement of the law of conservation of energy.

These days it is possible to construct designer metamaterials that actually get cooler when in direct sunlight. They are white for visible and near infra red radiation but black for thermal band emissions.

Becoming closer to a viable mass produced product now rather than a physics lab curiosity that takes days to fabricate a single 10cm tile.

[Nick Cat]

except for the other options of course.