Inside or outside -- that is the big question

Yes, there are thermal blinds. The subject is quite simple and at the same time frustratingly complex.

You're looking to reduce three heat losses: (1) air infiltration, (2) radiative losses, (3) conductive and/or convective losses.

It turns out it's better to do (1) with the window rather than with the blind or shutter. So the first priority is to make sure your window is completely draught proof.

(2) Needs shiny surface(s)* next to an air gap. So a shiny aluminium foil (space blanket type) roller blind is pretty good. If it's "fairly" well sealed at the edges and "fairly" close to the window, it also creates a still air gap that does pretty well for (3). A few multiple foil layers can help, perhaps mainly because of the air gaps.

(3) can be done with conventional insulation. If you're willing to look at "shutters" as well as blinds it takes a lot to beat a sheet of foil-faced kingspan or celotex or even EPS pushed into the window reveal. But it may not go with your perfect Victorian decor! For blinds, you can use thick quilt materials (duvet TOG is an SI unit of thermal resistance, 10 TOG = 1 m2/W :) ) Or fancy materials like aerogels, or bubble-wrap etc etc

The complications and frustrations come from several sources. Firstly, each blind or shutter typically needs to be made to measure so is time-consuming and/or expensive. Secondly, blinds and shutters need to be moved, so their benefits are ignored by "regulatory" energy calculations like SAP and PHI and hence no great corporate investments. Thirdly, it's pretty difficult to measure their performance so it's difficult to produce comparisons. For example, blinds and shutters are more valuable on single-glazed windows.

Internal blinds/shutters have some extra particular considerations. Firstly, they need to be close fitting because the extra insulation reduces the temperature of the window, making it more prone to condensation. Secondly, their appearance matters, unless you're a blind hermit, which complicates the design. Thirdly, they tend to be bulky and you have to find somewhere to stow them when they're not in use. Fourthly, you need to consider fire risk.

External shutters also have a couple of extra requirements. Firstly, they need to be weatherproof - impervious to wind, rain, hail, ice, sun and dead leaves, inhospitable to moss, lichen, spiders, rats, birds, wasps and vandals. Secondly, they need some mechanism to close them; useful on the ground floor on cold, stormy nights and vital at all times above the ground floor.

The commercially-made thermal blinds I have seen in shops seem to be more concerned with keeping heat out in the summer, though I expect they do something in winter.

There's an old book called "Thermal Shutters and Shades" by William Shurcliff. I just finished reading it but haven't digested it yet. There's also a current book called "Movable Insulation" by William Langdon, but I haven't seen that.

It's also worth mentioning the very special form of thermal shutter called secondary glazing. Adding another layer of glass or clear plastic to your windows can work well but again, it all depends on the particular situation.

Cheers, Dave

* That's a lie of course. It needs "low emissivity" surfaces. But "shiny" is close enough for now.

Posted By: Gotanewlifedjh, does the shiny surface have to face the heat?

No, doesn't matter. The more the merrier but with rapidly diminishing benefits. For an internal shutter/blind it often makes more sense to have it on the outside so (a) it can serve as a sunlight/heat mirror in summer and (b) you can decorate the inside :)

Posted By: fostertomGotta be an roll-up app for multifoil - even if you're sceptical for all other purposes.

You're quite right, Tom.

The one piece in the puzzle I'm still not quite sure about is preventing condensation but after I figure that out I may well try some multifoil.

Posted By: ken davis back of lining can be foil faced.

Posted By: djh Needs shiny surface(s)* next to an air gap. So a shiny aluminium foil (space blanket type) roller blind is pretty good. If it's "fairly" well sealed at the edges and "fairly" close to the window, it also creates a still air gap that does pretty well for (3). A few multiple foil layers can help, perhaps mainly because of the air gaps.
* That's a lie of course. It needs "low emissivity" surfaces. But "shiny" is close enough for now.

This is something I have learned from this FORUM: foil surfaces work because they have 'low emissivity' so no air gap no effect!!!

for a few hours but no good even for overnight when it is cold

I would advocate external blinds.

I bought some quilted miltifoil product from the caravan industry which has made superb internal thermal blinds. (used to insulate windscreens in mobile homes etc. cost about £10/m2 but you can just get what you need and i got it by post a couple of days later. sorry cannot remember brand/source.
It is a much more robust product than isofoil, and I have stapled it to the window frame and a broom handle with a couple of screw in eyelets and some strong cord. Rolls up and drops down easy peasy.
Quilted shiny foil to inside gets a few comments, but once you are past the initial shock, - generally gets very favourable comments, - and does reflect light back into the room, and given that we are accustommed to recieving light from the direction of the window it works very well.
WE do get condensation on the glass, and there is a major feeling of cold air falling out of the window reveal when you open up the blind in the morning. Works very well.

Thanks for all the comments - very interesting.... the "design" element is very important as it has to pass muster with the "boss".... who doesn't have the same interest in energy conservation that I have.

>it could be cheaper to specify passiv-haus windows...

do u have a link handy?

ta !

I have a couple of data points on the condensation question. Shurcliff says next to nothing about condensation as far as I can tell, but he does say that from a thermal viewpoint, absolute seals are not necessary and indeed even a 1/8" gap is OK. So I decided to investigate ...

I took a sheet of 50 mm EPS, which at 450 mm wide was just a couple of cm less than one of the fixed single-glazed panes in one of my windows. I cut it so it's a couple of cm less than the height as well.

I then simply held it up against the window with a couple of small (1 cm or so) blocks supporting it underneath and let the venetian blind down behind it to hold it in place. So it was in contact with the glass - what Shurcliff calls a 'face seal'. Leave overnight. In the morning, there was no sign of condensation around the polystyrene and definitely none running down onto the windowsill. When I removed the EPS, there was significant condensation behind it.

So what I suspect happens is that the vapour condenses and forms a film of liquid water that seals the remaining gap between the EPS and the glass, thus preventing further condensation.

To test that, and the effect of less-good seals, I stuck some scrap bits of foam sheet on the face of the EPS (they're about 4 mm thick). I've then tried again a few times with this 4 mm gap between the EPS and glass but so far the weather hasn't been cold enough to cause any significant condensation. The only thng I've noticed is some cat hairs stuck to the glass in the morning, aligned roughly horizontally and about half-way up the window. I've no idea what that means, apart from that we've got cats, of course.

Cheers, Dave

Have said this before (re. cheap and effective double glazing), but here goes again:
1. form level plane around your windows in softwood to suit existing glazing sub-division.
2. obtain double sided steel/magnetic tape.
3. buy 4mm glass to suit window dims.
4. install deeper (so that you can lift it out) than wide aluminium channel to head of window (this is only really necessary for safety and works rather like the top channel on simple sliding doors on kitchen units and the like).
5. stick one side of tape to sides of glass, other to sides of levelled up frame.
6. purchase rubber or plastic glazing gasket, cut to size and fit to bottom edge of glass sheets.
7. slot glass sheets up into top channel, sides will magnetically adhere to tape.
Hey presto, cheap, effective and frameless double glazing. Works especially well in listed buildings.
Only problem is making sure your pane sizes are nor too large too lift. About 1-1.5m.sq. will do it.

can't remember specifically where i bought mine but i have just googled it and there are loads of suppliers so shop around (ask for a small sample first). you should be able to get rubber gasket from any good local glazier. remember do not use plain 4mm glass below 900mm or in doors, it must be safety glass . your local glass merchant will have the BS for you to look at.

This Forum - The business - as usual!

Great link Ed, great info Ken. So this glass is removable right? So, maybe I'm dreaming here, but in theory I could have summer glass (max reflection of heat) and winter glass (max retention of heat), using appropriate glass films - right? Oh and 4mm glass would be a luxury, let alone safety glass - all my double width, fully glazed doors are 3mm - but since I live in Italy that's fine - right?! And I say dreaming because of the cost of the films!

I have not quite got the practical side of these SGs - intuitively, the channel should be at the bottom for safety. Should the glass be inside or outside? - what about the exposed sharp glass edges on the sides? Is condensation an issue? If outside then the does the 'plastic glazing gasket' have greater depth than the magnetic strips? I am sure I could work it all out eventually but it is MUCH easier with your help....

deep channel at top, just like sliding doors on old wall cupboards, principally its just for safety in case the magnetic seal on the tape at the sides is broken, glass can not then fall forward. rubber or plastic gasket at bottom just for the glass to sit in and take out any slight unneveness in the window cill. glass must be inside or it would not weather and/or wind would dislodge it.exposed glass edges: just ask your glass man to dull the edges with the diamond dust pads they all have (run up and down each edge a few times). saves having edges properly ground which costs lots.the magnetic strips run just up the two sides and stop just short of the channel at the top and the gasket at the bottom. i have had no trouble with condensation in the rooms where i have done this but i do have heat exchange ventilators in adjoining damp producing rooms i.e utility and shower room. the top channel can be painted the same colour as the frame so that it is lost visually. i made a little slidey/hooky devise to push under the gasket at the bottom otherwise these panes would be difficult to get hold off, though i guess a simple sink plunger might di it. good luck.

Now we've had some cold weather, here's an update on my experiments. Nothing remarkable I'm afraid, just a confirmation of the expected problem:

- When the polystyrene block is held slightly away from the glass, there's running condensation on the window behind it in cold weather

- Same goes behind a silver roller blind I made (from a hiking emergency blanket). It's also very difficult to make it roll up neatly without applying continuous correction tug on the bottom rail.

On the other hand:

- the polystyrene block held in contact with the glass is a bit better. There is some condensation but it can just be wiped off.

- I've tried covering one pane with heat shrink film held in place by double-sided sticky tape. I put it up last winter and it's still perorming well. Just a few square inches of condensation sometimes.

- my father used secondary sheets of glass held in plastic edgings and clips. That worked very well with no condensation over the winter as long as everything was clean and dry when it was fitted each year. As shown here but I've not used this supplier http://www.diyplas.co.uk/Info.asp?catid=0&subcatid=0&proid=536&unsid=6

Happy New Year, Dave