Custom-made authentic ones can be as efficient as any:
http://i30.photobucket.com/albums/c301/fostertom/Waterheadwindows.jpg
http://i30.photobucket.com/albums/c301/fostertom/Waterheadwindows002.jpg
Nowadays I'd increase the sash depth (measured from inside to outside) from 56 to 62mm, to allow optimal 16mm gap DG units.

True, sliding sashes are wierd, but they're here to stay. They're specially good when they're huge, like 3m high x 1.4m wide, as in posh Georgian 1st Fl - that's why they got popular - no way could you do that, sidehung, in 1736, or even now. These huge sashes used styles and top rail only 44x44mm; in Bath Georgian the meeting rails were only 16mm high x 56mm deep! - without horns, so barely any through-tenoning - many still going strong 270yrs later. Stupendous structural economy - structural glass, in fact. After the Great Fire of London, Building Acts outlawed all combustible material on facade, so window fixed frames were fully hidden behind a nib of masonry, leaving only flimsy sliding sashes exposed on face. Bath Georgian box frames don't have a cill either - the sash bottom rail 56-69 high x 44 deep comes right down onto the stone subcill.

Posted By: biffvernoneasier to make draught-proof.

Why/how?

Apparently Passive House Solutions are working with a uk window manufacturer to develop passive house standard sliding sash windows at present.

<blockquote><cite>Posted By: suecar</cite>Apparently Passive House Solutions are working with a uk window manufacturer to develop passive house standard sliding sash windows at present.</blockquote>

Anyone heard any more on this development since its nearly two years on from that post now?

Vrogum from Winchester Joinery do the most convincing hi-tech sash window I've seen, only 2G but still ... Vrogum tho expensive are something different amongst Scandinavian window manufs, superb looking and unique as they're glaze-in, not glaze-out. That fortuitously gives an external appearance that is near-indistinguishable from trad English cottage windows, also Georgian/Victorian etc, all in 3G (incl above mentioned 2G sash windows). And internally OK too. I was talking to Russell, and I'm sure there's no reason why they shouldn't source something similar from their pan-Europe suppliers' web, at a price that's no reason to be higher than their present lo-cost offering.

Swedish Timber Products do SP windows, next cheapest after Russell. Basically near-identical kit.

Posted By: spoonandforknot sure what you mean by 3g being indistinguishable, seems they're all 2g apart from glaslux which is 3g, but it has coloured perimeter glass to look like frame/hide inner frame. ie that's glaze out

Not sure what that means ... I meant that the outer face of the glaze-in timber frame, together with the outer glass face, are indistinguishable from English cottage - nothing to do with 2G/3G, not about hiding any spacers you can see thro the glass etc.

Posted By: dan_aka_jackI'm wondering if could use our local window makers to make the frames but provide them with a high performance double glazing unit?Jack

Hi Jack

We made our own up and down sash windows last Autumn, we used 4*12*4*12*4 Krypton filled glass with warm edge spacers. I think the overall U-value of the windows is close to 0.8 inc frames. The secret is to use warm edge spacers that are the same colour as the frames. The difference in the heating bills and comfort of our 1880's red brick house last winter was unbelievable. The convection currents have been totally eliminated.

I bought the Triple Glazed glass packets from an Irish glazing company Carey's Glass, they use St Gobain glass.

Interesting progress made yesterday: I phoned our local joinery company to say that I love the look of their windows but I'm not so happy about their u-value (2.0). I suggested that if they give me the precise size of the double glazing units they require then I'll source high performance DGUs to fit their specs.

At this point in the conversation, the guy stopped me and said: "Jack, may I ask why you want a u-value of 1.6 or better?" and I said "because we want to save on gas". It turns out that he received a letter from the local authority on Friday warning that the new Part L may require all windows to have a u-value of 1.6 or better. He wondered if I had seen that (I was aware that a new Part L was coming into practice but I didn't know the precise u-value requirements)

Anyway. The local joinery company was only too happy to take the few notes I'd made about UK suppliers of high performance DGUs (including Carey's Glass, many thanks @VikingHouse!) and now they're looking into getting new, higher performance windows. So, hopefully, everyone's happy: I get high performance windows and the local window company gets a bit of free research about high performance DGUs. Fingers crossed it works out...

Posted By: Viking HouseWe made our own up and down sash windows last Autumn, we used 4*12*4*12*4 Krypton filled glass with warm edge spacers.

Do you have any aesthetic concerns about using such wide glazing units in sash frames?

The reason I ask is because our local wooden window joiner is trying to persuade us that we should use the thinnest available DGUs because wider glazing units "look ugly in sash windows". His preferred DGU is a 3-4-4 (11mm total) DGU with a U-value of 1.6. Not too shabby, but I'd be happier closer to 1.2 (or lower).

So - is he correct that wide glazing units "look ugly"?

(It's worth noting that our local joiner spends a lot of time making windows for listed buildings and he has a strong interest in conservation, so he's obviously got a keen eye for the "authentic". But, to be honest, I'd rather have high performance in a window which looks, from a few meters away, authentic rather than a window that looks authentic when inspected from 10cm away).

Excellent info, many thanks. I'll discuss with the local joinery company.

Thanks,
Jack

Interesting reading these tips. I was also wondering who does the best mock-sash windows. Sliding sashes are always going to struggle to keep up with the performance of tilting or casement windows it seems but is there any reason why it shouldn't be possible to have a convincing (from a distance, until you open it) mock sash window that doesn't actually slide. The ones I've seen don't seem to look too good.

Beau,

Have you got any air leakage test results for your windows?

How do you get them past building control given they will not have an approved energy rating?

If you can accept PVC as a suitable material, Eurosash made by Dungannon Windows in NI have halved the effective air leakage compared to other manufacturers using the same (Spectus) profile. They will also fit triple glazed 28mm krypton filled units with a centre pane value of 0.7W/m2K and can achieve a whole window U value of 1.1W/m2K, subject to size and choice of glazing bars.

I would say these are among the "higher end" in PVC terms, having run through horns and mechanical butt joints rather than welded mitres. You have to ask for a deeper bottom rail if you would prefer it.

Another company I am seriously considering for our build is Baird's Windows in Scotland. They offer wooden windows, made from engineered sections (red grandis by default) and up to 44mm triple glazed units with argon fill for a centre pane value of 0.6W/m2K and an overall U value down to 1.0W/m2K, again subject to favourable sizing and a "one over one" design. *Be warned though that these are a rather chunky ~200mm deep window overall, although slimmer double and single glazed options are available.

Their software gives the U value for each window and although no formal permeability testing has been done, I was told that they could take a sample to a test rig if it proved proved pivotal in my decision. They do include a double seal at the meeting rail and use predominantly Q-lon seals, so hopefully they would fare well.