Double Glazing notes

Final Solution:

We went with Draught Busters Ltd to make and install the windows.  Great company; very easy to work with.  Mel spent a lot of time helping me to get what we wanted.  The final solution was to ask Draught Busters Ltd to make the frames.  If I remember correctly, the glass we specified was a 24mm unit from UA Glass (see below) with a center-pane U-Value of 1.1.


We have an end-of-terrace Victorian house in SE15, London; built around 1905.  We're looking to replace a total of 5 windows (which are uPVC at present) with sash windows (no vertical glazing bar) with the highest thermal performance we can get, without breaking the bank.  The sizes are:

Living room bay:

  • 3 windows, all 1.84m high
  • The side windows are 0.52m wide
  • The middle window is 1.09m wide


  • 2 windows
  • both are 1.56m high x 0.97m wide

Standard fittings.

We'd like the windows to look as much as possible like original wooden sash windows.

Ideal glass:

  • white warm spacers
  • toughened
  • soft coat
  • gas filled (argon or, better, krypton)
  • low u-value
  • apparently a centre-pane U-value of 1.1 is required for the DGU to get a whole-window U-value of 1.6 when using standard wooden frames
  • Need to get a certificate for the whole window if we tell BCO (local joinery company may not be capable of this).


Forums & Links

Suppliers of Double Glazing Units

Clayton Glass

Very helpful.  

Can deliver to London

Spoke to Jason on Tue 18th May

4-10-4 argon planitherm, toughened = U1.5 = £50 / m2 (advised that we can probably get this locally)

4-10-4 kryton filled, soft coat, toughened = U1.1 = £90/m2



Apparently their slimline product can achieve a u-value of 0.9 with an overall thickness of just 16mm but it's £204 per square meter!

They do lots of products and sent me a price list and details

Swedish Timber Products Ltd (in York!)

Spoke to lovely chap on 4th May (Steve).  Very helpful.
They don't do sliding sash but they do some windows which may look in keeping but it'd be a bit of an experiment.
Costs are £350-400 per m2 for whole-window U-value of 1.3  Add 40-50% for U=0.8
They would only provide the windows (plus bits and bobs); we'd have to use a local builder to install.

103mm front to back

Krypton costs a lot.  Krypton is required to get below 1.0.  Warm edge doesn't add that much onto the cost

Japanese vacuum glazing

(there's an interesting article on about a Japanese double glazing unit with an overall thickness of 6mm and a U-value of 1.4. It achieves this impressive U-value by maintaining a vacuum between the two panes of glass. )

Pearson Glazing

2 Pennethorne Road, Peckham
Tel: 020 77321511,  Email:
DGU charged at £71.50 m^2, £25 min per unit
I assume this is for regular glass

I called 4th May.  All she said is that a 24mm glazing unit in timber frame, argon filled, 4mm float, will "confirm to building regs" (which means >=2.0, I think), toughened = £125/m2

UA Glass, Norwich

Enquiries []
For 4 sealed units 376 x 671, in standard clear pilkington K glass come to £67.78 inc VAT, for all 4 units (hence, £15 each).
Anna Tomlinson, UA Glass Ltd, 01603 414557

I phoned UA Glass.  The lady who answered the phone at UA Glass was very knowledgeable and helpful.  They can supply DGUs from 14-28mm thick (changing the overall thickness just changes the gas-filled gap between the glass, the thickness of the glass remains the same and hence the weight remains fairly constant).  The wider the gap, the lower the u-value.  A 20mm thick DGU with their highest-performing glass coating achieves a U-value of 1.4; if you go a bit wider then it goes down to 1.2 (which is incredibly impressive; I was under the impression that to get below 1.4 you had to move from double to triple glazed, adding significantly to cost and weight)

20mm = 1.4 W/m2
24mm, white spacer available = 1.1 W/m2 = £50 / m2 float, £70 / m2 toughened

No performance increase above 24mm.

If thinner and lighter glass is required then they also do a product called "SashLite" with an overall thickness of 11/12mm and achieves a U-value of 1.6 (3mm or 4mm Clear / 4mm Krypton Gas Filled Cavity / 4mm Low E; warm edge).

Carey Glass, Ireland

Recommended on Green Building Forum.  Manufacturers very high performance double glazed and tripple glazed units.  Many options significantly below a u-value of 1.  Also make entire sash windows with a U-value of 0.7-1.4.

Their Elite TGUS have a u-value of 1.1 and their Elite SE get down to 0.7 for 28mm total thickness

They do have an office in Manchester.  I called.  The Manchester office only deals with large architectural orders of 100s of square meters and doesn't deal with the full windows (including frames) at all

Called their Irish office on Tues 4th May.  Waiting to hear back.

120mm wide weights
spring 70mm

sliding sash is 1.4, double glazed.

28mm minimum, 

euroes 1800 for all three; 5 weeks;

28mm thinnest triple
18mm 1.1-1.2 u-value

Living room bay:

  • 3 windows, all 1.84m high
  • The side windows are 0.52m wide and do not need to open  £890 + Transport each
  • The middle window is 1.09m wide £1119 + Transport each


  • 2 windows
  • both are 1.56m high x 0.97m wide £1028 + Transport each

+ 15% delivery + 21% Irish VAT


  • Found them on this GBF thread: Energy Efficient Sash Windows
  • 0845 602 5574
  • Apparently they can do wooden sliding sash windows with a whole-window u-value of 1.4
  • I need to send them an email with a photo of the windows & dimensions
    • c-windows and doors
      fao: ashley

      tel number 0845 602 5574



Mr Kelly


Further to your enquiry regarding 5 box sliding sash windows. Based upon the sizes provided the cost to supply the 5 units is £5,708.85 plus VAT & delivery.


The units are double glazed with an Ug of 1.1W/m2K with thermally insulated boxes and seals to the sashes. The opening operation is traditional pulley and cord. We can offer spring operation if preferred.


Delivery is 6 weeks from confirmation of manufacturing size and a 50% deposit being paid.


We trust the above is of interest, if we can provide further information please do not hesitate to contact us.




28 Winchelsea Road,

Rye, East Sussex,

TN31 7EL

Tel: +44 (0)845 602 5574



1. Triple glazed performance contemporary style windows (sorted low/high price order/ - exact details will follow in few weeks):
Rationell - good value timber or timber/alu with
Protec 7 - timber/modified fibre glass composite high performance windows with low U-values - in double glazed timber/alu version are price competetive with Velfac
ENERsign - timber/fibreglass passive haus certified with U value <0.7. Sleek design and minimum sigt line from the outside. My overall winner at the moment - will be getting quotes soon to find more detail on price.
Skyframe - sliding triple glazed windows bonded to slim fibreglass profiles for frameless experience. Decent U-values in triple glazed category.  Preffered choice of  bauhaus lovers with  deep pockets.
2. Triple glazed performance rooflights/skylights:
Fakro FTT U5 Thermo - triple glazed with U value of approx 0.95 (which beats  velux equivalent by approx 0.05)
Glazing Vision Skylights - flat roof skylights available in triple glazed version. More expensive than the above.
More types/info can be found here -

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