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
  • Bedroom:
    • 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.

  • 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!


Swedish Timber Products Ltd (in York!)

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


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).

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.

120mm wide weights spring 70mm sliding sash is 1.4, double glazed. 28mm minimum, euros 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
  • tel number 0845 602 5574