Slimpane Double Glazing for Sash Windows

Slim Double Glazing on Period House

Standard double glazed units (DGU’s) consist of two panes of glass separated around the edge by a spacer bar (usually in a gold or silver finish). These spacer bars were typically 10mm wide and that space was filled with an inert gas called Argon. This meant that to get a good thermal performance, the units had to be at least 18mm wide (0.7 inches).

However, in the last decade, it has become possible to reduce this width down from 18mm to only 12mm (0.5 inches) and by using a heavier inert gas, such as Krypton in this narrower space, one can achieve thermal and acoustic performances similar to standard D.G.U’s, while also offering the standard five year warranty.

This breakthrough in spacer bar technology has major implications for double glazing period buildings. Now we can make DGUs only the thickness of a Biro and by using a dark coloured spacer bar, it virtually disappears and looks more like a glass shadow, rather than double glazing, even under close scrutiny.

Slim Double Glazing Corner unit
Close up of Slim Double Glazing Window

It is worth mentioning at this point that, if a sash window has not been draught proofed, there would be very little benefit in double glazing it, as the heat loss from an unrefurbished sash window is mostly due to draughts (about 85%), while the single glass pane is only responsible for about 15% of the total heat loss (by conduction). This is a much slower process, when compared with a flow of cold air around the sashes into the room. This draught proofing and refurbishment work in protected structures does not require permission from the local Council. For more detail on sash window refurbishment, please refer to our website

Limitations and constraints

There are some constraints in the use of Slimpane units in sash windows, some technical and some regulatory. If the building in question is a “Protected Structure” and is on the local Council List of “Protected Structures”, then one is obliged to apply to the Council for an exemption from planning requirements, to change the glass from single to Slimpane double glazing. This exemption is called a “Section 5”. If there is historic glass in the window, the Conservation Officers normally refuse to issue a Section 5 for this purpose.
If there are glazing bars in the sash window, such as those with six panes over six (Georgian) or two panes over two (Victorian), we can only fit Slimpane DGU’s if these bars are more than 22-23 mm wide (measured from one pane across to the next pane). If the width is less than that, the black seal between the two glass panes will be visible and very unsightly.

Typical section of a slimpane glazing bar

Thermal Performance of Slimpane DGU’s

Thermal performance is measured in U-Values – the lower the value: the better the performance. Single glazing has a U-Value of about 5.4, while the regulation for double glazing requires it to be 1.9 or lower. The most obvious way of improving the U-Value of any DGU is to increase the volume of inert gas between the two panes, as you can see in this chart for Krypton gas:

Overall Thickness Pane thickness Gas Space U-Value
12mm 4mm 4mm 1.9
14mm 4mm 6mm 1.5
16mm 4mm 8mm 1.3

The other way of improving the U-Value of a DGU is to apply a coating on the outward facing surface of the inner pane and this is referred to variously as “Low E” (emissivity) glass, “K GlassTM S”, or “soft coat glass”. This coating is now standard on most DGUs.

Acoustic Performance of Slimpane DGUs

Sound is measured in decibels (dB). Busy street-side levels are about 80 dB, while in a sitting room you need 45 dB, or less, to be comfortable. So, where noise is a problem, you need to achieve the greatest sound reduction that one can.
There are four ways of reducing sound passing through DGUs:


Apart from keeping heat in and noise out, double glazing has another major benefit i.e. it prevents condensation on the glass in cold weather. Condensation accumulates on single glazing in rooms where warm moist air hits the cold glass. It condenses on the glass, forming pools of water at the bottom of the pane(s). When DGU U-values are 1.9 or lower, condensation is normally prevented, when the moisture levels in the air are not extreme.

Please contact us at if you have any other questions. 

Condensation on Single Glazing Window

Photo by Ozgu Ozden – Unsplash.

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