CN Heritage Slim Double Glazed Units

Conserving our Architectural Heritage

When looking to improve the thermal performance of a building the windows are a building component where there are many factors to consider. This is particularly the case for single-glazed windows in older and historic buildings. The outright replacement of the windows in Listed buildings is often considered inappropriate, for conservation reasons.

Simple energy-efficiency measures, such as draught-proofing, shutters or curtains can make a significant improvement to the energy performance of old windows. Secondary double glazing is also possible, but care must be taken to ensure glazing bars match the windows’ bars.

Repair is the first option to consider when seeking to maintain the character of a building, Sections of the frame which are no longer sound can be cut out and replaced. Sashes can be freed, cords replaced and draught-proofing materials added discreetly.

Most timber windows have been replaced because the homeowner has wanted to reduce energy bills by installing double glazing.

Retaining the original windows

Retain the original windows by replacing the original single-glazing with C N-Slim Heritage Double Glazed Units, this is a form of double-glazing with a cavity between the two glass panes that is thinner than that in conventional double-glazing units. The C N-Slim Heritage Double Glazed Units can be glazed into most existing single glazing glass rebates thereby allowing for the retention of the existing window casements and/or sashes, an important conservation aspect, as the installation of double-glazing in historic buildings may be more acceptable in situations where it can be incorporated within the original joinery.

A C N-Slim Heritage Double Glazed Unit generally has a less efficient thermal performance than conventional double-glazing, but performs thermally very much better than single-glazing. The thermal performance of slim profile double glazing can however be improvedsignificantly further by using technical glass and inert gas to fill the cavity.

Replacement windows

If we are truly concerned with conserving our architectural heritage then it is essential when replacing the whole window or door to install a true reproduction of the original using traditional materials.

The right replacement window will combine all the performance benefits of a modern window with the authentic look and feel of a traditional window. And, because they are factory-made, with factory-applied paints and glazing systems, they offer a long service life of over 60 years, with simple maintenance, providing excellent value for money. Modern replacement timber windows will combine authenticity with modern demands such as high energy-efficiency, security, low maintenance and a reassurance that they will last 60 years or more.

True replica windows involves real craftsmanship and therefore does not come cheap, and will possibly (but not necessarily) involve losing the character of the original glass although we do have the option of using restoration glass that is manufactured using a traditional blown glass method. A critical factor to be considered is the importance of the original construction methods with individual glazed panels to create a varied light reflection across the whole window in order to maintain the original character of any period property façade. Particular care is given to the detailing of traditional sightlines and elegance of glazing bars and beads. In conservation areas, it is possible to replace existing windows with modern energy-efficient timber equivalents which mimic the original and maintain the character and value of the building. And with modern paint systems, any maintenance of the window need not be required for 7 to 10 years after installation.

Retaining the original windows

Slim Heritage Double Glazed units

Retaining the original windows

CN-Slim Heritage Double Glazed Units

CN-Slim Heritage double glazed units have the appearance of single glazing therefore retaining the character, charm and heritage of the building. Only a very close inspection will reveal the glass to be a double glazed unit especially if Crown reproduction glass is used. When glazed traditionally into sash/casement window frames the units are virtually undetectable because slim units do not have the double reflection or the distorted reflection that is seen when you sometimes look at standard double glazing, making them perfectly suitable for conservation or listed buildings. (subject to local planning).

By also using the technical glasses available today and by filling the space between the two panes of glass with an inert gas a home owner may gain all the thermal and sound benefits of normal double glazing without compromising the appearance or the value of their property.

CN-Slim heritage double glazed units

CN-Slim heritage double glazed unit

Making a CN-Slim Heritage Double Glazed Unit


For a unit that will fit into a minimum 17mm rebate the outside glass will be 3mm thick and the inside glass 4mm thick this will produce an overall unit of 11mm depth. The glass can be simply clear or patterned annealed/float glass, for security it can be toughened, for an improved thermal performance it can be Low E Energy Saving glass.

For new replica timber windows, the rebate will be slightly deeper allowing for both internal and external glass to be 4mm therefore the overall depth of the unit will be 12mm.

Restoration Glass – Crown Glass

The only traditional method of flat glass making left is the cylinder process (known as broad or sheet glass). This is now used in restoration projects that would originally have utilised crown glass.

Glass is gathered on the end of a pipe and placed on a block of hollowed-out wood. The glass is blown and then re-heated. The blower swings the glass “pipe” in a vertical plane until it is elongated into a cylinder. The cylinder is then split down lengthways.

The split glass is then re-heated and flattened on a smooth stone and then annealed.
This process allows the colour range to be limitless; the glass is used ecclesiastically, commercially, domestically and for restoration purposes. It is also easier to make in comparison to any of the other processes.

This type of glass has no impact on the U-value of the unit, it is just the same as ordinary annealed/ float glass.

Edgetec Super Spacer

To produce our CN-Slim Heritage Double Glazed Units we use Edgetec Super Spacer bar. Edgetec Super Spacer bar introduces no-metal, pre-desiccated, structural foam spacer system that is a warm edge technology to suit all sealed glass unit applications helping to deliver windows of the highest standards.

Super Spacer bar is produced down to a minimum of 4mm therefore our CN- Slim Heritage units may be produced with a minimum overall depth of 11mm this means it will fit a 17mm rebate.

Traditional timber windows are glazed with low sightlines. This creates problems for standard aluminium spacer bars which protrude into the sightline of the window. Super Spacer has a lower sightline (equivalent to only 5.5mm from the edge of the glass) which eradicates the problem.

Its award winning, patented Thermal Set Spacer (TSS) technology gives the ultimate in high performance spacer systems. Super Spacer has 100% memory and allows for expansion and contraction, a major cause of fractures in conventional sealed units.

With Super Spacer® thermal bridging is all but eliminated, resulting in a massive 70% reduction in condensation.

Most of the heat loss through a window occurs at the edge of the unit, Super Spacer® blocks this escape route.


The introduction of gas into the cavity of a double glazed unit reduces the heat conductance between the panes, helping to keep more heat in during the winter and out in the summer. As a result living and working conditions become more comfortable the whole year round. The combination of Argon Gas and Low E energy saving glass can achieve an improvement in thermal performance in the region of 50% over and above that of a standard sealed unit.

Argon, which has 34% lower thermal conductivity than air, is the most commonly used. Apart from its heat insulation properties, Argon is non-toxic, sunlight stable, non corrosive, colourless and odourless.

Although argon-filled units cost around 5% more than air-filled units, they can improve a double glazing window’s U value or energy rating by over 30%. Argon fillings should last the lifetime of a double glazing window and over a period of 25 years should not lose more than 5%.

Krypton and Xenon are even more energy efficient than Argon but are seldom used in double glazing windows as they are more expensive.

It is important to note that sometimes you do have to use Krypton/Xenon when limited width is available. i.e.: in old buildings for example.



Super Spacer is produced down to a minimum depth of 4mm allowing us to produce the VERY THIN Double Glazed Units suitable for glazing in conservation areas and listed buildings (subject to planning permission). It is important to note that sometimes you do have to use Krypton/Xenon when limited width is available. i.e.: in old buildings for example.


Technical Glass

With the use of technical glass and by filling the cavity with an inert gas the U-value of CN-Slim Heritage Units can be as low as 1.0.

The reduction of heat loss through the glass is up to 77 per cent as when compared against single glazing

It is possible for traditional reproduction replacement windows installed with CN-Slim Heritage Double Glazed Units to achieve the new Part L building regulations for thermal efficiency. (Subject to the size of the glazed sections of the window)


U-Values (the term used to measure thermal efficiency)

A U value is a measure of heat loss in a building element, therefore, it is the measurement of the whole window and not just the glass. It can also be referred to as an ‘overall heat transfer coefficient’ and measures how well parts of a building transfer heat. This means that the higher the U value the less efficient the thermal performance of the building element. A low U value usually indicates higher levels of insulation.

The thermal efficiency benefits of CN Slim heritage Double Glazed Units over single glass can be seen below:
Single Glass – U-Value ….is approx. 5.6 W/m2K

Glass 1 (External)CavityGlass 2 (Internal)Centre Pane U-Value
4mm Float/Toughened4mm Krypton Filled4mm Planitherm Total Plus Low E1.9W/m2K
4mm Float/Toughened6mm Krypton Filled4mm Planitherm Total Plus Low E1.5W/m2K
4mm Float/Toughened4mm Eco Gas Filled4mm Planitherm Total Plus Low E1.7W/m2K
4mm Float/Toughened6mm Eco Gas Filled4mm Planitherm Total Plus Low E1.3W/m2K
4mm Float/Toughened4mm Xenon Filled4mm Planitherm Total Plus Low E1.4W/m2K
4mm Float/Toughened6mm Xenon Filled4mm Planitherm Total Plus Low E1.0W/m2K



We bed the units on silicone centralising the unit within the frame, fill the void between frame and glass with a low module silicone this allows both movement and a waterproof seal, allow the silicone to cure and then face putty or use a timber glazing bead to finish.

CN-Slim heritage double glazed unit

CN Glass glazing

CN Glass Edgetec super spacer

CN-Slim Heritage Double Glazed Units

  • Available from 11mm depth
  • Sightlines of 7mm
  • Use Planitherm Low E Glass for a warmer house in winter and cooler in summer
  • Use restoration Glass
  • Use Toughened Glass
  • Fill the cavity with an inert Gas for improved acoustic performance
  • Low U-Values
  • Compliant with Document L

Call one of our offices for more information on our slim double glazing for heritage and listed buildings – 01295 263364

More information on our Slim Heritage Double Glazed units can be found in the following article published by ‘The Listed Property Magazine’.