The Problem with Conservation Double Glazing

The Problem with Conservation Double Glazing

Slim Double Glazing in Heritage Windows

Conservation area double glazing has always been a sensitive area in the restoration of Listed Buildings and those in Conservation Areas.

The owners of the building want to install windows with modern performance to make their rooms more comfortable, and their heating bills more manageable!

However local planning officers and organisations such as English Heritage put conservation first. They much prefer to restore original windows that to replace them.

In this post we discuss the dilemma facing home owners and conservation officers when upgrading listed building to slim double glazing.

The Background:

Slim double glazing came into use about 50 years ago and comprised of 2 panes of glass separated by air. Whilst this was a major step forward from single glazing it was soon found that the insulating value was poor. As technology advanced, the space between the glass panes was increased which improved it’s thermal performance. Also the edge spacer technology improved to ensure there was sufficient secondary sealant to maintain the integrity of the air space and enough dessicant to control any moisture within. So the now standard 28mm Double Glazed Unit was evolved and has been a reliable addition to almost every home.

Conservation Area Double Glazing

People who live in period homes, Victorian and Georgian properties, could not benefit from this technology . The original timber frames in Conservation and Listed buildings simply could not accept such a wide unit.

Slim Double Glazing was not the solution it was supposed to be

So the glass industry created the first slim double glazed units that could fit into such frames. The reduction of air space between the glass panes was compensated for by injecting noble gases such as Argon and Krypton. This slowed down heat transfer between the panes. The desire to fit them into the small glazing rebates led to very low sightline edge spacers, sometimes as low as 5mm.

Why Slim Double Glazing Units Fail

Whilst this achieved the desired aesthetics, and good performance, the fact is the reduction in secondary sealant and desiccant has led to these units failing at an alarming rate. Not helped by contractors attempting to squeeze a ‘quart into a pint pot’ by reducing the bedding compounds etc. 5 years of service life seems quite common before the units ‘mist-up’. I recently heard of an installation that failed after 1 year.

Role of Conservation Officers:

Conservation Officers are currently stuck between a rock and a hard place. On the one hand they are doing their job by protecting and preserving the built environment and the country’s heritage. On the other hand they are under pressure from all sides by Climate Change targets, energy saving and performance targets, as well as the demand from listed building occupants who claim the right to the comfort and energy savings than others enjoy.

It’s also an unfortunate by-product that a great deal of rot on existing single glazed timber windows is caused by the constant running and pooling of water caused by internal condensation.

Some Conservation Officers will allow retro-glazing with slim double glazed units so long as the existing profiles and glazing bars remain. In-advertently they may be driving the demand for such units that fail far too early.

The environmental story is poor. Retain single glazing and have poor energy performance and possible timber rot. Insist on slim units and the regular renewal will fill landfill sites.

The solution: Vacuum Insulated Glazing

One solution for conservation double glazing is to look to the new technology Vacuum Insulated Glazing. Ultra -thin by design and not reliant on dessicant or gas. Design based on the principle of the vacuum flask, two panes of glass separated by a vacuum space of 0.3mm. These units allow for retro-glazing of existing windows with single large panes of glass and with glazing bars bonded on to achieve the multi-pane effect if needed.

Vacuum Glass is a solution for Conservation Double Glazing

Whilst I can almost hear Conservation Officers and English Heritage screaming, think about the benefits. Glazing bars as fine as the originals, some as slim as 16mm wide and profiles as ornate as the original. More efficient windows, as the glass has a higher thermal efficient than timber glazing bar and putty. Conservation area double glazing that is 10 times more efficient than single glazing. No funny reflections as the panes are so close, and a service life in excess of 25 years, so less landfill and customer frustration.


Some close up shots of Heritage Windows with Vacuum Insulated Glazing