The Challenge of replacing windows in listed properties

Replacing old wooden single glazed windows in listed or heritage property is always a question of striking the right balance between performance and aesthetics,  Andrew Madge, managing director of Gowercroft Joinery explains

The preference of Conservation Officers is usually to repair the existing timber frames, where possible, however if they are degraded beyond repair, then that is where the difficult choices and compromises begin.

Finding a like-for-like replacement window that sits well in a historic building is obviously of paramount importance, but most owners of period properties would also like to benefit from modern performance standards, including functionality of products, sustainability of materials, and the draught-proofing and energy-efficiency of the final window, in order to create a pleasant living environment.

Single-glazed replacement windows

Carefully crafted, singled-glazed timber windows, with a glazing thickness of 3–4mm, may be the perfect aesthetic replacement, but with a centre pane U-value of over 5.0 W/m2K, their energy efficiency is poor.

One way round this is to install secondary glazing, which may, help preserve the historic fabric of the building, whilst improving thermal and acoustic performance. However, if the original frames or seals are degraded, moisture will become trapped between the window and secondary glazing causing condensation. Secondary glazing also reduces the functionality of the windows and, as a result, can make the rooms “stuffy” and unpleasant.

In order to achieve modern performance levels, most specifiers tend to turn to double glazing.

Standard double-glazed windows

Standard factory fitted double-glazed units with an Argon gas-filed cavity typically between 16 and 20mm offer much improved energy efficiency, with centre pane U-values in the region of 1.2 W/m2K. However, the use of these units in heritage properties leads to a significant compromise on traditional aesthetics. The thickness of standard double-glazed units not only creates a giveaway ‘double reflection’, but the thicker frames required to support their size and weight, are typically not acceptable in period properties or in conservation areas.

Low sightline double-glazed windows

This has led to the popularity of ‘low sightline’ slim double-glazing. These are slimmer versions of standard double glazing, which combine a narrower cavity (to reduce the “double reflection”) with a heavier inert gas, such as Krypton or Xenon, in order to achieve centre pane U-values in the vicinity of 1.9 W/m2K. The sightline, (which is the area from the edge of the glass to the top of the spacer bar), is then reduced to approximately 5-6mm to allow for the thinner window sections.

Although it seems an excellent idea on paper, in practice there are major shortcomings. To achieve the slim sightline, it is necessary to reduce the amount of sealant and desiccant used in the perimeter of the unit, which can cause instability and significantly increase the likelihood of unit failure. This has prompted industry-wide discussion (including in the Glass and Glazing Federation (GGF)) as to whether some of these units even comply with the Construction Products Regulations (CPR). 1

Vacuum glazing

An innovative new option, which is attracting considerable interest in the heritage sector, is ‘vacuum glazing’. In these units, the air is extracted from between a pane of low emissivity (low-e) glass and a pane of clear float glass in order to form a vacuum cavity. With no air or gas between the panes, heat and sound cannot be transferred, resulting in excellent thermal and acoustic performance.

Its ultra-thin profile, reduced weight and high energy-efficiency compare favourably with double glazed units. For instance, Pilkington Spacia™ has a total thickness of 6.2mm, (the vacuum cavity being just 0.2mm) and offers U-values of 1.1 W/m2K on its standard units (and as low as 0.9W/m2K on its higher performance Spacia™ Cool units). This equates to roughly a quarter of the thickness of standard double-glazed units and half the thickness of a typical slimline double-glazed unit with approximately two thirds of the weight.

Authentic wooden frames

The advent of vacuum glazing reduces the weight and thickness of the glass and the problem of a double reflection, however manufacturing complementary timber profiles with a traditional aesthetic presents its own set of challenges.

The original features of traditional hand-made wood windows may look the ‘real deal’ but often do not match modern performance requirements. As an example, the use of linseed oil putty to fix the glass panes, although authentic, requires solvent based coatings, higher levels of maintenance and significantly reduces the overall security of the window. Similarly, the use of full pass-through glazing bars can reduce the energy efficiency of a window and significantly increase the cost to the end customer.

Modern enhancements, such as putty lines replicated in the outer profile and neatly applied glazing bars, can be un-intrusive, performance enhancing and offer considerably better value for money.

Forward thinking timber window manufacturers like Gowercroft Joinery are not only embracing vacuum glazing, but combining it with modern sustainable materials like Accoya® and ultra-low maintenance, water-based protective paint finishes to deliver windows with a traditional aesthetic as well as modern standards of performance and sustainability.

These products are gaining traction and are regularly being used in Listed properties and conversation areas across the UK, including the sympathetic renovation of Grade II Listed Templeton House, Winston Churchill’s former house on the edge of Richmond Park.

Of course, any attempt to provide an acceptable modern window solution for a listed property will inevitably involve some degree of compromise, but conservation officers, specifiers and owners of listed properties do now have a highly functional, future-proofed and sympathetic modern product to consider in that process.