Three top tips: installing windows in heritage buildings

The UK has a wealth of heritage architecture that public bodies like Historic England are committed to preserving. 

But as a building with listed or conservation status ages, it will need to be maintained or upgraded. Replacing windows, in particular, is often desirable or even necessary. This is in part due to decades of weather exposure causing damage to the glazing or frame, but more often now, it’s needed in order to improve an old building’s energy-efficiency, to reduce the occurrence of internal condensation, and to help ensure we live in healthy homes.

Heritage projects often come with a specific set of challenges to overcome, as conservation efforts can naturally conflict with measures to upgrade a building. So, what are the key parameters that installers and specifiers need to consider?

1. Ensure the right planning permission is secured

Before any work is carried out on a conservation or listed project, the property owner will need to get planning approval from the local authority and conservation officers. If the property is subject to listed building restrictions or an ‘Article 4 direction’ – a legal instrument councils can use to tighten the rules on buildings in a certain area or of a given type – detailed approval of the proposed refurbishment will be needed.  

It’s worth recognising that planning officers often perceive the loss of traditional windows from older properties as one of the major threats to a building’s heritage, as well as that of the area where it’s located.

As a rule of thumb, approval is more easily secured on conservation projects than they are on listed buildings, but officers can differ very widely in how strictly they apply or interpret the regulations.

2. Accurately match the original window

Planning consent will often stipulate that any replacement windows in a heritage building are accurate copies of the originals. But replicating the profile of older windows, which were usually single glazed, can be particularly challenging if a key aim of the project is to boost energy-efficiency by including double glazing in the replacement process. 

Glazing products with improved insulating properties are thicker in profile than single glazing due to the use of two or more panes of glass and gas-filled cavities. The use of two panes of glass can create double reflections and make visible the spacer bars that separate the glass panes and form the insulating cavity. Often, this may be unacceptable in the eyes of conservation officers – and it also creates the need for deeper frames to accommodate the thicker glazing. 

Many IGUs made using standard modern manufacturing techniques for the purpose of replacing single glazing in traditional frames, are manufactured with very low sightlines. This is to enable them fit into the narrow timber transom and mullion profiles. Unfortunately, this sometimes means that sealant depths are extremely low and may render those products vulnerable to failure due to moisture penetration.

A solution can be found in products like Pilkington Spacia™, an IGU that has the thickness of single glazing but the insulating performance of double glazing, having a U-value of 1.1W/(m2K) - the unit of measurement that determines the level of insulation an IGU offers.

It has an overall thickness of 6.2mm, which is roughly a quarter of the size of standard double-glazing units. As each IGU is bespoke in terms of size, most window styles can be upgraded with Pilkington Spacia™. Curves (in elevation) and many shapes can be accommodated for, with the exception of curved in plan windows.

Pilkington Spacia™, is a vacuum insulating product and is made by a completely automated process, different to that of standard IGUs giving it a naturally low sightline making it ideal for Heritage Glazing applications. It carries manufacturers guarantee of 10 years as standard.

In past projects, our installer partners have liaised with planning officers and demonstrated specially fabricated prototypes of a replacement window in order to reassure them that the appearance will be very similar to the original. This is encouraged by Historic England’s guidance on replacement windows.

While conservation officers and planners hold a lot of power when it comes to refurbing old buildings, many of them are also sympathetic to the need to improve their environmental performance – so it’s worth investigating the options for sensitive modernisation and checking with the relevant authorities. 

3. Know the regulations, speak to an expert

While most buildings are subject to energy performance requirements under Part L of the building regulations, listed properties, those in conservation areas and scheduled monuments are exempt from compliance where it would unacceptably alter their character or appearance.

We’re anticipating a consultation soon on Part L. As part of the changes, the supporting Approved Documents are expected to recognise that a building’s fabric, and in particular its windows, are crucial in the design of highly energy-efficient and healthy homes.

While we can only wait to see what the new proposals for Part L will bring, it will be interesting to see whether guidance will change in regards to what U-values are satisfactory when a historic building’s external appearance needs to be maintained.

Whatever happens, it will be important to speak with an expert to determine compliance. Distributors of Pilkington Spacia™, for example, are specialists in glazing for heritage buildings. They are also directly supported with advice and guidance by Pilkington, and will be happy to help with queries.

Image Courtesy of ENERGLAZE