Can You Double Glaze a Grade II Listed Building?

It is the question we are asked most by listed homeowners, usually on a cold January survey with condensation running down single panes. The honest answer is that double glazing in a listed building is frequently approved, but only in a specific form and only with listed building consent.

What decides it is not the listing grade on its own. It is the age and survival of the existing windows, and how visible the change would be from outside.

When officers usually say yes

Where the existing windows are later replacements of no special interest, most conservation officers will accept new timber windows with slim double glazed units. The units we specify are commonly 14mm overall with warm edge spacers, which allows glazing bars to stay slender and the putty line to look right from the street.

Approval is more likely when the drawing shows bars, sightlines and paint colour matching the originals, and when the double glazing sits invisibly behind that traditional face.

When they usually say no

• The frames are original and repairable, especially early sashes
• Historic glass survives, such as crown or cylinder glass with its ripple
• Standard thick units would force wider bars or a visible double reflection
• The proposal is uPVC on a principal elevation

Secondary glazing: the common alternative

Where the officer wants the original windows kept, a discreet secondary pane fitted inside the reveal is often accepted without much debate, because it is reversible and does not touch the historic frame. Combined with draught sealing of the existing sashes, it cuts heat loss and outside noise substantially.

It is not as neat as a single sealed unit, and cleaning means opening two layers, but it lets a home with irreplaceable windows become far more comfortable.

What slim double glazing will and will not do

A slim unit will not match the U value of a thick modern triple glazed window, and nobody should promise it will. What it does is remove the worst of the cold glass effect, reduce condensation on the inside face and stop the draughts that come with tired single glazing. For most listed owners that is the real goal.

Questions people ask