Secondary glazing vs double glazing: a practical DIY option for French homes

We have just had our windows replaced with double-glazed units.

I spent years thinking about how I could do it myself, but windows—especially those in older houses—are more complicated than they seem, and you notice them far more than you realise.

In the end, we got professional craftsmen to do it, and this was a good decision.

There is, however, an alternative to double glazing that is within the scope of the handy man or woman: secondary glazing.

This involves placing new panes inside existing windows to create two layers of glass.

You do not get all the advantages of double glazing—where the space between two panes is hermetically sealed to form a leak-proof insulating barrier between inside and outside—but you still gain extra insulation and noise reduction.

It is not suitable in all situations, but it can work well in certain buildings, especially where windows are regular rectangles and do not need to open.

Secondary glazing: when it works (and when it doesn’t)

The main disadvantage of secondary glazing is that it can look unsightly in an old French house if done badly, which is why I want to pass on some tips from my friend David, whom I consider a master carpenter.

He had a conservatory added to his house, with large windows on three sides giving views of the garden, but he could only use it in summer because it was difficult to heat.

It needed full insulation, but without any of the alterations being visible.

First, he insulated the walls and covered them with panelling. Then he built a second inner door.

The main job, however, was designing an unobtrusive secondary glazing system.

It is important to consider the finished effect before you start. The new windows had to match the old ones to make them less noticeable.

In particular, the verticals had to align so the secondary glazing would appear a natural extension of what was already there.

A carpenter’s method for discreet secondary glazing

To complicate matters, David wanted the windows to be removable for ease of cleaning, as condensation can build up between the panes.

He therefore decided to fit runners (two strips of pine forming an L-shape) along the window sills and overhead. Into these, independent window units are slotted, held in place by pieces of wood that could easily be mistaken for decorative trim.

He did not take the easy route, which would have been to buy pre-cut, pre-planed strips of wood. He had the right tools in his workshop, so he started with thick, rough pine planks supplied by a local timber merchant.

From these he cut every piece of wood he needed, planing them smooth as he worked.

If you do not have professional woodworking tools, you can still make elegant secondary glazing using timber from any DIY store—but it will cost more.

For the glass, he worked to two standard sizes and had it cut to order to avoid storing and handling large sheets.

Each removable window is a unit and can be assembled in the workshop. It consists of a broad rectangular frame held together with mortise and tenon joints at the corners.

Inside, a rebate is cut all the way around to take the glass, which is held in place by very fine wooden battens and a near-invisible smear of putty.

The new windows align with the old so there is no change to the appearance of the conservatory.

If anything, the secondary glazing enhances the space. It is now much cosier.