There is a short answer and a long answer to this question, when you ask if double-glazed glass can be put into sash windows.

Simply speaking, yes, you can put double-glazed units into most sash windows. We’re talking about timber windows, as that is what we specialise in.  Essentially, as long as there is enough space in the rebate to house the thicker units, it will be possible. We have even devised a unique system in the case of very fine Georgian glazing bars (down to 16mm in diameter), to strengthen them with steel inserts making them robust enough to hold the panes in place. Read more here: Timeless Sash WIndows Slimglaze system

Window legislation in Ireland

However, you need to take into account legislation, aesthetics and moral values, which is where the long answer comes in. In Ireland, since the year 2000, under the Planning and Development Act Part IV, windows are protected as part of our built heritage. This comes into account if your property is of historical importance and placed on the local authority List of Protected Structures, or if it is in an Architectural Conservation Area.

If this is the case, an expert survey will tell you if the glass in your windows is original or not. This will be a deciding factor on the options that are open to you. If the glass is original single glazing, you are obliged to keep it intact. However, we can still refurbish the windows, taking the old paint off, restoring any damaged timber, and repairing the putty in addition to a final three coats of water-based specialist paint with a durable finish. This service also includes full draught stripping with the best seals on the market. It is interesting to note that we have had a sash window independently tested in the UK and it achieved a Class 4 air leakage report – which is the best on the market.

Draught sealing

Draught sealing can potentially reduce heat loss from 40% as it prevents warm air from escaping, or indeed cold air whistling in. This will offer a profound improvement on comfort levels in the building.  This can in turn provide environmental benefits with a reduced carbon footprint through a reduced reliance on heating (of course, keeping energy bills to a minimum too!).

Increased property value

Maintaining your original historic sash windows, or indeed adding double glazing when permissible, can increase the property value and improve curb appeal. Sash windows are a sought after period feature, and those that are well maintained indicate an inviting, loved home. Original glass that dates back to the 1700s or 1800s was mouth blown glass, known as crown glass or cylinder glass, depending on the technique used to make it. Both are delicate and often have a waviness detectable in them. This is both reflective from outside and refractive from inside, throwing a soft light on the interiors and a shimmer when the light catches it from the exterior. Such glass is worth preserving as it offers greater beauty than the modern float glass. In tradtional, unlisted buildings, if this glass is still in the windows it is well worth preserving, and considering methods such as draught sealing or secondary glazing to improve thermal performance.

Secondary glazing

Another option that is worth exploring is secondary glazing. This system allows you to keep the original sash window yet adds a thermal layer for greater comfort in the home. The secondary glazed units made by Timeless Sash WIndows, may be timber or steel, and may be made as a sash or carsement style, and can be painted to match the building interiors and the original window. Thermally, acoustically, and for conservation, this works very well. You can additionally use impact resistant glass, especially if the building is being repurposed.

Other Options

The Irish government brough out guidelines for refurbishment in traditional buildings in 2023. It included an excellent report on the different U-values obtained from measures such as closing the curtains (from U-value of 5.5 of single glazing to 3.2), closing the shutters (from U-value of 5.5 to 2.2 with shutters closed),  secondary glazing (from U-value of 5.5 with single glazing to 1.7 with secondary glazing)  and draught sealing.

Double glazed glass in sash windows

To combine the short and long answers, in the case of retrofitting double glazed glass in sash windows, it is a bepoke solution and would ultimately need a survey to understand the condition of the windows, the measurements, the use of the building, the client’s expectations, and the location of the building from a conservation and weathering perspective.

To find out more, contact our highly qualified team by telephone on 046-9023323 or by email to [email protected] and we’d be happy to guide you the process.