The hidden problems with retrofit double glazing

Find out how to upgrade your old timber, wooden and sash windows to unlock amazing energy efficiency benefits. And how we can double glaze them with amazing new vacuum insulated double glazing.

The hidden problems with retrofit double glazing can be avoided with our revolutionary vacuum double glazing. This amazing new double glazing by Panasonic performs better than triple glazing and provide amazing new levels of energy efficiency for your home.

Retro fit double glazing – update June 2023

Since writing this a few years back, we’ve found a new high performance glass that has allowed us to install double glazing into old wooden windows. Glavenir vacuum insulated double glazing by Panasonic provides thermal and acoustic performance equivalent to triple glazing.

It’s amazing new technology that totally revolutionises traditional double glazing and avoids all the pitfalls we wrote about below.

For more information about double glazing your timber and wooden sash windows – click here.

The problems with retro fit double glazing to old timber, wooden and sash windows

The most common question Sealasash receives up front is “do you do double glazing for old timber, wooden and sash windows?”

Some people are surprised and disappointed that our answer is often “no” until we take them back to the first principles of our work.

The thing is that glazing is not the first, second, third or even fourth consideration to take on board when you are looking for improved comfort and efficiency from your old wooden windows.

Yes, there is a lot of heat loss and gain through glass. That’s why glass attracts so much attention. And yes, that’s why we offer a range of glazing options (including double glazing) to improve thermal and acoustic performance of your old wooden windows.

But your window is much more than the glass and we are much more than glass salespeople.

You will be offered improved glass as an option if you want it. But not before the most important stuff is done to improve your windows first.

Yes we can double glaze your old timber, wooden and sash windows, but

Think about it: it’s hard to imagine a business that has a stronger incentive to offer double glazing than Sealasash. We have researched the best possibilities in the market for our customers. But even in the face of these incentives, and with our industry knowledge, we haven’t done it. Why?

Well, it turns out there are compelling functional and aesthetic reasons to steer clear of retrofitting double glazing into old wooden windows.

These include:

* A large section of wood needs to be cut out of the original sashes, sometimes removing heritage moulding features and weakening the joints. The combination of a thick and heavy glazing unit sitting in a permanently weakened frame is a recipe for tears. Windows that have lasted a century or more certainly will not last another 100 years after that kind of treatment.

* Ripping timber out of the sashes is irreversible. This means that when the glazing units fail (a typical guarantee is only ten years) the value of the entire window is permanently compromised.

* A double glazed unit looks out of character. This is particularly the dry glazed types that have a special weep bead that protrudes at the bottom the the glass. The traditional slim look putty line will not exist anymore as beads will be nailed through your beautiful wooden sash. If you love the look of your heritage or old windows, if you want to maintain the charm and true value that original windows add to your property, then make sure you are aware and accept that double glazing will change that look and could possibly devalue heritage. We work in many old and heritage homes and we don’t want to be associated with destroying these features because it’s unnecessary when comfort can be achieved by less destructive means.

* Retrofit double glazed units are often attached by external wooden beads and these are nailed through into your sash. Every extra fastening placed into a wooden sash, particularly on the exterior, presents potential for water ingress and rot over time. The original sashes used putty (and these days modern sealants can be used) to secure single or laminated thermal and acoustic glass without compromising the future integrity of the wood. These are basic carpentry principles, so why would you risk rotting out your old wooden sashes?

* Double glazed units are only warrantied by the manufacturer for 10 years and this has a lot to do with the seal between the two panes of glass breaking down because of moisture build up in the sash frame over time. Some proprietary “dry glazed” systems overcome moisture build up by cutting angles in the wooden sashes (taking out more wood!), driving nails into the sashes and making moisture weep exits, thereby possibly extending the life of double glazed unit. That sounds good, but the installer does not necessarily extend the warranty of the double glazed unit that you purchase, let alone the sash itself. At Sealasash we want to provide outcomes that last generations not 10 years! Can you imagine spending all your money on retrofit double glazing and in 10 – 15 years later you need to replace the glass again? Let’s get real here, we are talking about old windows that had thin float glass put in 60, 70, 80, over 100 years ago and that glass is still functional, albeit not as good thermally as modern glass. Nowadays we can only get a warranty for 10 years out of a modern double glazed unit! These units are simply not designed to last.

* In Australia double glazing is thick (ranging 20-22 mm thick) compared to other options in the northern hemisphere (e.g. slim double glazing). And did you know that double glazing has already been superceded by thin vacuum glazing (down to 6.6 mm thick) which can be 3 times better thermally and 5 times better acoustically than double glazing? Sealasash is not interested in permanently damaging windows to install old technology when there are cutting edge solutions and other more sensible choices on the horizon.


Retrofit double glazing systems require significant changes to heritage window frames to accommodate the thick glazing units and unsightly external beads. 

* Some double-glazing companies only offer draught sealing as an optional extra, despite the fact that the performance of the double-glazed unit is severely compromised without high quality draught sealing. 

How to repair and upgrade timber, wooden and sash windows (including double glazing)

We know from undertaking work on 20,000 windows over the last 10 years in 5 states around Australia that the formula for success for improving comfort from old wooden windows follows some very basic and logical steps

1. Make your windows function properly (open and close evenly, service and overhaul mechanisms etc.)
2. Install the correct type of seals (draught and/or acoustic) in the correct places. Without these done properly you’ll never get the benefit from fancy glass anyway.
3. Undertake repairs to wood (the old growth wood in your old windows is superior to what you get today)
4. Invest the rest of your money (that you otherwise considered spending on new glass) to choose good quality thermally rated blinds/shutter/curtains to layer your window for thermal and acoustic benefit and also funk it up on the inside. We often mention to customers that spreading their spend like this provides such a great result that they do not need to even consider changing glass. Results speak for themselves as most of our customers do not change glass. There have been studies undertaken to clearly show these benefits of combining steps 1 to 4
5. Choose glass to suit your purpose if you think you really think you need it.

Yes, we can double glaze old timber windows

Most of the time, there is a very valid need to replace glass to address thermal and or acoustic issues.

But what many people don’t know is that modern single sheet or laminated glass, which fits into the existing sashes without butchering them, can be used in combination with high performance draught and/or acoustic seals to provide excellent results.

2023 update – fortunately now, we also have access to brand new, high-performance double glazing by Panasonic that has completely revolutionised retro fit double glazing for old timber, wooden and sash windows. You can read more here.

While these glass options have a new sheen compared to the old thin glass, the beauty is that windows can be made to fully work without altering the physical look of the window. Plus, because the sashes are not permanently ruined, the windows will be suitable for installation of vacuum glazing when it becomes widely available in Australia. 

After considering the full story, would you risk the heritage qualities and value of your old windows on the recommendation of a glass salesperson?

If you would like to talk to a timber window expert who understands all your options, and who will listen to your priorities, then please don’t hesitate to contact us.

Looking for double glazing for your sash windows or wooden windows?

Sealasash can double glaze your old timber, wooden and sash windows with new high-performance glass.  We also upgrade windows to include draught and acoustic seals that further improve the comfort and energy efficiency of your windows. 

Learn more about Sealasash:


Vacuum Insulated Double Glazing

Our ultra-high performance double glazing fits perfectly within your existing timber work, eliminating any bulky or expensive modifications.


Fix Those Annoying Rattles

With our draught and acoustic seals you’ll achieve amazing new comfort levels by reducing infiltration of outside air, noise and dust without the annoying rattles.

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Using traditional craftsmanship and modern materials, we’ll restore your windows and make them better than new.  This includes high-performance double glazing and draught seals that will dramatically improve the comfort and energy efficiency of old timber, wooden and sash windows.