Can You Double Glaze An Existing Window?

Double glazed windows are some of the best home improvements you can make, especially in the wake of Britain’s energy price hike, but it may not be cost-effective to buy entirely new double-glazed windows for every space in the home. But what about adding glazing to an existing window?

The answer is – yes. That being said, double glazing isn’t just adding a second pane, but also ensuring the insulating gas is placed between them. 

As a result, we’ve decided to put together a guide that explores the conditions required for double-glazing, as well as what goes into it. 

Feasibility of Double-Glazing Existing Windows

The possibility of double-glazing existing windows depends on a variety of factors, as seen below: 

  • Window age – The date of the window’s installation is important, because older windows were less technologically impressive, lowering compatibility. For example, a good deal of older casement windows have frames that cannot accommodate a second pane because they were designed with weight limitations. Typically, the rule goes, the older a window is, the less likely it is that you can add a second pane. 
  • Frame Material – The material of the window frame is a big factor. For example, in our experience, wooden window frames often require reinforcement before upgrading. Modern windows are usually constructed from aluminium or uPVC frames, which are quite hardy and can take loads much heavier than window panes. 
  • Building Regulations – Logistics and rules are the bane of home improvement. Some regions around the UK have specific requirements concerning thermal efficiency. For example, some areas that are protected by law may not allow the addition of double-glazing due to the fact that the architecture must be preserved (I.E. windows in a Tudor era home). 

What’s better – Secondary Glazing or Retrofit Double Glazing?

There are two main ways to double-glaze existing windows. We will go through both below.

Secondary Glazing

Secondary glazing is the installation of a second glass pane on the interior side of a window. Between the panes, a layer of air will be trapped. The air is, of course, less effective than the argon gas that comes with professionally installed double-glazing, but nonetheless, it does a much better job than single-pane at insulating your home.

This process involves measuring the window you already have, crafting a new frame, and fitting it alongside the secondary pane into the frame. It is not an intrusive operation.

It is suitable for historic or listed buildings, as well as for people whose budgets are constrained.

Retrofit Double Glazing

Retrofitting double-glazing may sound like just putting in a new window, but it’s not. Retrofitting double glazing entails getting rid of the single pane glass, and putting in a pre-made double glazing unit into the frame. This makes the work a lot more comprehensive and time-consuming, not to mention, it leaves you with a hole in the wall for the duration of the operation.

Retrofitting double-glazing has the unique advantage over secondary glazing in that some of the pre-made units come with Argon Gas. This is superior to air. That being said, secondary glazing is not only non-intrusive but is more accessible. With retrofit double glazing, you will likely be unable to put them in listed buildings, as well as window frames that are old/weak.

How do I double-glaze my existing windows?

This section will explain how to install double glazing on existing windows. 

Initial Steps

The following are the initial steps you should take for both Secondary and Retrofit Double Glazing. 

  • Assess (Secondary Glazing Only) – Examine the window frames. Look for visible signs of damage such as cracks, or rotting. Press the frame to test these spots for softness, as well as any other areas that are suspect. Also, test the joints, how well the window opens and closes, and if there’s any rust/corrosion on them. 
  • Measure Windows – Measure the inside height of the window from the left, middle and bottom, and the inside width from the top, middle and bottom. Finally, measure the depth, and cross-reference all of this information with your secondary glaze. Always give 3-5mm leeway. 

Choosing Material

There are several types of window material to choose from:

  • Acrylic is lightweight and impact-resistant but is prone to scratching and degrading in visibility over time. 
  • Polycarbonate is even more impact-resistant and ideal for those who are concerned about security but is more expensive whilst also being susceptible to scratches.
  • Glass has the best clarity, looks traditional, and is highly scratch-resistant. It’s also heavier, more fragile, and usually more expensive.  


The following are factors to consider: 

  • Thermal Insulation – Glass offers better insulation of all materials. 
  • Sound Insulation – Polycarbonate is thick and dense, soundproofing the best of all. 
  • UV Resistance – Some glass types come with UV resistance

How do I install secondary glazing?

First, we must clean the area: 

  • Clean – Remove any dirt, dust or debris from the frame, and clean the glass/frame
  • Clear – Move furniture and objects away to create a work environment with space. Cover the floor with sheets to protect against spills/dust. 

Install the Frame

Align the secondary glazing frame against the existing window frame and ensure it is evenly positioned. Mark the drill holes if you’re using a screw-fixed system, and ensure the marks are evenly spaced for secure and even support. Drill the points, and then screw in tight to secure it. 

If magnetic/adhesive strips are involved, start by peeling the protective layer off the strip. Then, carefully apply the strip along the perimeter of the existing window frame. Press firmly, and then use a level to check the strips are straight and level. 

How do I install retrofit glazing?

First, we must clean the area: 

  • Clean – Remove any dirt, dust or debris from the frame, and clean the glass/frame. 
  • Clear – Move furniture and objects away to create a work environment with space. Cover the floor with sheets to protect against spills/dust. 

Removing Old Panes

Remove any clips or fasteners holding the pane in place. Take a paring knife and cut through the sealant, freeing the pane. Gently remove it (suction cup lifters work best), if it’s stuck, gently tap outwards with a rubber mallet. Make sure you get rid of the old pane with the environment in mind.

Prepare the Frame

After the old pane has been removed, clean the frame thoroughly to remove dust, old paint or any sealant. Check for rot and damage, especially when using wooden frames. Allow it to dry completely, then sand the frame down for a smooth finish and apply a primer. Consider also applying a preservative on wooden frames to ensure their long lifeline. 

Install Double Glazing Units

Firstly, it’s best to put the unit into the frame to ensure that it’s a snug fit before you begin in earnest. But when you’re satisfied, place a big bead of sealant on the inside of the frame. This will create a tight anchor.

Set the double-glazing unit into the frame. Press it firmly into the sealant, but do not press too hard. Once it’s inside, you must secure the unit using glazing beads or clips. Place them evenly, and not too close, otherwise you will create pressure points.

Apply sealant on both sides of the unit for a weather-tight seal. Ensure there are no gaps in the sealant and check the installation for any gaps or misalignments when done. Clean any excess seal so you ensure the unit is clean and clear.


Here at Klic, we believe that home improvements should be tailored to the individual. This is why we’ve laid out the previous plans and explanation – to ensure that you make an informed, conscious decision. We hope this article helps you on your home improvement journey.

Can you double-glaze existing windows? – FAQs

How long does it take to double-glaze existing windows?

How long it takes depends on the method chosen: 

  • Secondary Glazing – This method is fairly quick. It involves adding an extra pane of glass inside the existing window. This could be completed within a couple of hours per window. 
  • Retrofit Double Glazing – This has a lot more construction-oriented work involved which not only makes it more laborious, but more time-consuming. It involves taking the frame out completely from the window and preparing it for the new unit. 

Ensure that when you’re retrofitting windows to do so in good weather, as without a window in place, you will have essentially a space in the wall. 

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