Short answer: fitting double glazing these days almost always means putting a modern low-e sealed unit or a triple glazed unit into your frame. Whether that will go into your existing frame depends on the depth of the glazing rebate: an old frame built for single glazing is often too shallow for modern insulating units. A good double glazed unit has a U-value of around 1.1, triple glazing around 0.5 to 0.7. If you are only replacing the glass in a sound frame, a handy DIYer can do that, but you lose the manufacturer’s guarantee on the unit and you rule yourself out of any grant. Those schemes only pay out when a registered firm does the work.
In this article I walk you through the whole job: which glass to choose, whether it fits your frame, what it costs, what you save, exactly how the grants work and where it goes wrong when you do it yourself. So that by the end you can make a choice you can back up.
What is a low-e double glazed unit, and how does it differ from triple?
Every modern insulating unit is made of two or three panes of glass with an airtight cavity between them, filled with an inert gas, usually argon. On the glass sits an invisible metal coating that reflects heat back inside. The better that build-up, the less heat leaks out.
You recognise the types by their insulation value, the U-value. The lower the U-value, the better the glass insulates.
| Glass type | U-value (glass, Ug) | Build-up |
|---|---|---|
| Single glazing | ± 5.8 | One pane |
| Old double glazing (1970s/80s) | ± 2.8 – 3.0 | Two panes, no coating, ordinary air |
| Early low-e double glazing | ± 1.6 – 2.0 | Coating, older generation |
| Modern low-e double glazing (Dutch HR++) | ± 1.0 – 1.2 | Coating plus argon fill |
| Triple glazing | ± 0.4 – 0.9 | Three panes, two cavities, argon |
| Vacuum glazing / heritage glass | ± 0.4 – 0.7 | Thin, vacuum between the panes |
The U-values for single and triple glazing come from Milieu Centraal; the other values depend on the product, so always check the data sheet or the unit’s markings. (Source: Milieu Centraal.)
The big difference sits between single glazing and a modern double glazed unit: that is a jump from 5.8 to around 1.1. The step from double to triple is much smaller in absolute insulation gain, but not in price. More on that below.
U-value: watch the glass (Ug), the window (Uw) and the frame (Uf)
This is where advertising trips people up. A salesperson quotes a low U-value but means the glass only. There are three values:
- Ug = the insulation value of the glass itself.
- Uf = the insulation value of the frame.
- Uw = the insulation value of the whole window: glass, frame and edge spacer together.
The Uw is what your house actually feels. Put a superb triple glazed unit into a draughty, poorly insulating frame and that frame drags your whole performance down. So if you go for triple, a well insulated frame is not a luxury but a necessity, otherwise you waste part of your investment.
Watch the edge spacer too, the strip that holds the panes apart around the perimeter. Traditionally it is aluminium, which conducts heat well and gives you cold and condensation right at the edge. A warm edge spacer in plastic or stainless steel insulates that edge better and cuts down condensation and cold bridging along the glass edge. Ask for it explicitly. It costs little and it makes a real difference to comfort.
Will a double glazed unit fit my existing frame?
This is the question most buying guides skip, even though it decides your budget.
Insulating units are thicker than single glazing. Double glazed units run roughly 20 to 24 mm thick, triple units easily 36 to 44 mm. Single glazing is only about 4 mm. Your frame needs a glazing rebate (the recess the glass sits in) deep enough for that thickness plus the sealant and the bead.
What that means in practice:
- Frame originally built for double glazing? Then a modern unit usually drops straight in. That applies to most houses from the 1980s onwards.
- Frame originally built for single glazing (often pre-war or 1950s/60s)? The rebate is frequently too shallow. On timber frames a joiner can sometimes rout the rebate deeper, but not always, because too little timber left means a rebate that is too weak.
- Triple in an old frame? Almost never. Triple glazing is so thick and heavy that as a rule you need a new sash or a completely new frame.
Measure it yourself: open the window and measure the depth of the rebate and the thickness of the existing glass. Not sure? Get a glazier round. They will see in five minutes whether new glass alone will do or whether joinery comes into it. That difference is hundreds of euros per window.
Double or triple glazing: which do you choose?
Triple insulates better, but the added value depends entirely on the rest of your house.
Go for triple if:
- you are fitting new frames anyway (then the extra cost of the glass is relatively small and the thickness works out from the start);
- the rest of your house is well insulated (roof, walls, floor) and you heat at low temperature, for example with a heat pump and underfloor heating;
- comfort and no cold downdraught at the window matter a lot to you.
Stick with double if:
- you are only swapping the glass in existing, perfectly good frames;
- the rest of your house is still moderately insulated (your heat leaks out through roof and walls anyway, so triple will not pay for itself);
- you face south, where a double glazed unit still lets in free solar gain in spring and autumn.
Milieu Centraal recommends triple mainly when you are replacing the frames as well. If you are only changing the glass, a good double glazed unit is usually the sensible call. The extra insulation gain of triple over double stays limited in an average existing home, while the cost is considerably higher. (Source: Milieu Centraal.)
What to look for in the unit itself
Not every unit is the same. A few options worth considering:
Acoustic glass. Do you live on a busy road or under a flight path? Then a standard low-e unit is not automatically quieter. For sound you need an asymmetric build-up (two panes of different thickness) and usually a laminated pane with an acoustic interlayer. Ask for the sound reduction in decibels (the Rw value).
Solar control glass. On a sunny elevation or with large areas of glass, solar control glass keeps overheating out. The downside: it also lets less free solar heat through in winter.
Safety glass. In doors, low windows and any glass people can fall against, laminated or toughened safety glass is required under the building rules. A glazier knows where it applies.
Heritage glass / thin insulating units. In listed buildings and conservation areas the thick modern units are often not allowed because of the appearance and the slim glazing bars. There is special thin insulating glass (vacuum glazing) that does fit a narrow rebate and still insulates well. In the Netherlands, listed buildings using this glass have qualified for grant support more easily since 2026 (see below). (Source: RVO, Milieu Centraal.)
Fitting double glazing: DIY or glazier?
Swapping just the unit in an existing, sound frame is technically within reach of a handy DIYer. But weigh these points up honestly before you start.
What DIY fitting costs you in certainty:
- The guarantee lapses. Insulating units normally come with a manufacturer’s guarantee on the cavity, meaning the seal stays intact. That guarantee almost always applies only when the unit is fitted to the glazing standards by a professional. Fit it yourself and the unit mists up after two years (condensation between the panes), and you are on your own.
- No grant money. Glazing grants apply only when the work is carried out by a registered firm. Fitting it yourself rules the grant out.
- It is heavier and more precise than it looks. A large sealed unit weighs a fair bit, because glass runs at around 2.5 kg per m² per millimetre of thickness. A double glazed unit of 1.5 m² soon comes to about 40 kg. Lift or set it wrong and the unit cracks, or worse.
What matters technically when you fit it yourself:
The unit must never rest directly on the timber or plastic. It sits on setting and location blocks that keep the rebate clear all round for ventilation and drainage, exactly as the glazing guidance sets out. Bad blocking, or a rebate sealed up so moisture cannot escape, is the number one reason units fail early. On timber frames you usually work with glazing sealant and beads; on plastic and aluminium with glazing gaskets.
In short: one straightforward unit in a healthy timber frame and a wish to learn, and DIY is defensible. When it is your whole house, and guarantees and grants are on the table, a glazier pays for himself quickly.
Step by step: replacing a unit yourself (timber frame)
If you are doing it yourself anyway, this is the broad order of work. Wear gloves and get a second pair of hands.
- Measure precisely. Measure the rebate in height and width and take a few millimetres off each side for clearance. Order the glass made to size, because you do not cut insulating units yourself.
- Take the beads off carefully. Ease them out with a chisel so you can reuse them. Number them.
- Take the old glass out and scrape the rebate completely clean and dry. Repair loose paint or rot first.
- Position the setting and location blocks correctly, so the glass stands clear of the timber and the bottom of the rebate can drain.
- Set the new unit in place, and level and centre it on the blocks. The unit must not bind anywhere.
- Apply the glazing sealant as the guidance sets out and bed the unit down neatly.
- Refit the beads and finish the joints. Make sure the bottom ventilation and drainage slots stay open. Never seal them up.
Unsure about the drainage or the blocking? That is exactly the detail that goes wrong. If in doubt, have a professional do it.
What does fitting double glazing cost?
Prices vary a lot with the size of the unit, the type of glass and whether frame work comes into it. The figures below are guide prices from the Dutch market. Always get several quotes.
| Item | Guide price |
|---|---|
| Double glazed unit per m², fitted | ± €90 – €160 per m² |
| Triple glazed unit per m², fitted | ± €170 – €330 per m² |
| Double glazing for a whole end-of-terrace house (22 m², existing frames) | ± €4,700 |
| The same, after the Dutch ISDE grant | ± €4,150 |
The figures for the end-of-terrace house come from Milieu Centraal (double glazed units in existing frames); the per m² prices are market indications and differ by supplier and unit size. Small units cost more per m² than large ones. (Source: Milieu Centraal.)
Watch out: if your glass still sits in rebates that are too shallow, or in rotten frames, the cost of routing, a new sash or a complete frame comes on top. That is often the difference between “just the glass” and “a full renovation”.
How much do you save with double glazing?
The saving depends on what you are replacing. Single glazing to a modern double glazed unit is an enormous jump; reasonable double glazing to a modern unit far less so.
| Replacement | Gas saved per year | Saving in money |
|---|---|---|
| Single glazing → modern double glazed unit | ± 260 m³ | ± €350 |
| Old double glazing → modern double glazed unit | ± 70 m³ | ± €90 |
Figures from Milieu Centraal for an average home; your saving depends on the glazed area, how you heat and the gas price. (Source: Milieu Centraal.)
The lesson: replacing single glazing almost always pays, and quickly. Replacing reasonable double glazing purely to save money takes far longer to come back. You do that more for comfort, less draught or less condensation, or because you are replacing frames anyway. Because alongside the gas you gain a lot in comfort: no more cold downdraught at the window and less misting.
Grants for glazing (2026)
Wherever you live, the same condition tends to come back: the work has to be carried out by a registered installer, and the new window has to meet a minimum insulation value. That is why DIY glazing falls outside every scheme.
In the UK there is no general grant for DIY glazing. The schemes that do exist, ECO4 and the Great British Insulation Scheme, depend on your household and property qualifying, and the work has to be done by a certified installer. Check what you actually qualify for before you order anything, because the schemes and their conditions change.
In the Netherlands the rules are spelled out in numbers, and those numbers show clearly what is at stake. The main conditions of the ISDE
, run by RVO:
- Modern double glazed units (HR++): €25 per m², with a maximum glass U-value (Ug) of 1.2 W/m²K.
- Triple glazing: €111 per m², with a maximum U-value of 0.7 W/m²K.
- You insulate at least 3 m² of glass, and the grant runs up to 45 m² maximum.
- The glass is on RVO’s Meldcodelijst Hoogrendementsglas, so it is a registered product.
- The work is carried out by a registered building or installation firm. Fitting it yourself does not qualify.
- Combine two insulation measures (or glazing plus, say, a heat pump) within 24 months and the grant per m² doubles.
(Source: RVO.)
Two current changes for 2026 are worth knowing. If you replace your frames while fitting the insulating units, RVO no longer sets any requirement for the insulation value (Uf) of the frame. And for listed buildings the permitted maximum U-value of the glass has been relaxed, so owners can now get grant support for glazing in the front or rear elevation too. (Source: RVO.)
Amounts and conditions change regularly. Always check the current scheme with the body that runs it before you place an order, and see whether your local council offers extra support or a low-interest energy loan.
Condensation on your glass: when is it a problem?
Condensation confuses a lot of people. It depends entirely on where it sits.
- Condensation on the outside (on the pane, on the street side, in the morning) is actually a good sign. It means your glass insulates so well that the outer pane stays cold and the morning dew settles on it. No problem, it clears by itself. (Source: Milieu Centraal.)
- Condensation on the inside usually points to too much moisture in the house and too little ventilation. Better glass can even make it worse at cold edges. The answer lies in ventilation, not in the glass.
- Condensation or misting in between the panes is always wrong. It means the cavity has failed and the unit is defective. The insulating effect is gone and the unit needs replacing. If it is still in guarantee (and was fitted by a professional), this normally falls under the manufacturer’s guarantee. Which is the single best reason to have units fitted to the glazing standards.
Common mistakes (and how to avoid them)
- Looking only at the glass U-value. A top unit in a poor frame performs moderately. Judge the Uw of the whole window.
- Choosing triple in an otherwise poorly insulated house. Your heat leaks out through roof and walls, so the extra cost never comes back. Insulate the big surfaces first.
- Not checking the depth of the rebate. Then you find out mid-job that your glass does not fit and you need joinery after all.
- Sealing up the ventilation and drainage slots. The rebate under the unit has to drain. Smear it shut and moisture sits there and the unit fails sooner.
- Fitting it yourself and losing both the grant and the guarantee. Work out whether the price difference against a glazier outweighs losing both.
- Forgetting to ask for the warm edge spacer. Standard aluminium gives more condensation and cold along the edge; warm edge costs very little extra.
Frequently asked questions
Can I fit a double glazed unit into my old frame?
You can if the rebate is deep enough. Frames built for double glazing (broadly from the 1980s onwards) are usually suitable. Older frames made for single glazing are often too shallow; on timber frames you can sometimes have the rebate routed deeper, otherwise you need a new sash or frame. Measure the rebate depth or get a glazier to look.
What is the difference between double and triple glazing?
A modern double glazed unit has two panes and a U-value of around 1.0 to 1.2. Triple glazing has three panes and sits at around 0.4 to 0.9, so it insulates better. Triple is thicker, heavier and dearer, and it mainly pays off when you are fitting new frames anyway and the rest of your house is well insulated.
Am I allowed to fit double glazing myself?
Technically a handy DIYer can replace a unit in a sound timber frame. But you generally lose the manufacturer’s guarantee on the unit, and you rule yourself out of grant support, because that requires a registered firm to do the work. For a whole house a glazier is usually the wiser choice.
Do I get a grant if I fit the glass myself?
No. Both in the UK and in the Netherlands, grant support only applies when a registered building or installation firm carries out the work. In the Netherlands that is around €25 per m² for a modern double glazed unit and €111 per m² for triple glazing in 2026, from a minimum of 3 m². Fitting it yourself is cheaper, but without the grant and without the guarantee. (Source: RVO.)
What does it cost to have double glazing fitted?
Reckon on roughly €90 to €160 per m² fitted, depending on unit size and supplier. For an average end-of-terrace house with existing frames, Milieu Centraal arrives at about €4,700, or around €4,150 after the grant. Add frame work and the figure climbs.
How much do I save with modern double glazing?
Going from single glazing to a modern double glazed unit saves you around 260 m³ of gas and about €350 a year, according to Milieu Centraal. From old double glazing to a modern unit it is far less, roughly 70 m³ of gas and €90 a year. So replacing single glazing pays back quickly; you replace reasonable double glazing more for the comfort.
Is condensation between the panes serious?
Yes. Condensation or misting between the panes means the unit has failed and lost its insulating effect, and it needs replacing. Condensation on the outside of the glass is normal, and in fact a sign that your glass insulates well.
Do you want to tackle not just your glass but your whole house in the right order, so you save as much as possible without spending money you did not need to? In the Energie Verpulveraar I show you step by step how to insulate, ventilate and cut your energy bill properly, including how to pick up the grant along the way. Have a look here: https://energie-verpulveraar.fredsbouwtekeningen.nl/