Windows are the weakest point in the shell of a home. A square metre of single glazing lets through roughly ten times as much heat as a square metre of well-insulated wall, and even the double glazing from the 1980s and 1990s performs worse than many people think. New insulating glass is therefore one of the most visible steps towards a lower energy bill and a better energielabel (energy label). But the choice between HR++ and triple glazing is not purely a choice of glass: it depends on the condition and depth of your window frames, on your ventilation and on whether your home is a listed building or stands in a protected townscape. This article helps you weigh it up.
What the U-value tells you
The insulation value of glass is expressed as the U-value: the amount of heat in watts lost per square metre of glass for each degree of temperature difference between inside and outside (W/m²K). The lower, the better. Milieu Centraal (the Dutch independent information centre on energy and environment) gives single glazing a U-value of about 5.8 W/m²K. Ordinary double glazing without a coating sits at around 2.8 to 3.0, depending on the cavity width. HR++ glass, double glazing with a heat-reflecting coating and an inert gas filling, comes out at about 1.1; for the ISDE subsidy, RVO (the Netherlands Enterprise Agency) sets the limit at a maximum of 1.2 W/m²K. Triple glazing, three panes with two filled cavities, achieves 0.4 to 0.9 according to Milieu Centraal, and only qualifies for ISDE at a maximum of 0.7 W/m²K.
Note the difference between the U-value of the glass (Ug) and that of the whole window including the frame (Uw). A poorly insulating frame drags down the performance of the best glass. If you put triple glazing in a thin, uninsulated aluminium frame from the 1970s, you lose a large part of the benefit around the edge.
Comparison of glass types
| Feature | Single glazing | Double glazing (old) | HR++ | Triple glazing |
|---|---|---|---|---|
| U-value of glass (indicative, W/m²K) | approx. 5.8 | approx. 2.8 – 3.0 | approx. 1.0 – 1.2 | approx. 0.5 – 0.7 (ISDE: max. 0.7) |
| Construction | One pane | Two panes, air cavity, no coating | Two panes, coating, inert gas (usually argon) | Three panes, two coatings, two filled cavities |
| Thickness and weight | Thin, light | Approx. 18 mm | Approx. 24 mm; often fits an existing double-glazing frame | Approx. 36 – 44 mm; heavy; usually needs a new or deeper frame |
| Fits the existing frame? | n/a | n/a | Often, provided the rebate is deep enough and the wood is sound | Rarely; only with a deep rebate and sufficiently strong hinges and locks |
| Condensation on the inside | Almost always in winter | Regularly | Rarely | Rarely; condensation on the outside on cold mornings is normal |
| Sound | Poor | Moderate | Better; even better with laminated or asymmetric glass | Comparable to HR++; not automatically quieter |
| ISDE possible? | No | No | Yes, when replacing single or old double glazing | Yes, when replacing single or old double glazing |
What stands out: the biggest leap is between single glazing and HR++, not between HR++ and triple. Milieu Centraal therefore advises choosing HR++ for existing frames and considering triple glazing as soon as the frames are being replaced anyway. In that case triple is almost always the better choice, especially on the north side. On the sunny side, triple glazing costs you some free solar heat in winter, which partly cancels out the benefit.
When does the frame need replacing too?
Whether new glass fits in the existing frame depends on three things. First, the rebate depth: the groove the glass sits in must be deep enough for the thickness of the new unit plus the sealant and spacer edge. A frame made for single glazing is often too shallow for this; a glazier can sometimes mill out the rebate in timber, but that has limits. Second, the weight: triple glazing is roughly one and a half times as heavy as HR++. In fixed sections that is rarely a problem, but in opening and tilt-and-turn windows the hinges and locking hardware must be able to bear the weight; Milieu Centraal points out that this is exactly where the frame, or at least the fittings, often has to be replaced. Third, the condition of the wood: new glass in a frame with the beginnings of wood rot is money thrown away.
A good window frame specialist or carpenter assesses these three points before making a quote. Ask explicitly whether they are assuming glass in the existing frame, new casements in the existing frame timber or a completely new frame, because those are three different jobs with three different price levels, lead times and VAT regimes. If you opt for new frames, that is also the moment to think about trickle vents in the frame, about the colour and material (timber, uPVC, aluminium) and about burglar-resistant hinges and locks.
Ventilation and condensation: the side effect of better glass
Better glass makes a home more airtight. Where the gaps around the windows and the cold glass surface itself used to provide air exchange and dehumidification, moisture stays in the house once HR++ or triple glazing is installed. The consequences are well known: misted-up panes on the coldest remaining spots, mould in external corners and behind cupboards. Milieu Centraal and the GGD (municipal public health service) are unequivocal: if you insulate, you must sort out the ventilation at the same time. Trickle vents in frames, working mechanical extraction in the bathroom and kitchen and vents that stay open in winter too are not a luxury but part of the measure.
Condensation on the outside of HR++ or triple glazing on a cold, damp morning, on the other hand, is not a defect. The Kenniscentrum Glas (Dutch glass knowledge centre) explains that it is actually proof that the glass insulates well: the outer pane stays cold because hardly any heat leaks from inside to outside. It clears as soon as the outside air warms up. Condensation between the panes is a defect: it means the edge seal is leaking and the pane must be replaced, usually under the glass manufacturer's guarantee. You can read what replacing such a pane involves in replacing glass.
Sound: not the same as insulation
Many people expect triple glazing to be much quieter than HR++ as well. That is only true to a limited extent: thermal insulation and sound insulation are different properties. Sound is mainly blocked by mass, by panes of unequal thickness (asymmetric glass) and by a laminated pane with a sound-dampening interlayer. If you live on a busy road or under a flight path, ask explicitly for HR++ or triple glazing with an improved sound insulation value (Rw), and have the supplier put that value on the quote. A new frame with good draught-proofing often contributes more to peace and quiet at home than the third pane.
Listed building or protected townscape: check with the municipality first
For a rijksmonument (national listed building), according to the Rijksoverheid (Dutch central government) you usually need an omgevingsvergunning (environmental permit) for renovation or restoration; it explicitly mentions replacing historic glass in a window as an example. Ordinary maintenance in which colours, materials, details, shape and profile stay the same requires no permit, but new insulating glass generally does not fall under that. Within a protected town or village townscape, too, you must contact the municipality, which grants the permit and usually puts the application to the heritage or design review committee. Similar rules apply to municipal listed buildings, laid down in the municipal heritage bylaw or the omgevingsplan (zoning plan).
That does not mean insulation is impossible. Many municipalities allow thin insulating glass within the existing glazing-bar pattern, Milieu Centraal mentions vacuum glass (approx. 8 mm thick with an insulation value comparable to triple) as an option for listed buildings, and secondary glazing on the inside leaves the historic window untouched. Discuss the options in advance with the municipality and the Rijksdienst voor het Cultureel Erfgoed (Cultural Heritage Agency of the Netherlands); you can read how a permit application works in general in When do you need a permit for a renovation.
VAT and ISDE
Fitting insulating glass in a home older than two years falls under the VAT scheme for insulation: the labour part is taxed at 9% VAT, the material (the insulating glass itself) at 21%. The Belastingdienst (Dutch Tax Administration) explicitly mentions insulating glass as an insulation material, but immediately adds that doors and frames do not count as insulation material. So if you replace the whole frame, the frame with the associated carpentry is at 21%, and the reduced rate only applies to the labour of fitting the glass. A good invoice splits this out. The full scheme, including the two-year limit, is in 9% or 21% VAT when renovating.
For HR++ and triple glazing there is also the ISDE, the Investeringssubsidie duurzame energie en energiebesparing (investment subsidy for sustainable energy and energy saving) from RVO. The conditions in outline: the glass replaces single or old double glazing in the existing thermal shell, HR++ has a U-value of at most 1.2 and triple of at most 0.7 W/m²K, there is a minimum surface area, the work is carried out by a construction or installation company (fitting it yourself does not count) and you apply for the subsidy within two years of the work being done. Keep the invoice showing the glass type, the U-value and the square metres. The current amounts and conditions are on rvo.nl and change regularly. You can read how glazing counts towards your energy label in Improving your energy label: which measures really count?
Which tradesperson, and what to look for in the quote
Replacing only the glass in sound frames is work for a glazier. New casements or complete frames require a window frame company or carpenter, who usually also supplies and fits the glass. In any case, ask for: the glass type with U-value and, if sound matters to you, the Rw value; the thickness of the glass unit and confirmation that it fits the rebate; whether the existing hinges and locks can bear the weight; how ventilation will be arranged; the VAT split between labour and materials; and the guarantee on glass and frame. If you assess the frames first and then choose the glass, rather than the other way round, you avoid the most expensive pane ending up in the weakest frame.