Replacing glass: HR++ or triple glazing, condensation between the panes and what the glazier does

The glass in a frame lasts less long than the frame itself. Insulating glass can start to leak after a number of years, a pane can crack, and many older homes still have single glazing or the first generation of double glazing, which insulates poorly. Fortunately, new glass does not always mean replacing the whole frame: in many cases a glazier can replace just the pane. This article is about that job. It explains how to recognise that a pane is due for replacement, whether your frame can carry the new glass, when safety glass or sound-insulating glass is sensible, what applies to listed buildings, subsidies, the VvE and rented homes, and how the glazier goes about the work. The choice between HR++ and triple glazing itself is covered in detail in HR++ or triple glazing: when does it pay off; if you are considering new frames, read Replacing window frames.

Condensation: between, inside or outside the pane

Misted glass is the most common sign that something is wrong with a window, but where the condensation is makes all the difference.

• Condensation between the panes. Insulating glass consists of two or three panes with a sealed cavity, sealed all round with an edge seal and a spacer containing desiccant. If you see moisture, a milky haze or streaks on the inside of the cavity that you cannot wipe away, the edge seal is leaking and the desiccant is saturated. The inert gas filling may also have escaped through the same leaking edge, so the pane insulates less well. Such a pane cannot be permanently repaired and must be replaced. There are companies that drill into the cavity and dry it, but that does not restore the original insulation value.
• Condensation on the inside. If the glass mists up on the room side, the glass surface is colder than the dew point of the indoor air. With single glazing and old double glazing this happens quickly in winter. With HR++ or triple glazing, condensation on the inside usually points to high humidity in the home, so too little ventilation, and not to a defect. A narrow strip of condensation along the bottom edge of the glass can also occur with a metal spacer; glass with a so-called warm edge spacer made of plastic or composite suffers less from this.
• Condensation on the outside. On cold, damp mornings in spring and autumn, well-insulating glass can mist up on the outside. According to the Kenniscentrum Glas (the Dutch glass knowledge centre), this is not a fault in the product but a sign that the glass insulates well; the condensation disappears by itself as the outside air warms up.

Other reasons to replace glass are a crack or breakage, for example from a burglary attempt or a ball, but also from stress in the glass. Insulating glass that is partly in the sun and partly in the shade, or that has a dark object close behind it or a film stuck on it, can crack due to temperature differences; this is called thermal breakage. So do not stick film on insulating glass without the supplier having confirmed that it is possible. Finally, there is the wish to save energy: if you still have single or old double glazing, you can gain a lot with new insulating glass.

From single to triple glazing: what the difference is

The insulation value of glass is expressed as the U-value: the lower it is, the less heat is lost. Milieu Centraal (the Dutch environmental information centre) gives a U-value of 5.8 for single glazing and 0.4 to 0.9 W/m²K for triple glazing. Ordinary double glazing without a coating insulates clearly better than single glazing, but much less well than HR++. HR glass and HR+ glass are intermediate steps with a coating; Milieu Centraal only recommends them if HR++ does not fit, and they are not eligible for subsidy. HR++ glass has a heat-reflecting coating and an inert gas filling, and counts as HR++ for the ISDE with a U-value of at most 1.2. Triple glazing has three panes and two filled cavities; for the ISDE it must have a U-value of at most 0.7.

Note that the U-value of the glass is something different from that of the whole window including the frame. A tradesperson who recommends a type of glass to you should tell you which value they mean. An extensive comparison, with the pros and cons of triple glazing on the sunny and shady sides, can be found in the article on HR++ or triple glazing.

Can your frame carry the new glass?

New insulating glass is thicker and heavier than single glazing, and triple glazing even more so than HR++. Milieu Centraal gives a thickness of about 5 millimetres for single glazing, 18 for double glazing, 24 for HR++ and 37 for triple glazing. Whether the new glass fits in your frame is assessed by the glazier on the basis of three questions.

• Is the rebate deep and wide enough? The rebate is the recess in the frame or window in which the glass rests. In timber frames made for single glazing, it is often too narrow for insulating glass. According to Milieu Centraal, the rebate of timber frames can usually be adapted by routing it out and fitting new glazing beads.
• Can the opening parts handle the weight? Windows that can open are often thinner than the fixed frame, and hinges and window hardware must be able to carry the extra weight. Milieu Centraal points out that HR++ sometimes does not fit precisely in the opening parts. In practice, triple glazing almost always requires a new or deeper frame.
• Is the wood sound? New glass in a frame with wood rot is not a good investment. Rotten spots in the bottom sill or the rebate must be repaired first, or the frame must be replaced.

In uPVC and aluminium frames, old double glazing can usually be replaced by HR++, because the glass opening there was already designed for insulating glass; if HR++ does not fit, what remains is HR+ glass, which insulates less well, or replacing the frame. A window frame specialist or carpenter can assess whether adapting or replacing is the most sensible choice.

Safety glass: where is it needed?

Ordinary glass breaks into large, sharp shards. Safety glass limits that risk in two ways: toughened glass breaks into small, fairly blunt granules, and laminated glass consists of two or more panes with a film between them that holds the shards together. Laminated glass stays in the rebate when it breaks and is also more burglar-resistant.

The Besluit bouwwerken leefomgeving (Bbl, the Dutch Building Decree) sets no general requirement for homes, including new builds, that glass must be injury-safe. The standard NEN 3569 describes where safety glass is needed to prevent injury and falling through, and is widely applied in construction, often declared applicable in contracts and specifications and used as standard by glass suppliers. As a rule of thumb, it concerns glass that you can bump into and whose bottom edge is low above the floor, such as glass doors, glass beside a door and floor-to-ceiling windows. The Bbl does set requirements for protection against falling. If you replace a complete window with frame on an upper floor and the sill height is lower than 0.85 metres, there must be a fixed barrier of at least that height (article 5.10a Bbl). If you only replace the glass, that requirement does not apply, but the situation may not become less safe than it was: if the glass serves as a barrier, the new pane must also be strong enough for that.

For every pane that is low, is in or beside a door or reaches down to the floor on an upper storey, ask the glazier whether safety glass is sensible and have the type stated on the quote. In homes with young children this is extra important.

Sound-insulating glass

If you live on a busy road, a railway line or under a flight path, noise is often just as much a reason to replace glass as heat. HR++ and triple glazing are not automatically much quieter than ordinary double glazing; other properties count for sound. Sound-insulating glass works with panes of unequal thickness, so that they do not vibrate at the same frequency, and often with laminated glass with a special sound-damping film. Its performance is expressed as a sound insulation value in decibels; have the supplier state that value for the chosen glass on the quote.

Glass is only one component. Noise also gets in through gaps around the window, through a poorly sealed junction between the frame and the wall and through ventilation grilles. If you have sound-insulating glass fitted, it is a good idea to have the draught strips and the grilles checked as well; sound-damping ventilation grilles exist.

Listed buildings and protected townscapes

In a rijksmonument (national monument), you usually need an omgevingsvergunning (environmental permit) to replace historic glass in a window; the Dutch central government explicitly mentions this as an example. Necessary regular maintenance aimed at preserving the monumental values is permit-free as long as detailing, profiling, design, type of material and colour do not change (article 13.11 of the Besluit activiteiten leefomgeving, the Dutch Activities Decree). Replacing a cracked modern pane with an identical pane may fall under this, but replacing old, slightly wavy glass with insulating glass generally does not. For municipal monuments, the rules are in the municipality's omgevingsplan (local zoning plan) or heritage by-law, and extra rules may also apply in a protected townscape or village view for anything that changes on the facade. So always consult the municipality first.

That does not mean insulating is impossible. Milieu Centraal mentions vacuum glazing, which at about 8 millimetres is so thin that it often fits in an existing rebate, as an alternative for listed buildings and protected townscapes. There is also special heritage glass, thin insulating glass that approximates the appearance of old glass, and secondary glazing on the inside can leave the historic window untouched. According to Milieu Centraal, those solutions perform roughly like ordinary double glazing. If the old glass is still intact, preferably leave it in place, or have the glazier carefully reuse it when the window is replaced.

ISDE subsidy for HR++ and triple glazing

Through the Investeringssubsidie duurzame energie en energiebesparing (ISDE), the sustainable energy and energy saving investment subsidy from RVO (the Netherlands Enterprise Agency), homeowners can receive a subsidy for insulating glass. The conditions change regularly, so check them on rvo.nl before you place an order. In broad terms, they are as follows:

• You own the home and live in it yourself.
• The home was built before 2019, or the omgevingsvergunning (environmental permit) was applied for before 1 July 2018.
• The glass meets the maximum U-value: 1.2 W/m²K for HR++ and 0.7 W/m²K for triple glazing, and is on RVO's list of notification codes (meldcodelijst).
• For the higher amount for triple glazing, the frames must also be replaced with insulating frames; if you fit triple glazing in existing frames, the HR++ category applies.
• You replace a minimum area of existing glass; there is also a maximum. Only the area of existing windows that you replace counts, not glass in a new opening.
• A construction or installation company carries out the work; fitting it yourself does not count.
• You apply for the subsidy within 24 months of the work being carried out, with, among other things, the invoice, proof of payment, a photo of the completed work and a glazing schedule.
• If within 24 months you combine insulating glass with a second insulation measure, for example cavity wall, roof or floor insulation, or with a heat pump, solar water heater or connection to a heat network, the subsidy amount for the insulation is higher. A combination with ventilation alone does not count for this.

Ask the glazier for an invoice that clearly states the type of glass, the U-value, the notification code and the square metres replaced; without that information the application will stall. How the ISDE relates to other schemes and to the reduced VAT rate on insulation work is explained in Sustainable renovation: subsidies and options. If your home has listed status, adjusted conditions apply for the ISDE.

Ventilation after better glazing: prevent damp and mould

Old windows with single glazing and gaps unnoticeably worked as ventilation and as a dehumidifier: the moisture in the air condensed on the cold glass. After HR++ or triple glazing is fitted, that cold surface has gone and the window closes better. The moisture you produce at home by cooking, showering, washing and breathing must then leave via the ventilation, otherwise it seeks out the coldest spot that remains: an external corner, the back of a cupboard or a cold bridge at a lintel.

If your home does not have balanced ventilation, ventilation grilles are needed for the supply of fresh air; when replacing glass, Milieu Centraal recommends having grilles fitted in the frame or above the glass. So have the glazier check straight away whether there are grilles and whether they are usable, and keep them open in winter. If you still have problems with condensation or mould after replacing the glass, read Recognising damp and mould in your home for the cause per symptom and the right tradesperson.

What the glazier does

A glazier replaces the glass in existing windows and frames. The work proceeds in a few steps.

1. Survey and measuring. The glazier inspects the frames, the rebates and the window hardware and measures each pane. Insulating glass is made to measure, with a small clearance all round; production usually takes from a few days to a few weeks. If a pane is broken, they temporarily close the opening with a board or emergency glazing.
2. Old pane out. The glazing beads are removed, old sealant or putty is removed and the pane is taken out. With old timber windows with single glazing, the glass is often held with putty on the outside; with insulating glass, glazing beads are usually used. Old sealant in homes built before 1994 may contain asbestos; a professional pays attention to this.
3. Preparing the rebate. The rebate is cleaned, repaired where necessary and, with timber, given a primer. With insulating glass, a ventilated and drained rebate is important: water that gets in must be able to drain away, otherwise the edge seal stands in the moisture and the pane starts leaking sooner.
4. Setting on blocks. The glass is placed on setting and location blocks, so that it nowhere rests directly on the frame and the weight is transferred in such a way that the window does not sag.
5. Sealing. With wet glazing, the pane is sealed with glazing tape or bedding tape and a glazing sealant that is compatible with the edge seal; putty is not suitable for insulating glass. With dry glazing, common in uPVC and aluminium frames, the pane is sealed with rubber gaskets in the frame and the glazing beads.
6. Glazing beads and finishing. New or reused glazing beads hold the glass in place. Beads on the inside are more burglar-resistant than beads on the outside, because a burglar then cannot remove them from the outside. With timber windows, new beads and the sealant joint must then be painted; agree who will do this.

The manufacturer usually gives a guarantee on the edge seal of insulating glass, often for a period of years, on condition that the glass has been fitted according to the instructions. Ask the glazier who gives the guarantee, for how long and under what conditions. The old glass should be disposed of separately so that it can be recycled.

VvE and rented homes

If you live in an apartment, first check the deed of division and the division regulations. These state which parts of the building are common. Frames in the facade often belong to the common parts, and in some regulations this also applies to the glass; in others, the glass, or damage to the glass, is the responsibility of the apartment owner. If you are allowed to have the glass replaced yourself, the VvE often sets requirements for its appearance, for example for the colour of the glass or the glazing beads, so that the facade remains a uniform whole. More about permission and rules in Renovating in a VvE apartment.

If you rent, under the Besluit kleine herstellingen (Minor Repairs Decree), replacing damaged panes is only the tenant's responsibility insofar as no significant costs are involved. A large pane of insulating glass often does not fall under this in practice; then it is up to the landlord, unless you caused the damage yourself. A leaking insulating glass pane with condensation between the panes is a defect that you report to the landlord. If you want to have better glass fitted yourself, first ask the landlord for permission in writing; for changes that you cannot easily undo at the end of the tenancy, and certainly on the outside of the home, this is generally required (article 7:215 of the Dutch Civil Code). If the landlord replaces single or old double glazing with insulating glass as an improvement, they may propose a reasonable rent increase (article 7:255 of the Dutch Civil Code); this does not apply to repairing a defect, such as a leaking pane. Make sure you are well informed about this in advance.

Choosing a tradesperson and the quote

For just replacing glass you can go to a glazing company, but many painting firms, carpenters and window frame companies also fit glass. The trade association for painting and glazing companies is OnderhoudNL, which has a separate glass sector. Membership is not a guarantee, but it is an indication that the company adheres to industry terms and conditions. Also check the registration with the Kamer van Koophandel (KvK, Chamber of Commerce) and ask for references.

A good quote states, per pane, the size, the type of glass with U-value, any safety, sound-insulating or solar control glass, the glazing method (wet or dry, glazing beads inside or outside), what happens with the rebate and any wood rot, ventilation grilles, painting of new glazing beads, removal of the old glass, the guarantee, and the details you need for the ISDE. What mainly determines the price is the number and dimensions of the panes, the type of glass, adjustments to the rebate and accessibility, for example on an upper floor. How to compare quotes is explained in Comparing quotes.

In summary: condensation between the panes means replacing, condensation on the inside of new glass means ventilating, and condensation on the outside means your glass is working well. Have the glazier assess the frame and the rebate first, choose safety or sound-insulating glass where necessary, consult in advance with a listed building, VvE or rented home, and keep the invoice for the subsidy. On VerbouwPro you post your job free of charge, and tradespeople from your region respond.