Glass in wooden windows, doors or frames largely determines how warm, quiet and energy-efficient your home is. A wooden frame may be very well made, but with the wrong glazing you lose unnecessary heat. Glass accounts for approximately 70 to 80 percent of a window’s surface area. In this blog, we explain which types of glass are available, how they differ and which glass best suits your wooden windows, doors and frames.
When it comes to insulating glass, everything revolves around one figure: the Ug-value. This indicates how much heat is lost through the glass, expressed in W/m²K. The lower the Ug-value, the better the glass insulates.
Single glazing
Single glazing consists of a single pane of glass, usually 3 to 6 millimetres thick. Until well into the twentieth century, this type of glass was standard in Dutch homes. It is still common in older houses, sheds and listed buildings.
The insulating performance of single glazing is very poor. Its Ug-value is around 5.8 W/m²K. In winter, you can feel the cold near the window, condensation forms quickly and energy bills can rise considerably. Single glazing also does little to block noise. Anyone replacing single glazing with modern insulating glass will notice the difference almost immediately in greater comfort and lower energy bills.
Insulating glass (double glazing)
Insulating glass consists of two or more panes of glass with a sealed cavity between them. This cavity is filled with dry air or a noble gas (often argon). A spacer around the edge keeps the panes apart and seals the unit airtight.
The first double glazing from the 1970s and 1980s had a Ug-value of approximately 2.8 to 3.0 W/m²K. That was already twice as good as single glazing. Later, versions with coatings and gas fillings, such as HR and HR+ glass, offered increasingly better insulation. Today, ordinary double glazing is outdated, and almost everyone replacing their glazing opts for HR++ or better.
HR++ glass: when is glass classified as HR++?
HR stands for High Efficiency. Glass may be called HR++ if its Ug-value is 1.2 W/m²K or lower. In practice, good HR++ glass has a value of around 1.0 to 1.1.
This insulating performance comes from three things working together:
Two panes of glass with a cavity of approximately 15 millimetres.
A heat-reflective coating: an extremely thin, invisible layer of metal oxide that reflects heat back indoors.
A noble gas filling, usually argon, which conducts heat less readily than air.
HR++ glass is now the standard for existing homes and an excellent choice for most wooden frames, windows or doors. It fits standard glazing rebates and insulates more than five times better than single glazing. You can identify it by the stamp or code on the spacer. Fun fact! You can also tell by looking at the reflection of a lighter flame: one of the reflected flames will have a different colour because of the coating.
Triple glazing (HR+++)
Triple glazing has three panes of glass and two cavities, usually filled with argon or krypton and fitted with coatings. Its Ug-value is approximately 0.5 to 0.7 W/m²K. This makes it almost another 50 percent better at insulating than HR++ glass.
Triple glazing is ideal for new builds, energy-neutral homes and houses with a heat pump. There are some points to consider, however. The glass unit is considerably thicker (often 36 to 48 millimetres) and heavier than HR++ glass, for example. The wooden frame therefore needs a deeper rebate and must be strong enough to support the extra weight. We take this into account when designing new wooden frames. Triple glazing often does not fit existing frames without modifications.
Vacuum glazing
Vacuum glazing is the latest development. Between the two panes of glass, there is no gas but a vacuum only a fraction of a millimetre thick. Small, barely visible support points keep the panes apart. Because a vacuum conducts virtually no heat, this glass achieves a Ug-value of approximately 0.4 to 0.7 W/m²K, comparable to triple glazing.
The main advantage is its thickness. Vacuum glazing is often only 6 to 10 millimetres thick and is also lightweight. This means it fits many existing wooden frames and even slender glazing bars, without the need to replace the frame. Vacuum glazing is more expensive than HR++ and triple glazing, but for anyone wishing to preserve the character of old windows while achieving maximum insulation, it is a very interesting option.
Heritage glazing
Heritage glazing has been specially developed for listed and characterful buildings. Listed buildings are often subject to strict requirements. The appearance of the windows must remain as unchanged as possible. Thick insulating glass does not fit the narrow rebates and slender glazing bars of historic frames and often looks too modern.
Heritage glazing solves this problem. There are broadly two types:
Thin insulating glass: slim double glazing or vacuum glazing that fits the existing rebate while still providing good insulation, with a Ug-value of approximately 1.1 to 1.4 W/m²K for thin double glazing and lower for vacuum glazing.
Glass with a historic appearance: glass with a slightly wavy surface resembling old drawn glass. This preserves the lively, irregular reflections on the façade that are so characteristic of old buildings.
Both features can be combined. This gives you the comfort of modern glass without compromising the building’s historic character. For a nationally or municipally listed building, always consult the municipality beforehand. Sometimes an environmental permit is required, although this is not always the case for privately owned homes.
Advice
Which glass is best depends on your home, your frames and your preferences. For most existing homes, HR++ is an excellent choice. Triple glazing is often worthwhile in new wooden frames. If you want to preserve the character of an old or listed building, vacuum glazing and heritage glazing are the best options.
Not sure which glass suits your windows, doors or frames? Contact us. We will be happy to advise you on the best combination of wood and glass for your home.
