Monument glass or vacuum glass: what do you choose for old window frames?

Old window frames often have a lot of character, but they are usually not made for thick insulating glass. If you want to replace single glass and retain the original appearance, you will quickly end up with monument glass or vacuum glass. Both solutions are intended for renovation, but they differ greatly in insulation, thickness, appearance and price.

The short rule: choose monumental glass if appearance and historic character are most important, and choose vacuum glass if you want maximum insulation in a thin glass structure. In this article we explain what monumental glass and vacuum glass are, when to choose which option and what you should pay attention to with old or monumental window frames.

Rule of thumb: appearance is leading = monument glass. Maximum insulation in thin glass = vacuum glass.

Why is standard HR++ glass often difficult to use with old window frames?

Many old wooden frames were originally made for single glazing. That glass was often only 3, 4 or 5 mm thick. Modern HR++ glass is much thicker, often around 24 to 28 mm. As a result, HR++ glass does not always fit into old window frames.

Problems that arise:

too little rebate depth
no room for glazing beads
change of appearance
too heavy glass for rotating parts
damage to the historic appearance
limited space at rods
vulnerable old wood
monumental rules

During normal renovations you can sometimes work with rebated slats or nose slats. But with characteristic window frames you often want to change the appearance as little as possible. Then monument glass and vacuum glass become interesting. You can read more about the thickness challenge in thicker insulating glass in an existing frame.

What is monument glass?

Monument glass is thin glass that is specially used for old, characteristic or monumental buildings. The goal is to preserve the appearance of old glass as best as possible.

Monument glass can have different shapes. Sometimes it concerns thin insulating glass, sometimes glass with a slightly irregular surface that resembles old drawn glass. It is often chosen for:

monumental buildings
1930s homes
farms
mansions
old sliding windows
stained glass windows
historic facades
protected cityscape
restoration projects

The main advantage is that it better matches the original appearance of the building.

What is vacuum glass?

Vacuum glass is modern insulating glass with a very thin glass structure. It usually consists of two glass plates with an extremely thin vacuum space between them. Because there is virtually no air between the glass plates, heat loss is greatly reduced.

The special thing about vacuum glass is the combination of:

thin structure
high insulation value
relatively low weight
suitability for old window frames
good comfort improvement

Vacuum glass is therefore popular for renovations where HR++ glass is too thick, but insulation is important. You can read how it works in practice in vacuum glass in practice: micro-spacers. All products can be found on the page vacuum glass.

The main difference

Monument glass is mainly about appearance and preserving character. Vacuum glass is mainly about insulation in a thin package.

Attribute	Monument glass	Vacuum glass
Main purpose	Historical appearance	High insulation
Thickness	Often thin	Often 8–12 mm
Insulation	Depending on type	Very good
Appearance	Classic/authentic	Sleek, modern thin glass
Suitable for monuments	Often	Often, depending on requirements
Price	Depending on version	Usually high
Best Application	Preservation of character	Insulating without a thick window

The choice therefore depends on what is more important: maximum insulation or maximum preservation of the historical image.

Image: cross-sectional comparison of thin monument glass next to a vacuum glass package with the same installation thickness

When do you choose monument glass?

Monument glass is especially suitable when the appearance of the building is leading. This certainly applies to monumental homes or facades where the exterior appearance may not change significantly. Choose monument glass when:

the building is monumental
the glass image must remain historic
the old appearance is more important than maximum insulation
want to imitate your existing glass image
the frame is very narrow
the municipality sets requirements for the appearance
you want to keep stained glass or rods
the window is part of a historic facade

Monument glass can provide a nice balance between improvement and preservation.

When do you choose vacuum glass?

You should mainly choose vacuum glass if you want to keep old window frames, but still want to insulate considerably better. Choose vacuum glass when:

HR++ glass is too thick
you want to keep existing window frames
you have a lot of heat loss
comfort is important
you are looking for insulating glass that is as thin as possible
you want to reduce condensation and cold traps
you do not want to install new window frames
the house is old but not extremely protected
you want a high insulation value

Vacuum glass is often a smart choice for old wooden frames where standard HR++ glass does not technically fit well. See also the comparison vacuum glass versus HR++ glass.

Is monumental glass also insulating glass?

Sometimes yes, but not always. The term monument glass is used in different ways. This makes it important to look carefully at the exact structure. Monument glass can be, for example:

single monumental glass
thin insulating glass
glass with historic appearance
glass with slight distortion
glass suitable for restoration
insulating glass with slim construction

Therefore, always ask about the total glass thickness, the U-value, the glass structure, the safety structure, the appearance and the applicability in your frame. Not every monument glass insulates equally well. Some variants are mainly intended for appearance, not for maximum energy savings.

Is vacuum glass suitable for monuments?

Vacuum glass may be suitable for monuments, but that depends on the requirements of the municipality or monument committee. The advantage is that vacuum glass is thin. This means it can often be installed in existing window frames without major adjustments — beneficial for monumental windows.

However, there are points of interest:

the glass image may be different from old drawn glass
small spacers may be visible
the edges and details must match
not every monument committee accepts every solution
permit or permission may be required

You should therefore always check in advance what is permitted at monuments.

What about the appearance?

Appearance plays a major role in old homes. Standard modern glass can sometimes look too sleek or too flat in a historic frame. Monument glass can deliberately have a slightly more lively or older glass image, making it a better fit for classic facades.

Vacuum glass is technically modern: thin and sleek, but in some products it may have small dots or spacers. These are needed to keep the glass plates apart. Usually they are not noticeable, but in certain lighting you can see them. Therefore, do not only look at insulation, but also at what the glass looks like in your frame.

How important is the U-value?

The U-value indicates how well glass insulates. The lower the U-value, the better the insulation. Vacuum glass usually has a very low U-value and therefore insulates much better than single glass and often also better than standard HR++ glass.

For monumental glass, the U-value depends on the type. Thin insulating monument glass can provide an improvement, but is usually not as good as vacuum glass.

Do you mainly choose energy savings and comfort? Then vacuum glass is often stronger.
Do you mainly choose to maintain appearance? Then monument glass may be a better fit.

Calculate the difference yourself with the U-value calculator. More background on the modern standard can be found in HR++ glass: what is it?.

Does monumental glass fit in existing window frames?

Monument glass is intended for existing old window frames, but you still have to check carefully whether it fits. NB:

rebate depth
glass thickness
space for glazing beads
old putty
condition of the wood
rods
rotating parts
ventilation and drainage
desired finish

Restoration frames often use narrow glass structures, so that the appearance is preserved as much as possible.

Does vacuum glass fit in existing window frames?

Vacuum glass often fits well in existing window frames because it is much thinner than HR++ glass. However, vacuum glass must also be able to be installed correctly. Check:

rebate depth
minimal imposition
space for sealant or glazing tape
glazing beads
condition of the frame
flatness of the rebate
weight of the window
edge protection
instructions from the supplier

Old window frames can be crooked or uneven. That's why accurate measuring is important — see measuring glass.

What if your window frame is in poor condition?

Installing new glass in a bad window frame is usually not a good idea. This applies to monument glass and vacuum glass. First check:

wood rot
cracks in the wood
loose connections
poor paint finish
leakage
bad sealant seams
sagging windows
poor locks

If the frame is bad, it must first be restored or replaced. Otherwise, the new glass may not function properly and moisture problems may return.

Monument glass in stained glass

With stained glass you often want to maintain the original appearance. Monument glass can play a role in this, but additional glass or insulating glass with stained glass are also options. Possibilities are:

restore existing stained glass
incorporating stained glass into insulating glass
install additional glass
making new stained glass
use thin insulating glass
use vacuum glass as an alternative

Which option is suitable depends on the condition of the stained glass and the insulation requirements. Read more in replacing stained glass.

Monument glass at sliding windows

Old sliding windows often have little space for thick glass. The weight must also be limited, otherwise the sliding window will no longer work properly. With sliding windows, please note:

glass thickness
weight
counterweights
balance system
rebate space
draft proofing
monumental appearance

Vacuum glass can be interesting here due to its thin construction. Monument glass can be interesting when the original appearance is more important.

Sound insulation: which option is better?

Sound insulation does not only depend on monumental glass or vacuum glass. It mainly depends on the glass structure, the gap seal and the frame. Important for sound insulation are:

glass thicknesses
layered foil
gap sealing
cavity or superstructure
rubbers
frame quality
ventilation grilles

Vacuum glass can improve comfort, but is not automatically the best solution against noise. If you want less street noise, ask for a sound-resistant structure.

Condensation on old window frames

Old single glazing often causes condensation to form on the inside. This is because the inside of the glass becomes cold and moist indoor air settles on it. Better insulating glass can reduce condensation because the inner pane remains warmer.

Please note: old homes often also have ventilation problems. If you improve glass and reduce gaps, ventilation must remain properly regulated. Preventing condensation requires better glass, sufficient ventilation, good gap sealing, a healthy frame, good heating and dry indoor air. More about this in condensation on windows: inside, outside or between the glass.

Subsidy for monument glass or vacuum glass

Subsidies may sometimes be possible for insulating glass, but this depends on the current conditions and the performance of the glass. NB:

does the glass meet the U-value requirement?
is a reporting code available?
is the glass professionally installed?
how many square meters will be replaced?
is the correct information on the invoice?
is the application submitted within the correct period?

Vacuum glass can often be interesting for a subsidy if it meets the requirements. With monumental glass it depends greatly on the exact insulation value and product structure. Always check this before ordering — see ISDE subsidy for insulating glass.

What is more expensive: monument glass or vacuum glass?

That depends on the implementation. Vacuum glass is usually more expensive than standard HR++ glass. Monument glass can also be expensive, especially if it requires restoration, special appearance or custom work. The costs are determined by:

dimensions
glass structure
insulation value
restoration
assembly
accessibility
frame work
whether or not there are monumental requirements
safety glass
number of windows

Don't just look at the price per square meter. If vacuum glass prevents you from having to replace window frames, the overall picture can be more favorable.

Common mistakes with old window frames

Only look at insulation — with old window frames, appearance is often just as important as insulation.
Try to install standard HR++ glass — HR++ glass is often too thick for narrow old rebates.
Frame condition forgotten — new glass in a bad frame does not provide a lasting result.
Do not check monument rules — not everything is allowed at monuments or protected cityscapes.
Reject vacuum glass because of price — if you can keep existing frames, vacuum glass can still be interesting.
Not taking ventilation into account — better insulation without ventilation can cause moisture problems.
Forgotten the glass image — modern sleek glass may fit less well with a historic facade.

Selection aid: monument glass or vacuum glass?

Situation	Best choice
Historical appearance is leading	Monument glass
Maximum insulation in thin glass	Vacuum glass
HR++ glass does not fit	Vacuum glass
Monumental facade	Monument glass or approved vacuum glass
Retain old window frames	Vacuum glass or monument glass
Glass must look old	Monument glass
Energy saving is the most important goal	Vacuum glass
Low budget	HR++ examine glass
Preserving stained glass	Restoration, front glass or insulating glass construction
Sliding windows	Often vacuum glass or thin monument glass

Image: selection aid diagram with the most important decision points between monumental glass and vacuum glass

Checklist before you choose

Is your home monumental or protected?
Should the outside view be preserved?
How deep is the rebate?
Is the frame in good condition?
How important is maximum insulation?
How important is historical appearance?
Maybe HR++ glass will fit after all?
Is vacuum glass technically possible?
Is monumental glass sufficiently insulating?
Do you want to apply for a subsidy?
Is safety glass necessary?
Is there condensation or a moisture problem?
Should ventilation be improved?
Are permits required?
Are the measurements recorded exactly?

Conclusion

Monument glass and vacuum glass are both interesting solutions for old window frames, but they have a different purpose. Monument glass is mainly chosen when the historic appearance is important. Vacuum glass is chosen when you want high insulation in a thin glass package.

Standard HR++ glass is often difficult for old and narrow frames because it is too thick. Vacuum glass often fits better and insulates very well. Monumental glass is particularly strong when the glass image must match a classical or monumental facade. Always check the condition of the frame, the rebate depth, the appearance, the insulation value and any monument rules.