Why old buildings fail when we try to make them better

We have become very good at improving buildings, but when it comes to thgose that were constructed 100, 200 or even 300 years ago, the definition of “improvement” is not always as straightforward as it appears. An old building can look cold, inefficient and poorly insulated by modern standards, so the natural response is to seal it, insulate it, replace its windows, repoint its walls and introduce materials designed to make it perform more like a modern building. The problem is that many traditional buildings were never designed to behave like modern buildings in the first place. When we change one part of the construction without understanding the way the original building dealt with moisture, heat, air and movement, an intervention intended to improve performance can often create an entirely new problem, writes John Ridgeway.

This is one of the great challenges of working with historic buildings. Their apparent simplicity can be deceptive because traditional construction often relies on a relationship between materials rather than on a series of sealed, independent layers. Brick, stone, lime mortar, timber, plaster and roof materials each have their own characteristics, but they also interact with one another. Moisture can be absorbed, redistributed and released. Walls can warm and cool gradually. Small amounts of air movement may be part of the way the building manages its internal environment. What we might regard as an imperfection in modern construction can therefore be an important part of how an older building functions.

One of the biggest differences between traditional and modern construction is the way moisture is managed. Many older buildings have solid masonry walls constructed from materials that are relatively permeable. Rather than relying on an impermeable barrier to prevent moisture from entering, these buildings can accommodate and release moisture through the materials themselves.

That does not mean that damp walls are acceptable or that traditional buildings somehow do not need protection from water. It means that the route moisture takes through a building can be fundamentally different from the route it takes through a modern cavity-wall construction. A traditional wall may absorb some moisture and subsequently allow it to evaporate. Its lime mortar may accommodate movement and moisture differently from a hard cement mortar, while lime plaster can interact with the masonry behind it rather than simply forming an impermeable skin.

Problems can arise when modern materials are introduced without considering that original behaviour. An impermeable coating, cement-rich render or inappropriate pointing can restrict evaporation and alter the moisture balance within the wall. Water that once had a route out can become trapped, increasing the risk of deterioration to masonry, finishes and timber. In other words, the intervention may not have caused water to enter the building, but it may have removed the mechanism that previously allowed the building to deal with it.

Insulation can create a similar paradox

Improving thermal performance is an important part of responsible refurbishment, particularly given the energy required to heat older buildings. The mistake is to assume that insulation is simply a matter of adding as much thermal resistance as possible.

Where insulation is placed can be just as important as how much insulation is installed. Changing the temperature of a wall can alter the way moisture behaves within it, while poorly considered details around windows, floors, roofs and junctions can create cold spots where condensation becomes more likely. Internal wall insulation can be particularly sensitive because it changes the thermal conditions experienced by the original masonry.

None of this means that historic buildings cannot be insulated successfully. They can and well-designed retrofit can make a significant difference. The important point is that the insulation needs to be compatible with the existing construction and considered as part of the building as a whole. A technically impressive product cannot compensate for a design that ignores the way the original fabric responds to moisture and temperature.

Modern construction has rightly placed enormous emphasis on airtightness. Reducing uncontrolled air leakage can improve energy efficiency, comfort and indoor environmental conditions. The problem comes when airtightness is treated as an isolated objective rather than as one part of a wider building strategy.

Consider something as apparently straightforward as replacing the windows in an old property. A modern window can be vastly more thermally efficient than the original, and properly sealing the junction can prevent unwanted draughts and heat loss. However, the surrounding wall, plaster, timber and existing ventilation arrangements still have to deal with the consequences of that change.

The same principle applies throughout the building. Blocking gaps, sealing joints and adding membranes can all be appropriate interventions, but each one changes the way air and moisture move. If ventilation is reduced without considering how the building and its occupants previously dealt with moisture, condensation and indoor air quality can become problems elsewhere.

The objective should therefore not simply be to stop everything moving through the building. It should be to control that movement intelligently.

Stronger does not always mean better

Perhaps nowhere is this more obvious than in traditional masonry repairs. It is tempting to assume that a stronger repair material must provide a better repair. If an old mortar joint is soft and weathered, replacing it with a harder cementitious mortar can appear to be an obvious improvement.

But traditional masonry often relies on the mortar being more sacrificial than the brick or stone. A softer lime mortar can accommodate movement and moisture, while allowing the masonry to remain relatively protected. A much harder mortar can change that relationship. Instead of the joint deteriorating gradually and being relatively straightforward to repair, stresses and moisture may be transferred into the surrounding masonry. The repair may therefore be stronger while the wall becomes less resilient.

This is an important distinction because conservation is not necessarily about preserving every original material indefinitely. It is about understanding why a particular material was used and what role it plays before deciding what should replace it.

Old buildings are not defective versions of new buildings

Perhaps the biggest conceptual mistake is to look at an old building through the standards and expectations of modern construction and conclude that it is simply an inferior version of what we build today. It isn't.

A Georgian townhouse, a Victorian terrace, a timber-framed house and a nineteenth-century stone cottage may all have very different ways of dealing with moisture, heat, movement and ventilation. Even buildings constructed at the same time and from similar materials can behave differently depending on their location, orientation, exposure, alterations and previous repairs.

Traditional builders did not have modern thermal modelling or sophisticated building-performance software, but they had extensive practical knowledge of local materials and construction methods. That knowledge developed through experience, observation and repetition. They understood that a building had to work with its environment rather than simply isolate itself from it.

We should not romanticise that knowledge or assume that every traditional method was better. Many old buildings genuinely are thermally inefficient, poorly ventilated or in need of substantial repair. Modern materials and technologies can provide enormous benefits when they are used appropriately. The lesson is simply that the starting point should be understanding rather than assumption.

The best retrofit may be the one that changes the least

The challenge of improving an old building is therefore not to prevent change, but to make the right changes in the right places. A successful refurbishment should improve comfort, energy performance and durability while respecting the way the existing building works.

That requires looking beyond individual products and asking how the whole construction behaves. Before insulating a wall, understand what it is made from and how it deals with moisture. Before sealing a junction, understand what the seal is changing. Before replacing a mortar, understand the relationship between the mortar and the masonry. Before improving airtightness, consider ventilation. Before replacing an original material because it appears outdated, establish whether it is actually causing a problem.

There is a useful principle here that applies well beyond conservation - a building should not be judged simply by how closely it resembles the buildings we construct today.

Some of the apparent imperfections in an old building may be evidence of how it has survived for generations. Others may be genuine failures that need correcting. The skill lies in knowing the difference.

That is why working with historic buildings requires more than finding modern solutions and applying them to old problems. It requires an understanding of materials, construction, moisture, movement and, above all, the particular building in front of you. Because the irony is that an old building can sometimes survive perfectly well for two centuries, only to begin experiencing problems after we decide to make it better.

Frequently Asked Questions

1. Why can improving an old building sometimes create new problems?

Traditional buildings often rely on materials such as brick, stone, lime mortar and timber to manage moisture and movement in ways that differ from modern construction. Introducing impermeable materials, insulation or airtightness measures without considering those relationships can alter the building's behaviour and create problems such as trapped moisture, condensation or deterioration.

2. Does an old building really need to “breathe”?

“Breathing” is often used as shorthand for the ability of traditional materials to manage and release moisture. It does not mean that walls should be draughty or that buildings should be left uninsulated. The important consideration is how moisture moves through the construction and whether refurbishment allows the original materials to dry appropriately.

3. Can you insulate a solid brick or stone wall?

Yes, but the appropriate approach depends on the construction, exposure, materials and condition of the building. Internal and external insulation affect the existing wall differently, so the design needs to consider thermal performance alongside moisture movement, junctions, ventilation and the condition of the original fabric.

4. Why can cement mortar damage an old brick wall?

Traditional bricks can be relatively soft and porous, while lime mortar was often designed to accommodate movement and moisture. A hard cement mortar can behave differently and may cause moisture to remain within the masonry or transfer stresses into the bricks. In some situations, a mortar that appears stronger can therefore contribute to the deterioration of the surrounding wall.

5. Is replacing old windows always a good way to improve energy efficiency?

Replacing poorly performing windows can reduce heat loss and improve comfort, but the window itself is only part of the equation. The installation needs to consider the surrounding wall, ventilation, moisture and detailing. In some buildings, repairing and upgrading existing windows may provide a better balance between performance and preservation.

6. Why can sealing draughts sometimes cause condensation?

Reducing uncontrolled air leakage can improve energy efficiency, but it can also reduce ventilation. If moisture generated by occupants is no longer adequately removed, humidity can rise and condensation may occur on colder surfaces. Airtightness therefore needs to be considered alongside ventilation rather than treated as an isolated improvement.

7. Are modern materials always better for historic buildings?

Not necessarily. Modern materials can provide excellent performance, but they are not automatically compatible with traditional construction. The important question is whether the material is appropriate for the existing building and whether it allows the construction to manage moisture, movement and temperature effectively.

8. Why shouldn't an old building be treated like a modern building?

Traditional and modern buildings can operate according to very different principles. A modern cavity wall, for example, is designed around controlled layers and barriers, whereas a traditional solid wall may depend more heavily on the properties of the masonry and mortar themselves. Applying modern solutions without understanding those differences can create unintended consequences.

9. Does making an old building more energy efficient damage its historic character?

It doesn't have to. Good retrofit can improve energy performance while retaining important historic fabric and architectural features. The challenge is to identify where intervention will have the greatest benefit and least impact, rather than applying the same solution throughout the building.

10. How can you tell whether damp is caused by a building defect or an inappropriate repair?

Damp should be investigated rather than assumed to have a single cause. Roof defects, gutters, drainage, ground conditions, plumbing, condensation, external coatings and changes to pointing can all contribute. Understanding the construction and tracing the movement of moisture is usually more useful than simply treating the visible symptom.

11. Should old lime plaster or mortar always be replaced with modern products?

No. If traditional lime materials remain sound, there may be no reason to replace them. Where repairs are necessary, compatibility with the existing construction is important. The appropriate material should be selected according to the building's original construction, exposure, condition and the function the repair needs to perform.

12. What is the most important principle when renovating an old building?

Understand before you intervene. Before changing insulation, windows, pointing, finishes or ventilation, establish how the existing building works and identify what is actually failing. The aim should not be to make an old building behave exactly like a new one, but to improve its performance while allowing its traditional construction to continue working effectively.