Traditionally constructed houses (ie those built before 1919) were created from soft, permeable and flexible materials – typically earth, lime or clay mortars and plasters binding soft bricks, stone or timber.
These buildings don’t have vapour barriers or damp proof membranes like modern structures do, but manage damp and moisture in a different way, through absorption and evaporation. This is known as breathability and before you undertake any renovation work on a heritage property, it’s vitally important that you understand this concept.
Learn more: Follow a real life period renovation
You must make sure that the materials you use maintain the breathable performance of the building fabric. Most modern products don’t do this because they are designed to restrict moisture movement.
As a result, incorporating materials such as cement, gypsum plaster, vinyl paints, impermeable membranes or vapour closed insulation could result in very serious long-term damp problems in old buildings, leading to decay and mould growth.
The problem is that many products used nowadays, particularly those containing cement, are rigid and thus incompatible with the highly flexible underlying structure of older buildings.
Applying the wrong materials could lead to cracking, water penetration and even structural failure. As well as being bad for the building, these problems create a very unhealthy living environment for the occupants.
Natural resources were used; these were handcrafted and sourced locally. These materials make an important contribution to the overall character, history and feel of a heritage home and its surroundings. Period houses often feature a lack of uniformity, unevenness, signs of age, and wear and tear – and these are the elements that make them special, desirable and valuable.
It can be very tempting to sweep away much of the old, worn material, often described as defective by modern surveyors or builders. Lime plaster, timber with signs of past woodworm attack and old windows are stripped out and replaced without much thought to the consequences.
But taking this approach unfortunately means that much of what made the house interesting has disappeared by the time the renovation is complete. The best advice is nearly always to aim to retain as much as possible.
It’s important to choose products that are compatible with the existing makeup and structure of the building. You must consider breathability, but flexibility and dimensional stability capabilities should also be similar to original interface materials to avoid problems such as cracking and delamination. Plus, don’t forget visual impact – matching colour, texture and style can be important to maintaining the character and feel of your house.
The best way to achieve this degree of compatibility is often to use the same material as the original – obviously the more closely matched it is, the more similar its characteristics will be. However, this is not always as straightforward as it might seem. For instance, mixes for lime plaster, mortar and render can vary considerably and accurate matches can be hard to achieve without expensive analysis and formulation.
Another example is structural timber, as behaviour varies greatly depending on speed of growth, grain direction, method of conversion and which part of the tree it comes from. And some historic materials, such as old growth softwood, are no longer available.
There are occasions where it’s a good idea to consider different materials from the original – for example, to improve thermal performance or moisture management. Plus, there is an increasing range of newly developed heritage materials out there.
These include lime plasters with hemp or cork for improved thermal performance; clay plasters with very high capability for moisture buffering and improving indoor air quality; and acetylated wood for external cladding that is resistant to decay. The important characteristic is that these are natural materials that are compatible with the breathability of the structure.
For new additions to old buildings the choice of materials is different, especially for the exterior. You’re likely to use modern construction methods, so compatibility of performance and breathability aren’t necessary. Instead, your choices will be based on the aesthetic and design.
It’s often perfectly appropriate to create a contrasting, contemporary extension and almost never suitable to try to match the new work exactly to the old using the same style. The most successful outcomes generally incorporate similar materials to the old building within a contrasting design so that the elements are visually linked but distinct.
The use of heritage materials – such as oak, handmade bricks, clay tiles and lead – can be very effective. Quality of craftsmanship, authenticity and attention to detail are essential for this approach to work properly; the selection of mortar, fixings and construction details can be just as important as bricks, tiles and timber.
For internal finishes, you need to consider performance characteristics. Natural materials like clay or lime plasters have very different acoustic properties from modern products in that they have noticeable temperature and moisture buffering capabilities. These factors create a distinctly different feel and sense of comfort.
If the internal finishes of a new extension are harder and more modern than in the parent building, there can be a disconcerting change as people move from one to the other. The new element can feel less comfortable than the old part of the house. To avoid this it can be a good idea to continue the same feel through into the new build element by using heritage materials for the internal finishes.
The same principles of breathability and moisture management apply to insulation products, which can be described as either vapour open or closed. Most modern systems are the latter type – they will not allow the passage of water vapour, tending to trap it in permeable, traditional building fabric.
Vapour open materials do the opposite, aiding the breathability of the structure. It’s a pretty reliable rule of thumb that synthetic materials are vapour closed and natural materials open.
Natural options include sheep’s wool, hemp, woodfibre, cellulose, cork and sisal.
These items are also hygroscopic, which means they have the ability to absorb and release moisture without any reduction in their insulation value. Because of this they can buffer internal humidity levels, reducing condensation risk and creating a healthy environment for both the building and its occupants.
Insulating floors has a significant impact on comfort because the floor is the element of the building with which you are in most contact. Suspended timber floors can often be carefully lifted without causing damage.
It is then quite straightforward to drape a membrane between the joists and fill the space between them with insulation before replacing the boards. Any of the materials listed above can be used but loose fill cellulose insulation works particularly well.
If solid floors have been replaced with uninsulated concrete ones, these can be removed and a breathable, insulated floor comprising a base of foamed glass aggregate covered by a lime screed applied. Of course, surviving historic solid floors should be left undisturbed.