How to let your old walls breathe

how-to-let-your-old-walls-breathe Grade II listed thatched cottage in Cambridgshire
Grade II listed thatched cottage in Cambridgeshire

If you live in a period property, it's important to understand how the building works and how the moisture is managed. The walls of your heritage house need to be able to 'breathe', in other words, be able to dry out when they get wet. It is one of the keys to avoiding dampness and decay.

Before about 1919, houses were built using weak and porous mortars, plasters and renders. A process of absorption and evaporation made it possible for old buildings to deal with moisture. When it rained, moisture was absorbed but could easily evaporate due to the porous nature of the walls. Indoor moisture from cooking and bathing could also easily evaporate through the walls. This process was helped by the use of open fires.

It's very easy for this delicate balance to be upset by introducing modern cement based renders and mortars, 'plastic' paints and waterproof sealants. These modern products will trap the moisture, causing damp and rot.

Historically, the internal walls in traditional vernacular buildings usually consisted of stud partition construction which were made of wood stud framing with laths nailed across to form a base for the plaster coat.

Lath and plaster wall. Image credit: Chris Costello

A thick application of plaster was applied to the interior cottage walls. The plaster consisted of clay and animal hair with some lime to bind it together. The ingredients would have varied according to the locally available materials. Sometimes chopped straw or reeds were also used.

If the walls and ceilings of your period house have been re-plastered with modern materials, or if you have damp walls, it might be worth thinking about replacing the modern materials with a 'breathable' product such as lime or clay plaster.

Clayworks in Cornwall manufactures breathable clay plaster finishes for internal walls and ceilings. Amongst the most sustainable wall finishes available, they are recyclable, compostable and contain no toxic ingredients, VOC's or synthetics.

Below are examples of clay plaster finishes from Clayworks.

From left to right:

Pigmented Rustic Top Coat without straw, Smooth Pigmented Top Coat without mica and Demi-Rustic Smooth Top Coat.

Of course, there is little point in going to the trouble of applying a natural plaster and then using a vinyl emulsion paint for decoration. This will simply seal in the moisture and lead to damp, damage and decay.

Using renewable raw materials such as beeswax, linseed oil, earth and mineral pigments, Edward Bulmer Natural Paint is an artisan business producing 100% natur