Use of Natural Materials in Retrofitting Houses

By Barry Tylee

As the owner of an old, stone house I needed to know which materials are the best to use for retrofitting, so I attended this course on how to work on houses built with heritage materials.
Houses made out of traditional materials such as brick, wood and stone and then covered in render (on the outside) or plaster (on the inside) must be able to breathe to evaporate the moisture which we produce during normal occupation. When houses were built previously, they used natural materials. Often these have fallen out of fashion; and that’s a pity as they certainly may have a role in retrofitting buildings to improve insulation and reduce heating costs. A typical family produces a few pints of water (as moisture in air) a day just being in a house; and then there’s washing and drying clothes and baths and showers – all producing more moisture. Also, houses often have problems with water ingress due to leaking gutters and roofs or rising or penetrating damp. There’s no way that these can be sealed in and will disappear – the only solution if the fabric of the house is damp, is heating and ventilating it – and this might take years to dry the structure once the problem is fixed. So double glazed windows must have a trickle flow of fresh air and care must be taken not to block air bricks. Also, the outside surface should be at least 150 mm below the internal surface.
Building Surveyors have seen increasing problems due to trying to seal basements (tanking) and reconfiguring areas for multiple occupations (more people in smaller areas means more moisture). Often sealing a wall, say with some wonder plastic solution, just leads the moisture to move upwards maybe to wooden joists which may then rot, bringing structural problems. The more that a building becomes air-tight, the more that mechanical air extraction is required, otherwise moisture won’t be extracted.
Natural materials such as sheep’s wool or hessian are ideal for loft or underfloor insulation as they will naturally breathe. The sheep wool must be specially heat treated – this chemically changes each fibre making it unattractive to moths – a big problem with untreated fibre! Look for a company where the wool is heat treated eg  Ionic Protect | Benefits of Sheepwool Insulation Once in your house, moths are incredibly difficult to eradicate.
When it comes to pointing stonework or brickworks or sealing joints in old houses, then lime mortar is recommended, once set this enables moisture to evaporate out of the wall. However, cement won’t breathe like this and if the fabric is softer than the cement, it will be damaged when the mortar pulls/falls away with age.