January 24, 2019

Managing Damp in Older Buildings

Managing Damp in Older Buildings

A solid walled structure, built with breathable materials, needs to breathe.

It’s inevitable that you will find dampness somewhere in an old house. Nothing lasts forever, but despite British weather, our old buildings stand the test of time better than any modern house. The worst enemy of old houses is we humans – it’s what we do to them, or fail to understand about them, that causes problems and makes you think you have to ‘Damp Proof’.

Dont Stop Walls breathing

When a wall warms up after a cool night, air contained within its pores expands as it warms and a small proportion moves out of the wall via the connected pores. As the wall cools down the air within contracts and air moves back into the wall from the atmosphere. So masonry walls ‘breathe’ out as they warm up, and in as they cool. Breathing occurs on a daily basis, or more frequently in periods of variable weather; breathing is shallow when there is little temperature variation and deepest when the daily range is greatest. Walls don’t actually breathe in the human sense: they sit there while changes in temperature (and air pressure) do the work. The ‘breathing’ analogy is a convenient way of understanding frequent exchanges of air from masonry to atmosphere and back again. If air drawn into the wall is humid, and the wall cools below dew point then water vapour in the humid air condenses as water droplets in the pores of the masonry, though the wall will still appear ‘dry’. During warmer and drier times, some of this water will evaporate and leaves the wall as it breathes out. Apparently dry walls commonly contain water, the amount varying with changes in the season and climate. If there are salts or other hygroscopic (moisture-attracting) materials in the masonry, the amount of water drawn into (and retained in) the wall can be sufficient to make the wall visibly damp, even in dry weather. This is the same as happens when you leave a pot of salt on the kitchen table – it gets wet.

Anything that prevents a masonry wall from breathing will reduce its life expectancy. Coatings designed to seal the surface of masonry walls (and so ‘protect’ them)  trap moisture behind the coating and cause a damp problem elsewhere, such as on the other side of the wall. If there are appreciable salts in the wall, damage caused by inappropriate use of coatings can be dramatic . Coatings themselves – cement renders, gypsum plasters, plastic emulsion paints – will eventually be forced off the wall.

Most problems of damp in pre-1920’s buildings have been caused since the war, when cement and gypsum plaster became widely available – these two materials are responsible for over 90% of the damage that we see. They are impervious, they trap moisture and cause rot – they are death to an old house. Add silicone sealants to a timber frame, and you have instant rot of oak which is probably 400 years old. I have seen new timber frames start to rot in 2 or 3 years when the owners seal the panels using mastic instead of oakum, which can breathe.

Understanding your build environment

Building restoration is all about understanding the built environment, and how it affects materials which make up your home. By understanding this, it becomes a relatively simple task to sort out problems causing damp, and manage them for the long term.

We are trained in the Build Environment.  We study the materials old houses are built with.  We understand them. We work with clients to restore old buildings back to beautiful, warm and dry places.  There is something very special about the look and feel of a lime plastered wall with clay paint – it is almost luminous in quality – very soft and welcoming.  All the materials used in old homes are nice, friendly and green.  By using them, not only will you feel good, but you will be doing good – they are carbon friendly, and contain no toxic chemicals – lime mortars and plasters are carbon neutral, clay and linseed paints similarly – sheepswool and hemp fibre insulation are natural and nice (they are also just as good as chemical insulation), timber is a renewable resource…

We can always find the cause of damp, whatever form it may take, and I have to say that in our collective experience, we have never had to drill hundreds of holes into the lower courses of brickwork, thereby destroying them. We never inject chemicals which are supposed to stop water from ‘rising’ up the walls, which never did anyway.

Common causes of Damp

  • Insulation
  • Modern paints
  • Cement render
  • Gypsum plaster
  • Ground levels outside higher than inside
  • Broken guttering or missing downpipes
  • Vegetation growing near the wall
  • Trees creating shade and moist air near a wall
  • Lack of ventilation – double glazing, no vents
  • Blocked chimneys – fireplace blocked up, no vents
  • Furniture against walls creating cold, damp areas

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