Damp Problems in Cavity Walls

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Cavity Walls - how to identify and fix damp problems

One of the most effective ways of stopping damp traveling through the walls of a property is to build with a cavity, but sometimes the cavity can actually cause the damp problems. Below is a list of ways a cavity can be compromised and allow damp through to the inside. If you believe you have damp walls, check first with a meter available from www.dampness-info.co.uk/meters.htm . The back of your hand will also give you a good indication of damp within a wall when you make comparisons between areas.


Problem: Damp course could be 'bridged' or covered by high ground, pathway, garden or abutting new structure, (e.g. Conservatory, steps): the damp proof course must be a minimum of 6 inches (150mm) above outside ground level, in order to avoid rainwater splash up causing penetrating dampness, although even in this case a clear cavity should prevent transfer of dampness to the inside wall.

Remedy: Remove the obstruction or inject a new damp proof course at the proper level

Problem: Cavity 'bridged' or filled with debris: usually brick ends, sand and mortar, all dropped by the 'Brickies' during construction.

Remedy - remove a brick or block from the wall and clean out the cavity.

Problem: Wall ties dirty: can cause damp spots to appear on the inside plaster, sometimes with 'salt' or black mould.

Remedy - use a metal detector to locate the ties on the outside, examine with a borescope and remove a brick or block from the wall and clean the ties.

Problem: Wall ties rusting: can cause damp spots to appear on the inside plaster, sometimes with 'salt' or black mould.

Remedy - use a metal detector to locate the ties on the outside, examine with a borescope and if necessary fit replacement ties before isolating or removing the old ones.

Problem: Poor cavity insulation job: can cause cold spots on the inside plaster, sometimes with 'salt' or black mould.

Remedy - Try re-injection first, but if this is not successful remove a brick or block from the wall and check the cavity insulation.

Problem: Service fixing, pipe or wire penetrates the cavity: pipes, wires, bolts and other fixtures that pass into the cavity can carry moisture inside, which may emerge at that level or drip down and show elsewhere.

Remedy - remove the obstruction, or carefully seal the entry point. Pipes and wires should always be routed to climb up the wall immediately before entering the duct or hole, to avoid chandelling the water by gravity.

Problem: Water pipes concealed inside the cavity: usually heating or mains water, but can also be boxed in foul water pipes or cistern overflows in more modern houses.

Remedy - examine with a borescope and remove a brick or block from the wall to expose the defect.


Indoor Walls - ground floor, a quick summary checklist:

Symptom - white, fluffy salts, plaster possibly 'blowing' off the wall - 'efflorescence' - you have a water source which needs to be found and eliminated. Use our Water Leak Checklist

Remedy - Salt Neutraliser might work if you can find and stop the leak, or the plaster may be too damaged and need replacement to our Specification.

Symptom - damp patches (at low level, right down to the skirting board) that come and go - you have 'hygroscopic' salts that are attracting moisture from the air.

Remedy- you may have Rising Dampness, or your damp proof course may be 'bridged' by high outside ground, plaster on the inside or debris in the cavity (cavity walls only). Rectify any defects and if necessary inject Quick Cream at the correct level to form a new Damp Proof Course. The plaster will need replacement to our Specification.

Symptom - damp patches on or around chimney breasts - sometimes showing yellow or brown stains in wallpaper or paint - you have 'hygroscopic' chimney salts that are attracting moisture from the air, which may include tars and other chemicals from burning wood or coal.

Remedy - these 'salts' are very difficult to hold back in the wall, even with the replastering to our Specification carried out to perfection. We recommend using Mesh Membrane - a type of dry lining - to allow you to replaster using normal, lightweight plaster, with no risk of further 'salting'.


Products mentioned above:

Salt Neutraliser - a clear liquid used to neutralise 'salts' in render, plaster or masonry, either to attempt to save otherwise sound plaster or as a preparation prior to re-plastering to our Specification. Apply two coats, wetting in between with fresh water.

Mesh Membrane - easy to install dimpled plastic 'air gap' dry lining membrane, supplied in rolls like wallpaper. Designed to cover damp or salty walls and provide a fresh surface for replastering with a modern lightweight plaster (usually 'bonding').

Quick Cream - new, easy to install, cream damp proofing product - no pump required, no mess, no smell - includes injection nozzle extension. Designed to gently diffuse into the wall to form a water repellent layer - a 'damp proof course' (DPC). It is approved by the British Board of Agrèment - BBA Test Certificate No: 02/3961 - as used by the professionals. The BBA is a Government Approved organisation which has been testing building materials for over 30 years. Note - not all damp proofing materials have passed these tests - beware of cheap imitations).

click here for a check list if you want to diagnose your damp

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Site written by: David Moore

David Moore, B.A. (Hons.), C.T.I.S., C.R.D.S. Technical Author

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