Cavity Insulation Damp

What causes dampness in cavity walls

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How to diagnose the causes of damp cavity insulation

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Damp Cavity Insulation:

Defects to check and eliminate before replastering or lining with an Air Gap Membrane.

A 'damp meter' and a surface thermometer (see www.dampness-info.co.uk/meters.htm) are useful tools for this diagnosis - if not available use the back of your hand to assess the relative dampness of one affected area to another unaffected area.

For help with damp cavity insulation call our experts on 01626 331351.


Bridging of the Damp proof Course

A damp course is said to be 'bridged' when it is 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.

For help with bridged damp proof courses call our experts on 01626 331351.


Cavity filled with building debris

A cavity that is 'bridged' or filled with debris: usually brick ends, sand and mortar, all dropped by the 'brickies' during construction, or sand gradually falling from soft mortar joints.

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

For help with blocked cavities call our experts on 01626 331351.


Dirty wall ties

The wall ties are 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.

For help with dirty wall ties call our experts on 01626 331351.


Rusting wall ties

The wall ties are 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.

For help with rusty wall ties call our experts on 01626 331351.


Patchy cavity fill

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.

For help with patchy cavity fill call our experts on 01626 331351.


Penetration by services

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.

For help with leaking services in a wall call our experts on 01626 331351.


Leaking pipes in the cavity

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.

For help with leaking pipes in a wall call our experts on 01626 331351.


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 (click here).

Answer - 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.

For help with leaking services in a wall call our experts on 01626 331351.


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.

Answer - 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 Ultra DPC Cream at the correct level to form a new Damp Proof Course. The plaster will need replacement to our Specification.

For help with hygroscopic salts in a wall call our experts on 01626 331351.


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.

Answer - 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'.

For help with damp patches in a chimney wall call our experts on 01626 331351.


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).

380cc tube with extension nozzle - makes a maximum of 3.5 metres/11 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Can be applied with a DIY, standard 380cc version mastic/sealant gun or a professional 380cc skeleton gun (see our Order Form)

1000cc tube with extension nozzle - makes a maximum of 9 metres/30 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Requires a professional 1000cc skeleton gun (see our Order Form)

3 litre bulk pack, for professionals - makes a maximum of 27 metres/90 feet of damp proof course in single thickness masonry (spacing interval 120mm/4.5 inch walls). Requires a pump up dispenser.


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

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Go straight to Price List and Order Form


Most Frequently purchased Items:

- Replastering Additive Renderproof - part of the Replastering Specification

- 'Tanking' waterproofing liquid BondAcryl - for walls and joints below the DPC

- Salt Neutraliser - to kill 'efflorescing' (fluffy, white) salts

- Anti Mould paint - to prevent Mould, Guaranteed

- Certificate of Product Guarantee


Salt Neutraliser - in 5 litres and 1 litres

Salt Neutraliser for walls affected by dampness

Renderproof Additive for Rendering.

Use in sand and cement plastering to

make a 'salts' barrier, then 'skim' with

Finish Plaster.

Renderproof additive, for sand and cement mixes

Go straight to our Damp Shop


Learn from the UK's top Expert: become your own expert - save money.

Quick DIY Master Class in Dampness and Condensation - become your own Expert in 1 hour! Buy our "Dampness in Buildings" reprinted book by the great expert Graham Coleman. A great read, short and to the point.

Click here to buy the reprint.


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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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