1. The eye of experience is the most useful instrument for the recording and diagnosis of damp and timber decay problems in buildings.                                   

·           2. When analysing damp and timber decay problems in buildings, it is generally best to survey for moisture penetration and movement from top to bottom and from outside to inside, following the usual route of water penetration in the UK.

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·           3. When analysing damp problems in buildings, make a note of moisture sources, moisture sinks and moisture reservoirs.

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·           4. Particular attention should be given to sources of concentrated moisture penetration such as roof drainage and ground drainage systems

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·          5. Particular attention should be given to defective moisture sinks (e.g. drains, cavity ventilators, evaporative surfaces and structures).

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·          6. The course of moisture from sources to sinks should be plotted, when analysing damp problems in buildings.

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·          7. When analysing damp problems in buildings, check for moisture vapour movement through the structure from source to sink, and air movement from warm to cold or from wet to dry areas.

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·          8. Measurements of relative humidity are not generally very useful, except with reference to dew point temperatures. The moisture content of air equilibrated timber is often a more useful indicator of chronic environmental problems.

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·         9. Generally, relative humidities over 70 % in occupied buildings is likely to cause condensation if not vented to the exterior. Timber at equilibrium with 70% r.h. will have a moisture content of approx 15%

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·         10. Prolonged relative humidities of over 80 % allow mould growth and will provide the conditions for microbial  decay of materials. Timber at equilibrium with 80% r.h. will have a moisture content of approx. 18%

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·          11. Prolonged relative humidities of over 85 % will result in bio deterioration. Timber at equilibrium with 85% r.h. will have a moisture content of approx. 20%

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·          12. Material moisture content rather than relative humidity is the most important factor to be measured in the analysis of damp and decay problems

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·          13. The significance of the percentage gravimetric moisture content varies between materials due to differences in their density and the extent to which water is bound hygroscopically.  The significant factor is the partial water vapour pressure in the material or the available moisture.

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·          14.In building surveying, a convenient approximation to available moisture is the timber equivalent moisture content (EMC) which may be defined as the moisture content that would be found in a piece of timber that was allowed to come into equilibrium with the material in question.

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·         15. As a rule of thumb for site investigations, the timber equivalent moisture content is approximately 10 times the gravimetric available moisture content of in-contact masonry.

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·          16. The most cost effective way of estimating timber equivalent moisture content on site is to use resistance-based moisture meters, but the readings from such meters are commonly misinterpreted.

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·          17. The deep moisture content of a material is more important than the superficial moisture content.  In order to estimate the deep moisture content of materials with a resistance-based moisture meter, deep probe electrodes are required.

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·          18. The readings from resistance-based moisture meters are commonly misinterpreted for the following reasons:-

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·          19. The limits of superficially measured wet or dry readings are not appreciated.

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·           20. Artificially high readings are taken due to conductive salts in the material.

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·           21. Artificially high readings are taken in timbers due to previous chemical treatments.

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·          22. Artificially high readings are recorded in composite materials, such as plywood, masonry containing carbon (e.g. PFA etc.), some surface printed finishes and metal foil!

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·          23. Useful rules of thumb for assessing the significance of resistance-based moisture meters during site surveys are as follows:-

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·          24. Readings of less than 20% timber moisture equivalent indicate that moisture is too dry for active fungal decay.

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·         25. Readings of less than 13% timber moisture equivalent indicate that timber is too dry for significant insect decay (except in the case of House longhorn and Powder post beetles).

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·         26. The internal materials in an occupied building will usually have moisture contents of less than 12 % timber equivalent moisture content.

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·         27. Roof timbers in an occupied building commonly have moisture contents of 16 % or less.

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·         28. Roof timbers in an occupied or intermittently occupied building may have moisture contents varying between about 12% and 25 % throughout a year, but these may not be significant. Experience has shown that an annual average of 14% results in ‘safe’ conditions.

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·         29. Timber equivalent moisture contents of 30 % (timber fibre saturation point) or greater are damp enough for wet rot decay or for dry rot germination to occur ie there must be available liquid water

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·          30. Surface capacitance meters are useful for initial survey of masonry or plaster structures, but readings are poorly correlated to gravimetric or available moisture contents

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·          31. Low surface capacitance readings are a useful negative finding, but high readings require further investigation.

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·         32. Sampling and gravimetric analysis of materials is the only accurate and cost effective surveying technique for determining the true moisture content of building materials.

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·         33 . The use of the oven drying technique, as described in BRE Digest 245,  1986 is the standard technique.  The formulae for determining the percentage of gravimetric moisture content is:-

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·                                 Ww - Wd x 100% where Ww is the wet weight and Wd is the dry weight The hygroscopically bound moisture content is significant in most masonry and plaster due to water bound to salts.  This is defined in BRE Digest 245 as the moisture bound to a material at equilibrium with air at relative humidity of 75 % at 20oC.  The formulae for the percentage of hygroscopic moisture content is:-

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·                                 W75 - Wd x 100% W75 where W75 is the weight after equilibrium at 5% RH at 20oC

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·         34. The available moisture content is the most useful indicator of water available for deterioration and biodegradation  processes. This is defined as the total gravimetric moisture content less the hygroscopic water.

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o        35. The ‘Speedy’ meter is of limited use because it can only determine total moisture content. However, the results are obtained on site and are relatively accurate. Slavish reliance on total moisture content as an indicator (eg the 5% threshold estimate given in BS 6576 1985) is usually a mistake and experience shows such values are frequently misinterpreted. The ‘Speedy’ does have the advantage of  providing sub-surface readings that can be replicated through or over a structure, thereby illustrating moisture content gradients. This is perhaps its most useful function.

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o        36. A sub-surface moisture gradient correctly taken using a Speedy meter or, preferably, the gravimetric method can point to the source of dampness. With experience and intelligence, surface electronic readings can sometime be used in the same way. However, in the case of ground water penetration this method is notoriously unreliable because of the influence of salts

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o        37. In the case of rising dampness the source of moisture is the ground, and moisture contents will always decrease with height above ground. This will generally not be shown with surface reading electronic meters.

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o        38. Condensation will often cause dampness that resembles rising dampness because heat rises leaving the base of walls preferentially cool. Air circulation at the base of walls is often very slow. Drying of condensate may be very slow. This results in a net increase in moisture content in the wall surfaces over time. However, the interior of the wall may be dry.

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o        39. Condensation may occur at the internal surface of the wall or within the wall. When the surface of a wall is dampened by condensation which is not allowed to dry out the rate of condensation will increase because the wall will loose heat more rapidly. The ‘wet front’ advances into the wall. Over time, this often resembles rising dampness.

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o        40. Water soluble salts translocated to the wall surface by water penetration and evaporation over time will affect electrical conductance. Calcium sulphate from the building material will be translocated and may appear at the surface as efflorescence. Salts from the moisture source way also be translocated. In the case of water from the ground nitrate and chloride salt combinations are commonly found. These salts become hygroscopic in relative dry air (30-50% r.h). Hygroscopic salts are often blamed for dampness but without formal assessment of available moisture (using the gravimetric method) such statements are meaningless. Hygroscopic moisture is not available to cause decay. Hygroscopic salts in plaster are therefore not a significant structural issue

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