Damp FAQ

Selected topics that have generated a lot of interest.
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NT
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Damp FAQ

Post by NT » Fri 20th Oct, 2006 1:35 am

If you need damp information, start here.

This thread is for reposting some of the posts with the most key points about damp in period properties. Please discuss damp in other threads, and we can keep this for a long term condensed reference section. Regulars here will then benefit from much better informed questions being asked, and less wear and tear on fingers.


NT

NT
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Post by NT » Fri 20th Oct, 2006 1:37 am

Matt Green wrote:

| We need to know:
|
| floor types (on either side for int. walls)
| ground heights (for ext. walls)
| plaster type
| wall construction
| problem being experienced
| what the room is used for

NT
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Post by NT » Fri 20th Oct, 2006 1:54 am

This Introduction
-----------------


This Introduction is intended as an initial guide to damp problems and their treatment. It is not intended to be exhaustive, to cover all possible situations, to address uncommon problems, nor to reflect every possible school of thought or opinion. It is not paid professional advice, and you have no recourse if you find it does not answer your question, does not solve your problem, or causes your dog to spontaneously combust.

If you read this guide before requesting help with a damp problem on the forum, your chances of a successful resolution will hopefully be noticeably higher, and your odds of paying for inappropriate, unsuccessful or damaging work lower.


NT
Last edited by NT on Fri 20th Oct, 2006 2:54 am, edited 1 time in total.

NT
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Post by NT » Fri 20th Oct, 2006 1:58 am

Diagnosing damp
---------------

Generally one should not rely on reports of damp, since misdiagnosis is so widespread. The symptoms of a damp problem are:


Bubbling disintegrating plaster
Black mould, especially on lower part of walls
salt crusting on plaster
rotting wood
visible condensation on wall
stain patches on walls
musty mouldy smell

Only severe damp will demonstrate all symptoms at once.


NT

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Post by NT » Fri 20th Oct, 2006 2:10 am

Damp meters
-----------


Damp meters were designed to measure water content in wood, and are now widely used to diagnose damp problems, despite some significant issues:

1. They measure conductivity, not damp. Conductivity is caused by any of:
damp,
partially conductive building materials,
foil lining due to historic problems, real or imagined,
transient surface condensation.

2. They can not be calibrated for use on walls because reading versus water content varies significantly from one wall material to another.

3. They use no means to give consistent probe to wall contact area

4. They measure only surface conductivity. Transient surface condensation is a harmless phenomenon which does not indicate a damp problem.

5. Readings are heavily affected by presence of salts on the wall (which may be present due to causes other than a damp problem).

6. For the less honest, the probes are suitably positioned for those looking for work/money to place a finger across, increasing the reading.


Thus damp meter readings do not diagnose damp problems for several reasons.


NT

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Post by NT » Fri 20th Oct, 2006 2:11 am

Damp vs damp problem

I think its often not appreciated that theres a difference between damp and damp problem. Typicaly old house walls have higher water content than new build, but in most cases the water content is low enough for no symptoms of any kind to occur, and thus there to be no damp problem.


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Post by NT » Fri 20th Oct, 2006 2:21 am

Dynamic equilibrium
-------------------


Water content of walls is a dynamic equilibrium between water ingress and evaporation. Thus to dry damp walls we need to do 2 things:

1. Remove any sources of unnecessary ingress - eg faulty rainwater goods etc.

2. Increase evaporation by removing the barriers to evaporation, impermeable wallcoverings. These are cement render, cement tanking, proprietary wallcoatings, plastic based wallpaper, and non-breathable paints.


Many have sought to stop water ingress by covering walls with impermeable coatings, but this is not generally successful in the long term, because it has more impact on evaporation that it does on ingress. Allowing the rain onto a wall also allows 24/7 evaporation, and perhaps counterintuitively, this results in a drier wall overall.


NT

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Post by NT » Fri 20th Oct, 2006 2:33 am

Wood floors, rot and damp
-------------------------


Suspended timber ground floors, common in period properties, need a few basic measures to keep them dry and healthy.

1. There should be air vent bricks in the exterior wall below the floor. The holes in these bricks should be kept clear. Occasionally it can even be necessary to dig down to them due to rising ground levels. Bricks that have been blocked for some time can cause rot in the floor or the floor joists.

2. Water shold not run through the airbricks into the underfloor void.

3. Exterior ground levels should be 6" below interior floor level. This avoids penetrating damp, reduces rain splash at floor level, and allows some wall area for evaporation below the floor.


The above assumes the floors are correctly constructed, with a ventilated void under the floorboards. Very occasionally other constructions may be encountered, which may require a different approach.


NT

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Post by NT » Fri 20th Oct, 2006 2:40 am

Causes of damp on internal walls, in approximate order of likelihood:

1. condensation.
Cause 1: interior RH too high. Solution: find cause/s of high RH and improve them.
Cause 2: 4" thick walls with no insulation creating very cold surfaces in winter. Solution: Insulation + vapour block membrane on the 4" walls.

2. Faulty roof or rainwater goods dumping water onto the wall. Solution: clear gutters and hoppers, check rain drainage works ok, or repair roof. Look out for corrosion through the rear of iron rainwater goods, which can sometimes be hard to spot even when looking.

3. Cement floor or dpc in floor: prevents evaporation of water from bottom of wall, thus causing damp build up. Possible solution: replace floor with permeable floor if all other easier measures have failed.

4. Leaking plumbing

5. Salt contamination: this tends to happen to chimneys mostly, but can also result from iron pipe fixings or spillages. Solution: remove and replace affected plaster

6. Peeing: animal or small child peeing on wall. Solution: retrain.

7. Rising damp. If every other cause has been ruled out, and every necessary measure taken, inserting a dpc might work. But these are very much the exception, nearly all rising damp diagnoses are false.


NT
Last edited by NT on Tue 26th Jun, 2007 7:10 pm, edited 1 time in total.

The Damp Man
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Damp FAQ

Post by The Damp Man » Fri 20th Oct, 2006 8:01 am

NT Wrote

7. Rising damp. If every other cause has been ruled out, and every necessary measure taken, inserting a dpc just might work. But these are rare cases, almost all rising damp diagnoses are false. [/b]

So where is your evidence and can I assume from this statement that you have individually reviewed every single damp investigation made?

TDM

The Damp Man
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Damp FAQ

Post by The Damp Man » Fri 20th Oct, 2006 8:05 am

Oh and before you state the obvious response.....

No I don't make a living out of damp proofing!

Lets see your evidence to back up the statement youve made!.

TDM

NT
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Post by NT » Sat 21st Oct, 2006 3:18 am

Glossary
----------

RH: relative humidity. This is the amount of water vapour in the air expressed as a percentage of the max amount of vapour the air can hold. Note that this max amount varies with temperature, so for example 50% RH air at 30 deg C holds more vapour than 90% at 5 deg C.

DPC: damp proof course. This is a thin horizontal layer of non-permeable material found below floor level in walls and floors of modern buildings and some older properties. DPCs are typically made of slate, plastic sheet or bitumen. A lot of older houses have no dpc.

LBC: listed building consent. Required for many works to listed buildings.

Breathable: Relatively porous building materials, through which water can travel and evaporate form at a relatively high rate. Most materials allow moisture migration to some extent, but how much varies widely.


more things to be added if anyone thinks of them :)


NT
Last edited by NT on Tue 26th Jun, 2007 7:13 pm, edited 2 times in total.

NT
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Post by NT » Sat 21st Oct, 2006 3:43 am

Does Rising Damp exist?
----------------------------


There is some controversy surrounding the question of whether rising damp exists. This space is bookmarked for a set of links for those wishing to read the various discussions and evidences.

If you have a link you'd like to see added, please PM it to me or post it in another thread. Please do not post it in this thread, as 10 or 20 posts on each subtopic would make the thread very user unfriendly.

Thank you.


NT

NT
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Post by NT » Sat 21st Oct, 2006 9:16 am

Dehumidifiers
-------------


<> When should I use one?

When you need to reduce water vapour produced by showers, cooking or in bedrooms.

When damp is bad enough that it will take a while for the house to dry, and mould would present a health risk in the meantime.

When other practical measures have failed, or it is known they will not work.

When damp is severe.

In preference to external venting fans when the fresh air is not required. Fans have a higher total cost of ownership than a dehumidifier, due to throwing heat out of the building.



<> Which model should I choose?

Use a standard compressor based unit for most tasks. For use in cold locations a desiccant wheel unit is needed, as compressor dehumidifiers are ineffective below around 10 deg C. Do not use the £5 units that are simply boxes of lime chips, these have so little effect as to be pointless.

For small and medium houses the smallest models extract enough water. Note that water extraction rate is often not specified under the same conditions as end use, and real extraction can be rate much less in practice.

A humidistat is important, otherwise the RH is uncontrolled. This can result in woodwork warping, wasted electricity, and no ability to respond to the changing vapour load of cooking, shower use & occupancy. Humidistatic units cost more than the cheapest dehumidifiers, but this feature is a safeguard to avoid wood damage.

A mouldproof water tank is a good idea. Older units may lack this feature, and some types of mould might pose a health problem.

The ability to connect a continuous drain hose occasionally gets used, so this facility is a minor plus point.

Units with water tanks under a gallon should be avoided, as they require unnecessarily frequent emptying. A gallon tank should fill in anything from 1 day in severe damp to several days when used to dry a shower room.



<> How should I use it?

If using it to reduce overall house RH, set it to reduce RH slowly and gradually. Wood dries out slowly, so quick air RH reduction would cause a humidity gradient between the inside and outside of all woodwork. This can cause woodwork warping & cracking.

If using it to dry out shower rooms, set it so it just fails to come on during dry conditions in the room. Any increase in RH should trigger it, and it will keep going until RH is back down to normal dry level.

If using it to dry the whole house, put it in the middle of the house to minimise the distance of any location from the unit, and thus maximise effectiveness and minimise RH variation.

If drying a severely damp house, the use of a large fan may increase air circulation and evaporation, and thus improve extraction rates.



<> What will it cost?

At time of writing, basic non humidistatic units go for around £100, and humidistatic units for around £150. Second hand units are much cheaper when available, but many are not humidistatic. Desiccant wheel units are more expensive, despite being simpler.

There is no fitting cost, just plug it in.

If the unit has been tipped over, let it sit upright for a day before plugging in. Omitting this can sometimes cause the compressor to fail.

Run cost depends on machine rating and amount of use. During summer the unit can be switched off and ventilation used instead, and during winter any electricity used will become heat for the house. A typical unit might use 200w, and when used to dry a shower room be used for an hour a day. This is 0.2kWh a day, or approx 2p. A 200w dehumidifier running continuously day and night in a severely damp house would use around 40p a day, and release that electrical energy as heat into the house. Heat is also given up by the water vapour as it condenses. Total heat production is very low, but the 200w is not wasted.

When using electric heating, the dh adds no run cost at all, as all its electricity becomes heat. When using gas heating, only part of the dh run cost is subtracted from gas consumption cost.

A dehumidifier is cheaper than an extractor fan because the fan will throw a fair amount of heated air outside, costing higher heating bills.

Dessicant wheel units have in the region of twice the run cost of comparable compressor units.



<> Are there any cheaper options?

There are 2. If a unit is only needed short term, and a low extraction rate is acceptable, a fridge or freezer can be plugged in with its door propped wide open, and the plastic drain hose diverted from the compressor tray into a water container. A desk fan is placed by the fridge to increase extraction rate. Extraction rate is poor, but a 90w fridge only costs a penny a day to run.

Desiccant wheel dehumidifiers are not difficult to make, and the parts are simple and cheap. High sale prices seem to be down to low sales volume more than anything else.

Turning the heating up and opening the windows is not normally a cheaper option. A small 15kW heating system can use 15x24= 360kWh per day. @10p/unit electricity this is £36 per day, or at 3p/unit gas with 80% efficiency this is £13.50 a day. Medium and large heating systems would cost more. When comparing for temporary use, dont forget resale value of the dehumidifier.
Last edited by NT on Tue 26th Jun, 2007 7:26 pm, edited 1 time in total.

NT
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Post by NT » Sat 21st Oct, 2006 9:18 am

Listed Buildings
----------------


If your property is listed, you should seek advice before carrying out works discussed in this thread. Some works on listed buidings, including quite minor ones, would constitute a criminal offence if carried out without LBC. Deciding which approach to take with a LB is also not as straightforward.

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