Damp FAQ

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

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

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

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

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

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.


NT

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

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

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

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

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

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

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

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.

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.

First actions for damp properties
---------------------------------


If your property is damp, there are several simple actions that can be taken first which may solve or improve the situation.




A good place to start in case of damp problems is to check your underfloor vents. Timber ground floors are normally suspended, and the exterior walls have airvent bricks to allow air exchange between the underfloor void and the outside world. This air exchange carries away water vapour. If the airbricks are blocked or buried, damp trouble is likely. Clear the holes in the bricks with a stick to ensure free airflow. If the vents are buried they must be dug clear and kept unobstructed. Sunken vents will require regular clearance of debris to maintain free airflow.




Rainwater goods.

Does a gutter overflow in heavy rain? If so, clean it out and check again. (Looking up during heavy rain is always fun.)

Does a downpipe discharge water onto a wall? If so, the pipe may be corroded through at the back, or partly blocked. Sometimes this corrosion can be difficult to see.

Are there open hoppers fitted? If so clear them out, as these are prone to accumulating materials causing a blockage or flow restriction.

Are your ground level drains taking away all water, or is standing water visible during heavy rain? If not draining freely, drains might need rodding or repair. There are also drain clearing chemicals such as caustic soda and sulphuric acid, but choose these with caution, as they can damage to some drain systems, and can be violent when mixed.

Also check for pipes that have moved slightly out of place, causing a leak where downpipe meets gutter, or they may discharge water that misses the hopper.



Ground levels:

Outdoor ground level should be at least 6" below interior floor level. Ground level too high is a common cause of penetrating damp at the base of walls. If the ground is too high, remove earth next to the wall to reestablish a healthy level.



Sun & wind:

Direct sunlight & free air movement both help to evaporate water. If your damp wall is shielded from the sun or free air access is obstructed, the obstructions may be removed if practical. Common problems here include plants, wood piles, binbags and junk.



Ground drainage:

Check the ground immediately next to the building drains properly in rain. If there is standing water, the remedy will depend on the details of the situation.



Heat & ventilation:

Buildings that have remained unoccupied and unheated are prone to becoming damp. Normal heating & ventilation may be all that's needed to resolve such situations.




Once all relevant first line actions have been carried out, the building will need time to dry and settle to its new moisture equilibrium level. Drying may take months.

Often first line actions are enough to resolve a damp problem, but in some cases they aren't. In these cases, further damp resolving measures which require significantly more work than the first line actions may be needed.

Second line actions for damp properties
---------------------------------------


If first line actions have not resolved your damp problem, and you've given the building enough time after such works to dry, then its time to move on to second line works. These are further damp resolving measures which require significantly more work than the first line actions.

Taking good advice before embarking on such measures is only sensible. This post is a description of measures used rather than any specific recommendation for your property.



Roof:

Look up at the roof, ideally wih binoculars, and look for any slates or tiles missing or out of place. This does not mean look for holes, but rather look for a change in the normal pattern which would indicate a place only 1 tile/slate thick instead of 2. Any area 1 slate thick will let in water. A tile or slate that has slipped down lower than its normal position is also liable to cause water ingress, and fall out entirely.

Inspect the flashing or fillets.
Flashing is (usually) lead sheet fitted to direct water from walls and chimneys onto the roof, where it will run down the roof and drain away. Leadwork can be detached, broken or even missing.
Fillets are strips of mortar applied on top of the roof where it meets a wall or chimney, and do the same job as flashing. These are more prone to trouble because they are inflexible and the mortar has a limited life. Look for fillets that are broken or have pulled away slightly from the brickwork. Rain only requires a tiny gap to enter the building, so fillet trouble spotting is best done either close up or using binoculars.

Some roofs have valleys that also need inspecting, and clearing, repair or relining if necessary.



Rain water goods:

Check for missing gutters. All roof sections should be fully guttered. Absent sections mean water from the roof is discharged onto the ground at the foot of the building, soaking the ground and splashing walls. When necessary use matching guttering. Cast iron gutter is still available new. Wood gutters must be made from the right wood types to be durable.

Check where rain water discharges. If not fed to a mains drain it should be dumped at least 5m from the house, preferably further away.



Pointing:

Check condition of external pointing (the mortar joints between the bricks). If it is falling out in places, this is liable to permit penetrating damp to enter the wall. Before repointing, realise that renewing all the pointing with cement mortar can result in damage to the wall and exacerbate damp problems. Nearly all period properties should be repointed using lime mortar rather than cement.

Traditional Lime Mortar:
3 volumes sand, perferably sharp sand
1 volume lime putty

Biff's Instant Lime Mortar:
3 volumes sand, perferably sharp sand
1 volume bagged hydrated lime (dry)
add water to make a stiff mortar.

Bagged (hydrated) lime is available from builders merchants and many but not all diy sheds.
Lime putty can be purcahsed ready made, or made from dry bagged lime. Note that lime putty should be made before it is needed, as it is best stored before use.

Some houses may have mortar wih other ingredients added. Where these were used, replacement mortar should be made up to give the same apprearance. Mortar can be analysed.

Cement should not be added to lime mortar, as it is liable to cause premature failure, and reduces the mortar's ability to evaporate water.

Raking out whole walls and replacing all mortar is not generally recommended. Mortar need only be replaced where missing, broken, disintegrating, turning to sand, or at risk of falling out.

Exterior lime mortar should not be applied when there is a risk of frost or heavy rain, unless you're willing to cover the work temporarily with sacking etc.



Exterior cement render:

This is a common contributor to damp problems. Cracks allow rain water in, but the cement gives very little opportunity for evaporation, so wall water content gradually increases.

The only real solution to cement render is to remove it. However removal can sometimes be a problem, as soft some bricks and stones are liable to be broken when firmly adhering cement is removed. Use only the minimum amount of force needed to remove the cement, and if damage occurs, stop. Do not damage your brick or stone, as damaged bricks sometimes steadily deteriorate until they require replacement.

If cement can be removed without damage, this will make a real difference to the wall's damp handling abilities. The wall can either be left unrendered, or if an eyesore it can be lime rendered. Lime render allows evaporation to continue.

If the cement can't be removed without damage, it is an option to remove only areas where the cement is weaker or not properly attached. Find these areas by tapping the cement: a hollow sound indicates where the cement is not stuck to the underlying wall. Removed patches may be patched with lime render, giving patchwork evaporation areas. It is fair to say though that it may be difficult to hide such patchwork completely, as lime and cement renders are not identical in nature. Also there may be a risk of slight cracking at the boundaries due to differing thermal expansion coefficients.

Warning: Cement rendered earth walls can (rarely) reach the point where the earth inside has become wet enough to lose structural strength, resulting in the building being held up by the cement render. If this were to occur, removal of cement render could then cause a collapse. Since cement render on earth causes this risk, it is important that these houses have their render removed, but it must be done with care to avoid the risk described. Cement may be removed in areas, allowing enough time for the wall to dry before progressing to the next area. The earth wall material may be tested for strength to detect when risk is present, and when the building is sound enough to support itself without its render. If you have cement rendered earth you will need to take advice specific to your building.



Showers & Cooking:

These activities can produce large quantities of water vapour. Reducing vapour production from these sources can make a real difference.

Showers have 5 main solution options.

Closed shower cubicles reduce vapour release into the room.

Window ventilation exhausts vapour during and after a shower. This may be done by opening the window by hand, or fitting a locking mechanism that allows it to be locked a 1/4" - 1/2" open. This can often be done at minimal cost.

Extractor fans remove some damp air during showering. Adding an overrun timer causes them to continue extracting for some time after room use. The principal downside of fan use is wasted heat. There is also noise.

Dehumidifiers extract water vapour from the air. Although unit purchase cost is higher, fitting cost is zero and no heat is thrown out. This results in lower total cost. A humidistatic unit should be chosen, as non humidistatic units fail to respond to shower use and can overdry the air during drier times.


Cooking has 2 options for reducing vapour output:

1. Fit an exterior venting cooker hood. Interior venting recirculating hoods do not remove water vapour.

2. Use lids on pans when cooking, turning the heat down low. This makes heat setting more critical, glass lids or pans make setting the heat correctly much easier. It also saves money & energy, and reduces risks of burning food and fire.



Impermeable coatings & paints.

Damp proof paints, bitumen and proprietary wall coatings all stop evaporation, and thus raise water content in exterior walls. Purchasers of these products see their ability to stop rain penetration, but fail to realise the effect on the other side of the equation, evaporation, or fail to realise there even is an equilibrium situation in effect.

Impermeable paints such as masonry paint have a similar effect. Buyers should be aware that paints sold as breathable, microporous or similar terms are often mostly-impermeable paints, ie their level of permeability is often much too low to make them suitable for most period properties.

Removing these coatings enables walls to evaporate dry. How the coating is removed depends on the coating and wall type. Sand blasting is rarely appropriate.



Breathable interior paints:

Gypsum plaster is breathable, but most modern paints are low breathability. Interior emulsion paints reduce evaporation. Many lime based paints give the inside of a wall more opportunity to dry, but it is necessary to remove any existing impermeable paints first to gain this advantage.

If replastering, lime plaster has some advantages over gypsum, and is generally recommended for most period properties.



If these actions dont sort your damp problem out, either you've got an unusual situation, or a cause that has been missed or not correctly resolved.

Wall types
------------



Solid vs cavity walls
---------------------


Solid and cavity walls are both found in period properties, despite the often heard assumption that old houses used only solid walls. The 2 types of wall handle damp to some extent differently.

Cavity walls have 2 inner surfaces from which damp can evaporate, and are thus relatively tolerant of cement render, impermeable paints and wallcoatings, and other contributory factors to damp walls.

Solid walls however have no such hidden evaporation surface, their visible sides are the only places where damp can evaporate. Consequently it is more important with these walls to ensure that damp can evaporate, and inappropriate wall finishes or treatments can more easily cause damp troubles.

When assessing wall construction, it should be born in mind that many Victorian houses used several types of wall construction in one building.


4" walls
--------

The majority of house walls are 9" brick, but various other types also exist. One of the more troublesome from a dampness point of view is 4" brick walls. 4" walls offer poor insulation, and are thus prone to winter condensation and ensuing damp trouble. Where 4" walls suffer from damp, insulation may help stop the problem, as well as provide a welcome reduction in heating costs.

NT Wrote......

The last and least desirable option is to have shorter showers or use the bath instead.


You couldn't make this up could you!

We're all still waiting for an answer to my previous posting NT!

TDM

Actually, we are all getting a tad impatient with you as

a) you have been asked to post anything which is a discussion as part of the other thread;

http://periodpropertyshop.co.uk/phpBB2/viewtopic.php?t=6779



b) you seem to have ignored Biff's question.

This is a serious attempt to get some help to people, not an argument.

NT - I am sure this entire thing can be tidied up and reposted eventually, with extra stuff removed.