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What is Relative Humidity (RH)?

Relative humidity if is often talked about in flood restoration and dealing with wet building assessment and repair.

Air always contains some moisture in the form of water vapour, however most of the time this is not noticeable to our senses.  The amount of vapour that can be held in air depends on temperature, with a greater holding capacity in warmer air.

When air is holding the maximum amount of vapour possible at a given temperature it is considered saturated.  This may feel stuffy and noticeably humid to the senses.  Dry air, which contains a small amount of moisture, will feel crisp and refreshing when cold but when warm will dry the throat and can lead to a hard cough.

Relative Humidity is the degree of saturation, as it expresses as a percentage the amount of water vapour that air is holding relative to its saturated level.  Saturated air is therefore at 100% RH.  Air at 50% RH is holding only half of the vapour it could do at that temperature.

If that same sample of air is cooled, the RH will increase because the maximum weight of water vapour that air can hold is reduced in the cooler air. Conversely, if the air is warmed, RH reduces as the air at is capable of holding higher levels of moisture.

When measuring water vapour, it is expressed in terms of weight of water in a given weight of dry air.  Usually this is in grams of water per kilogram of dry air.

What is dampness in buildings?

When left untreated, damp can lead to deterioration of the building structure.  Materials decay and mould can develop, which is a risk to health.

Materials either absorb moisture from the atmosphere, or emit water into the atmosphere.  This is the process by which they find equilibrium with the relative humidity of air.  When humidity changes, the moisture content of materials will change, and the moisture content of different materials in a given relative humidity will not be the same.

Moisture content of materials is often expressed as a percentage scale called ‘wood moisture equivalent’ or WME.  This is used as the benchmark because between different types of wood, there is not as great a difference in moisture content as compared to other building materials.

The % moisture content of materials is an expression of the weight of water as a proportion of the dry weight of the material.  Therefore a heavy material such as brick or concrete has a lower % moisture content than a light material with the same amount of moisture contained within.

Under normal, dry conditions, different materials within the same structure will therefore have different moisture content. For example, equilibrium moisture levels in a brick wall with wood battens and plasterboard on one side, and plaster on the other, may show moisture content of around 10% in the wood, 8% in the plasterboard, 3% in the bricks and less in the plaster.  These are different levels, but the materials are in equilibrium with the humidity of the local environment.  Under damp conditions, all these materials may see an increase in their moisture levels, but they will remain very different levels and they will again find a new equilibrium with the relative humidity of the environment.

Dampness defined:

The definition of dampness in buildings could be considered to be when a material is wetter than ‘air-dry’, where ‘air-dry’ means in equilibrium with a ‘normal’ relative humidity in the atmosphere (30-70% RH).

Dampness is often most visible as a result of mould developing, which decays wood and spoils decorations.  These biological organisms have a dryness limit below which they can not multiply and live.  This limit is the line between dry and damp in buildings, and although not precise, can be considered to be between 75-85% relative humidity.  Above this level, mould, mites and fungi can develop quickly.

Damp is therefore an atmosphere more moist than 85% RH and a material is damp if in equilibrium with this humidity.

Dampness in materials is not always caused by moisture in the atmosphere.  For example, while air is dry, walls can still be wet.  This may be from penetrating or rising damp.  In this case, if the wall is damp (wetter than air-dry) the thin layer of air immediately next to its surface will be in equilibrium with the wall, regardless of the general humidity level of the room. Mould can therefore grow here, however if the wall dries, and room air remains dry, the mould will die.

How do we measure dampness in buildings?

Measuring humidity emanating from a wall is difficult in practice, so often electrical moisture meters can be used to measure the free water in a material.  This gives an indication of the relative dampness of different materials as they are measuring only the ‘free’ water. A high reading would indicate damp in whichever material is being assessed.  These meters give a reading of ‘dry’, ‘at risk’ and ‘damp’ expressed often as a percentage reading which corresponds with the humidity equilibrium of most non-metallic materials.

What type of dehumidifier do I need? Desiccant or refrigerant?

Dehumidifiers remove moisture from the air.  There are two main types of dehumidifier technology that do this:  Desiccant and refrigerator coil technology.

Refrigerant Dehumidifier:


These work by condensing moisture out of the air.  Damp air from within the building is drawn into the dehumidifier and passed over a cold evaporator coil which cools the air below its dewpoint temperature.  This results in condensation that can be collected from the cold coils.  This water is collected in a pan and either removed manually or some dehumidifiers have a hose through which the water is automatically purged.

The dry air in the machine is then passed over warm condensing coils, heating the again before exiting the machine.  This dry air is in many machines pushed out at pressure so can be directed at specific damp affected areas to accelerate drying.

Desiccant Dehumidifier:

CTR300XT photo

These operate by way of passing air through a rotor which contains moisture adsorbent desiccant material.  Once water is removed from the air, dry air can be blown back into the building, thereby encouraging drying.  Water collected within the desiccant wheel is removed by adding heat so the vaporised moisture can then be ducted out of the building.

The desiccant material is typically silica gel, which is not of the usual ‘gel’ texture but is a porous form of granular silica, the internal structure of which comprises a network of microscopic interconnected pores.  These can adsorb moisture by attracting it within each granule.  It is by the addition of heat that this moisture can then be released from the desiccant by evaporation and pushed out of the building, leaving the desiccant wheel ready again to collect further moisture.

Comparing Performance:

1.  Extraction Rates:

The main benefit of refrigerator coil dehumidifiers over desiccant is that they can remove a larger volume of moisture per day.  They are therefore particularly useful at the early stage of flood restoration when saturated materials require drying rapidly.

However, extraction rates do depend on the make and model of dehumidifier and we suggest researching the manufacturers’ guidelines of extraction rates of the equipment you are considering.

2.  Operating Temperature:

Importantly, the two technologies perform differently in the lower temperature range.  Desiccant dehumidifiers are capable of operating in cold temperature conditions, as the silica gel desiccant still adsorbs moisture regardless of temperature.

Conversely, refrigerant dehumidifiers suffer a fall in water extraction at colder temperatures, as the dew point is harder to reach and condensation of moisture out of the cold air is more difficult to achieve.

2.  Running Costs:

Generally refrigerator coil dehumidifiers consume less electricity so they are cheaper to run.  If you provide us with the cost of your electricity in pence per kilowatt hour (KWH) then we can indicate the cost of running equipment for the duration of the drying program.

3.  Noise:

Refrigerant technology uses compressors which generally make these dehumidifiers slightly louder than desiccant.

4.  Size / weight / manoeuvrability:

Refrigerated coil dehumidifiers usually are heavier as they contain weightier components, and they are often larger in size.  However, the ones we use come with wheels so although larger then the desiccants are in some ways easier to manoeuvre around.  We can give guidance on the use of each machine.

Tips on Dealing With Flooding – What Do You Expect From Your Home Insurer?

Hi there. Long time, no blog… we’ve been busy drying buildings!

A recent Association of British Insurers guidance note detailed how to respond to major floods, in particular what to expect from your home insurer in the days, weeks and months afterwards. An extracted summary of its main points is as follows:

If your home has been flooded:

If you have to move out of home due to flood damage and have building or contents insurance, your insurer will offer to provide or pay for the cost of appropriate alternative accommodation and other related additional expenses, such as removal and storage of undamaged property. If you have buildings cover, your insurer will dry, clean, repair and restore your home.

Contents cover usually allows for the repair or replacement of contents.

Step 1: Immediate aftermath

Contact your insurer as soon as possible.

Your insurer will assess the extent of damage to your home and possessions and will advise whether you need alternative accommodation.

A Loss Adjustor may be appointed to assess the damage in more detail and oversee the claim and restoration, depending on the scale of the damage.

Step 2: Assessing damage and finding temporary accommodation

Depending on the extent of the damage, your Loss Adjustor will either visit you or liaise over the telephone. If the home is severely damaged, the Loss Adjustor may appoint a surveyor to provide additional expertise.

If you need to move out of home, your insurance will usually cover the whole of your stay in alternative accommodation.

Most building and contents policies include up to 20% of the total insured value to cover alternative accommodation needs and associated costs. If you rent your home, your Landlord or Building Manager will know about the relevant insurance.

Step 3: Cleaning and stripping out

The Loss Adjustor will typically appoint a disaster restoration company to undertake this work.

Firstly the silt and debis left by flood is removed, together with checking gas and electricity supplies and appliances.

Stripping out will occur where necessary – likely to include removing damaged and wet furnishings and fittings and hacking off damaged plaster and woodwork, in order to facilitate proper drying out.

Separately, cleaning any valuable or personal items affected will need to be discussed with the Loss Adjustor.

Step 4: Disinfecting and drying your home

Your insurer/Loss Adjustor will most likely appoint a specialist drying company to disinfect and dry your home until it is certified dry enough for repair work to start.

Drying homes can take some time as floodwater can penetrate deep into the fabric of properties even if the flooding seems minimal and shallow. This deep-seated moisture can take a long time to move back through walls and floors, even if the surface appears dry. Full dryness is required so that repair work will be sustainable and damage doesn’t reappear in future.

Drying machines need to be left on for the duration of the period, as any downtime will extend the drying out period. Additional electricity costs incurred should be covered by your insurance.

Time required for drying your property can range from weeks to many months.

Step 5: Repair and reconstruction work

A building contractor is likely to be appointed to undertake the repair and reconstruction work. Your insurer is responsible for the work of builders they appoint. If you prefer, you may be able to use your own builder.

Your insurer can also discuss with you whether you want the repair works to include those which improve resistance and resilience to future flooding. If this doesn’t cost more than the cost of repairing your home to its pre-flood condition then your insurer will not charge for these changes.

Most contents policies will pay for the full cost of replacing damaged items with the equivalent new ones. If the value of your contents is more than the sum insured the settlement of your claim may be reduced to reflect this.

Step 6: Moving back into your home

You can discuss with your insurer the possibility of moving back into your home before work is completed. Your claim is settled once all the work has been completed and the outstanding payments have been made.

What if things go wrong?

If you feel unhappy about any aspect then contact your insurer. Beyond this, a formal complaint to your insurer and then to the Financial Ombudsnam Service are the remedies.

The above is extracted from an ABI document. If you require any further information on dealing with water damage issues or require dehumidifier hire then please contact us on 0800 078 6999.

Salt Contamination In Wall Surfaces Caused By Rising Damp

Soil or subsoil is the source of rising damp as these are always wet. Soil water is not pure water as it contains the soluble materials from its composite parts; from decaying plant materials and bacteria, moulds and soil-living animals.

Of the soluble materials in water, nitrogen containing salts (which eventually become nitrates) are the most prolific, and chlorides are also generally universal. Rising damp is therefore a rising, dilute solution of various materials including nitrates and chlorides. When water evaporates these are left behind because they can’t evaporate.

Although rising damp is a slow process and the solution is dilute, if it continues for a number of years the result can be high concentrations of certain nitrates and chlorides at the surfaces from which evaporation occurs.

This is the basis for diagnosis of rising damp by analysis of wallpaper or surface scrapings of plaster, for the presence of nitrates and chlorides. Typically the highest concentration will be in wallpaper, a the highest point to which the dampness has been rising, however this is not always the case as the paper may not have been in position for many years. Typically, though, wallpaper will contain the greatest weight of salts, followed far behind by the finishing coat plaster and then the bonding plaster. Often the salt content deep in brick walls is relatively low.

Soil salts are hygroscopic, meaning they absorb water from the atmosphere (unless it is very dry) and form a solution. This means they may keep wall surfaces damp even if the rising damp which deposited them may have been cured. The evidence of dampness from hygroscopic salts is that occupants of the building will vary with the weather, becoming more pronounced when the atmosphere is humid.

These salts are electrically conductive and will give high readings on an electrical type of moisture meter. Therefore if a high meter reading is given, this is a condition that should be investigated even if the wall appears dry. It should be noted that their presence is a sign of continuing trouble if the source is not stopped. Therefore removal of and replacement of contaminated plaster is usually required.

Non-hygroscopic salts can sometimes be contained within the inner surfaces of walls particularly in very old buildings, but this concentration of salts is not due to rising damp. These can cause high readings on a conductance type meter, and are considered to be due to occasional rain penetration under exceptional circumstances over the years, which has leached these salts from the fabric of the building.

If you require any further information on the above, require help with a damp problem in your property or think you may need to hire a dehumidifier to dry your building, then please contact us on 0800 078 6999.

Tips For Preventing Burst Pipes

Cold spells and empty properties are the most common conditions for pipes to burst. If you are going away from your home during a cold spell then bear in mind the following tips, which are only some of the things that can be done:

Insulate pipes with lagging in the crawl spaces and attic where the chance of freezing is at its highest. Typically, exposed pipes in these areas lead to freezing pipes.

Central heating should be kept at a minimum of 12 to 15 Celsius if it is expected to freeze.

Allow warm air to flow into the loft space by leaving the loft hatch open so warmer air can prevent the cold tank from freezing.

Cabinets and cupboards that contain pipes should have their doors left open; this will allow warmer air to circulate and reach pipes under sinks and also in outside walls adjacent.

Water supply to outside taps should be turned off. Also mains water at the stopcock should be turned off unless a combination boiler requires constant mains pressure.

Consider draining the cold water system if you’re going away for long periods over the winter and want to save on heating bills.

There’s loads of information out there on the web about preventing burst pipes. Perhaps one to look at is British Insurance Brokers’ Association and Ageas have a free factsheet on protecting properties against burst pipes. This can be downloaded at

If you need any further information on how to help prevent burst pipes in your property or if you have suffered from a burst pipe that needs repair then please don’t hesitate to contact one of our technicians on 0800 078 6999. We offer expert advice on how to dry your building and repair water damage following a burst of water.

Top 30 Causes of Residential Water Damage

The National Flood School have identified the 30 most common causes of residential water damage. These are listed in no particular order as follows:

1. Pipes in the loft – broken or frozen;
2. Blocked guttering;
3. Storage tanks – leaky or over-flowing;
4. Badly fitted bathroom tiles;
5. Broken or poor fitting shower screens and curtains;
6. Shower trays – broken or badly fitted;
7. Baths – poorly plumbed in or unattended;
8. Poorly maintained sealant around bath and shower;
9. Poorly maintained or open chimneys;
10. Flashing around chimney – damaged or poorly installed;
11. Roof tiles – missing or broken;
12. Poorly installed or broken windows;
13. Defrosted fridges and ice machines;
14. Damp proof course – failed or missing;
15. Insufficient or low lying air bricks;
16. Damp proof course – bridged;
17. Old corroded pipe work;
18. DIY ie nailing into pipes;
19. Leaking radiators;
20. Corroded ball valves in toilet cicterns;
21. Blocked sewers and sewage back flow;
22. Unattended or blocked sinks;
23. Appliances – faulty or corroded;
24. Storm drain overflow;
25. Drainage – poor or insufficient (around foundations);
26. External flooding;
27. Insufficient threshold on external doors;
28. Poorly maintained boilers;
29. Hot water cylinders – poorly maintained;
30. Broken aquariums.

So to prevent costly and inconvenient water damage to your home make sure you maintain your property and keep an eye on the above!

Self Help For Victims Of Flooding – What Can YOU Do?

Welcome to this blog of advice for those seeking help with fire or flood damage restoration needs and advice. This post is taken from an information document by the British Disaster Management Association. For further information from them have a look at

Where flooding occurs on a wide scale the emergency services, insurers, local authorities and other support agencies can be overwhelmed. In these circumstances advice and assistance may not be immediately available.

While you will want to improve your situation as quickly as possible, you may feel unsure about what you should or shouldn’t do and will want to understand the steps you can take without jeopardising your insurance claim.

This information will also be useful for those who are not insured.


Stick to Basics:
Remember much of the work needed to recover your property after flooding will require specialist knowledge and will need to be undertaken by professional technicians.

Be Aware of Health Risks:
Flood water is often contaminated and, even if it appears ‘clean’, may include elements that can cause a range of illness. Personal hygiene, including washing hands and covering cuts or scratches, is very important. Always wash hands before preparing food, eating, drinking or smoking.

Within a few days mould can form, due to the damp environment and this can affect the throat, nasal passages and lungs.

Young children, the elderly and those with immune system deficiency should be kept away from properties that have been flooded until they have been passed safe for habitation. Medical advice should be sought immediately if any health concerns are identified.

Be Clear About Your Objectives:

Your main objectives should be to:
1. Ensure safety first;
2. Prevent further damage by reducing the effect of any remaining water or residue;
3. Safeguard possessions that have not been damaged, and;
4. Establish whether unaffected parts of the property are useable.

Ask for Help and Advice:

Don’t feel you have to deal with this on your own. It will be a new experience fro most people and you should take advantage of support available from the many agencies, authorities and organisations who can provide back up information and services.

Talk to neighbours and others in the same situation. You will often pick up useful tips from sharing experiences. Engage a professional to deal with any gas, electrical, plumbing or structural work.

Gather Tools and Equipment:

To carry out essential work to keep further damage to a minimum you are likely to need brooms, scrubbing brushes, mops, buckets, detergent, disinfectant, rubber gloves, Wellingtons, protective clothing, tools such as hammers, nails and screwdrivers, strong refuse bags and shovels. You may also need appropriate face masks if carrying out work on your property.

Assess Status of Mains Electricity and Gas Supplies:

This can depend on the height and location of the flood water. Get professional advice if equipment or sockets have been affected by water. Do not attempt to switch on any device that has been affected. It may be necessary to arrange for an electrician to install a temporary supply board.

Take Pictures of Your Property and Any Damaged Possessions:

If you do not have a camera, or your camera has been damaged in the flooding, you may be able to buy cheap disposable cameras at supermarkets, chemists, etc.

Keep a Record of the Damage and Any Action Taken:

Write down a description of the overall state of the property. When the water has subsided mark the high water point, including the date, on each wall. Make a list of damaged items and their condition. Note down actions you have taken in detail.

Remove Standing Water and Mud Where Possible:

Consider, if mud or debris is piled up against an internal or external wall this could be affecting the structure. Only attempt to move this type of material if the amounts are minimal. Once water and mud have been removed floors can be rinsed down.

Remove Saturated Carpets, Rugs and Furnishings:

To reduce health risks these should be taken outside the property and, if possible, disposed of. Carpets can be cut up to make removal easier. Take pictures and keep a small sample of carpets and other materials as evidence of the damage.

Protect Furniture and Possessions from Further Damage:

If you have access to a freezer – important documents, photographs and books that are water damaged should be wrapped in polythene or plastic bags and frozen for restoration at a later date. Undamaged furniture and possessions should be moved to a higher level where possible. Furniture that cannot be moved from water affected rooms should be raised off the floor on blocks. Plastic bags should be placed under the legs of wooden furniture to avoid further water being absorbed.

Dry With Care:

Open windows and doors. Do NOT attempt to dry out property with the use of central heating or other heating appliances. Be aware that a combination of heat and damp can cause further damage to the property and can encourage mould growth.


Provided you are able-bodied, keeping busy may help you to cope with the situation but you need to adopt a reasonable approach.

You should only carry out work you can manage comfortably and safely. Don’t work for too long without a break. Recognise that the stress of your circumstances can make you more vulnerable to health problems. If in any doubt, wait for advice from an insurer, loss adjuster or professional contractor.

Note: You may find further helpful information on the BDMA website, including links to flood warnings, weather and travel information.

If you require any advice on how to deal with flood water damage, how to clean up your property, dry out your building and restore it to its original condition then please contact our friendly staff on 0800 078 6999.