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Condensation Humidity and Mould. A little bit of what you already know, what you don’t know and what you knew but have forgotten.

October 9th, 2019

All posts, Condensation, Damp / Rising Damp / Damp Proofing /, Energy Efficiency / Insulation Improvements, Mould Remediation, Surveying, Thermography / Thermal Imaging

As the winter season approaches here’s an article on condensation, humidity and mould to keep you up to date and refresh your memory.  A little bit of what you already know, what you don’t know and what you knew but have forgotten.

It is often stated that 80% of all damp complaints are the result of condensation although I’m not exactly sure where this statistic came from. As a surveyor with over twenty years’ experience specialising in damp and moisture related problems, condensation surveys certainly don’t account for 80% of my work however, it is clearly a problem that is trending and is on the increase. In my experience this damp related issue is often miss understood, therefore miss diagnosed which inevitably leads to failed repairs and reoccurring problems.

I suspect if you felt interested enough to click on this article the likelihood is you already know a little bit about condensation dampness and how the process occurs however, for those who may not be as knowledgeable, here’s a brief explanation. 


Boiling water produces water vapour – water vapour is basically water in its gas phase

Water basically has three phases, a solid phase as ice, a liquid phase as water and a gas phase as water vapour. Believe it or not the air around you is not completely dry. All air will contain an amount of water, basically molecules of water floating around in the air in its gas phase. This is water vapour and is perfectly normal. The amount of water within the air is referred to as humidity.

Air though only has an ability to hold so much water before it becomes fully saturated and this ability is directly associated to its temperature. Warm air has a much greater holding capacity than cold air and as such is capable carrying more water vapour.

We refer to this relationship as Relative Humidity, the airs ability to hold water vapour relative to its temperature. This is usually expressed as a percentage (%RH). Therefore, when we say the humidity level is 60% RH (Relative Humidity) we’re basically saying the air is 60% full of water vapour in comparison to it total holding capacity at that temperature. That may sound like quite a lot however, at normal household temperatures it isn’t really. What we consider dry air in our homes often varies between 50 – 60% Relative Humidity. 


When the air reaches its maximum holding capacity, we refer to the air as saturated. This is the limit where the air is unable to hold any more water vapour and as such, excess water vapour is dumped from the air as liquid water. This process is referred to as condensation. There are two main causes for the process of condensation to occur.

Firstly, a reduction in the air temperature and therefore its holding capacity or secondly an increase in humidity, basically more water vapour being added to the atmosphere.

There is a calculated temperature of when air will reach saturation and this temperature is referred to as the Dew Point temperature. This is the temperature at which condensation will form in the current environment. The dew point temperature will change from environment – environment dependent upon the conditions although a basic hygrometer or atmospheric monitoring device will provide you with the calculated dew point temperature.


Relative humidity is only a measurement of the occupied volume of water in the air at a given temperature and does not relate specifically to the mass of water present. The actual mass of water vapour is referred to as the airs specific humidity and is a direct measurement of water molecules in the air. This is not related to temperature and is usually measured in grams of water per kilogram of air or grains per pound. 


In addition, molecules of water suspended within the air also exert a force and increase pressure within the atmosphere. Therefore, warm humid air with lots of suspended water molecules will exert a greater pressure than that of cooler, drier air. As such, the pressure of the atmosphere as a result of its water content will cause the air to move. High pressure forces will always overpower low pressure forces therefore, hot humid air will always move towards regions of lower pressure or regions of cooler drier air. This is important to remember. 


Specific humidity vs relative humidity

To put the above into a practical sense, let’s consider the following. You arrive at a petrol station to fill up your car. Your cars fuel gauge reads 1/2 full, this means your tanks basically at 50% of its holding capacity. This would be 50% Relative humidity. You wouldn’t however know how many litres of fuel were in your tank unless you knew the size of your tank. If you had a 2000 Litre tank, 50% would be 1000 Litres. 1000 Litres would be your Specific humidity.


Now let’s put pressure, evaporation and condensation into practice. Consider a basic weather cycle. Water on land or in the sea is heated by the sun, when the suns energy is powerful this is enough to cause the water to evaporate as a gas (water vapour) into the air. This warm heated air, now laden with moisture rises into the atmosphere where the air pressure is lower. As the air rises it cools and eventually is unable to hold the water vapour it once did before. Clouds form, precipitation (condensation) occurs and the water falls back to land or sea. 

The weather cycle

So now we understand a little bit more about water vapour, humidity and vapour pressure why is this knowledge important within buildings. 

Well, basically similar cycles occur within our homes. As occupants of a building we produce water vapour directly through breathing but also through our activities, such as cooking, washing and drying clothes. This water vapour is suspended in the air within our homes and is managed through means of temperature control and ventilation. Problems however, will start to occur if there’s is an in balance between the two. 

Let’s first look at the role temperature and air movement plays on the living environment. 


Infra-red thermometer – to record surface temperatures

The air within the building has to be sufficiently warm and capable of holding the water vapour we produce as occupants. The air temperature must also be warm enough to ensure the fabric of the building, walls, floors and ceilings are also sufficiently warmed, keeping their surfaces above the dew point temperature of the atmosphere. Generally, we expect the air temperature within an occupied property throughout winter to be 21 degrees C and above. 21 Degrees is not only a comfortable temperature for the occupants but provides conditions capable of holding up to 15.7 grams of water, per kilogram of air, at idyllic humidity levels between 50 and 60% RH.

Provided there are no serious obstructions to the walls surfaces within your property you should find the surrounding wall and ceilings surfaces are approximately 2-3 degrees lower than the air temperature. Under normal household conditions this should mean the surfaces are well above dew point. It’s isn’t however unusual to find these conditions exaggerated particularly on older properties with solid external walls and ventilated suspended timber floors where the temperature variance across a wall can vary in excess of 5 degrees from floor – ceiling.

The temperature of most properties is-controlled by a heating system and regulated often through a thermostat or timer, which control the radiators or wall heaters. The location of these heating devices in rooms can be crucial to reducing risk of having condensation and mould related problems. Radiators / heaters should be sufficiently positioned to achieve maximum radiation of heat within a room, therefore not behind couches or curtains. In most older properties you’ll often find radiators are positioned on the coldest external walls and beneath windows however, on modern properties it’s much more common to find heat sources on internal dividing walls often back to back in adjacent rooms. This is a poor consideration to the heating of a building and was implemented basically as a cost saving measure on labour and material during installation rather than performance. A prime example of design considering expense before performance. 


The orientation of a building can increase it’s risk of moisture and mould related problems on North Facing elevations.

The most problematic elevations are likely to be the North facing elevation of any building and this is due to its orientation and absence of solar gain. As such, the walls and fabric of the building do not benefit from the heat of the sun making the North a much colder elevation. It’s is however, important to mention that any elevation which is shaded can be at similar risk, opposing gable ends on terraced properties and large tree lined boundaries can also be problematic for the same reason.

Similarly, internally the position of furniture and belongings, particularly built in wardrobes and cupboards which restrict the ability of warm heated air to pass over a walls surface have a similar effect. With the exception of possibly being able to reposition furnishings there may be little that can be changed regarding orientation and shaded elevations. In this circumstance the placement of heat sources internally such as radiators play a more crucial role is reducing the risk.

Options are available to help raise the surface temperature of vulnerable surfaces such as; internal, external and cavity wall insulation however, diligence needs to be undertaken with any of these improvements as they can sometimes create further problems.


Sustained high humidity and poor air movement can lead to extensive problems with mould and condensation.

If we refer to vapour pressures above, you’ll remember that moisture laden air excerpts a pressure on the atmosphere and that high pressure will always shift towards regions of low pressure. As such, you can expect the water vapour you produce internally will naturally shift towards regions of lower pressure. In this county 90% of the time the air vapour pressure externally will be lower than the air vapour pressure internally. This means that the air within your building is always trying to escape. That’s why most condensate / mould related problems are most likely seen on external walls and cold ceilings. This is basically warm, wet air trying to escape. 

To a lesser degree these heated air presure changes also exist internally. Air within the centre of the room is often much warmer with lower humidity levels than the air adjacent your walls, ceilings or window surfaces. This is simply the result of the surrounding surfaces being slightly lower in temperature than the air. As such when the air which is central to the room moves towards these cooler surfaces the air then cools and its humidity level increases. This can result in a layer of cooler more humid air usually within an inch of the surface. We refer to this as the boundary layer of air. Usually this air will be prevented from becoming stagnant adjacent walls and ceiling surfaces by convectional air currents although obstructions from furniture may impede. That’s why where possible you should ensure furniture is stood back from walls by a few inches to help the air behind circulate.

Thermal image captures cold region within the corners of a room.

Air also tends to move in a circular motion and as such the corners of rooms are the most vulnerable where the air tends to sweep around the internal corners of external walls leaving the boundary layer air undisturbed. It doesn’t help that most homes often have televisions, wardrobes, couches and other items of furniture close to the walls. In this circumstance you’ll often notice a sweeping tide mark of mould or surface moisture deposits either from the floor upwards or ceiling level downwards, this is simply a cold region of lower pressure where air movement is limited. 


Insulation is a crucial component of a building ensuring surfaces within the building remain warm via the radiated heat of the heating system. Areas of absent or disturbed insulation within walls, floors and ceilings can create vulnerable cold spots across an otherwise uniformed temperature surface.

Thermal image captures the absence of insulation with eve skielings

Sloping skielings which protrude into rooms are common in post war properties and can be extremely problematic due to absent insulation, an issue which is often highlighted when the accessible sections of a roof void are insulated from above.

This scenario creates a significant thermal variance across the ceiling between the warm insulated section and the cold uninsulated section which inevitably becomes problematic with mould. Similarly, flat, roofs, protruding bay windows and poorly designed garage conversions can also be problematic for similar reasons.

A lot of the time in this scenario the insulation is blamed as the cause however, in my experience this isn’t always true. The insulation may well reduce the permeability of the of the structure encouraging warm moisture laden air to remain within the property. A similar scenario exists with retro cavity wall insulation. I often find however that these retrofit insulation measures are poorly considered and only highlight inadequacies in ventilation that existed within the property prior.


Mould affected furnishings adjacent a cold wall

Contrary to the belief moulds are not plants, moulds actually belong to the kingdom of living organisms Fungi, moulds are simply micro-fungi.

Moulds and fungi, also unlike plants do not contain chlorophyll, therefore do not derive their energy from sunlight, hence the reason why most moulds prefer dark, damp conditions. It is common nonetheless, to see moulds/fungi living symbiotically with plants, as each contribute to this mutual relationship. Moulds and fungi perform the laborious task of breaking down complex organic substances into simple food compounds for plants in exchange for sugars and energy derived by the plant through photosynthesis.

It’s important to mention that moulds are the result of high humidity and not necessarily condensation, therefore it’s perfectly possible to have mould problems without physical condensation. Both are nevertheless an issue. Most Moulds don’t require the presence of physical water to germinate only humidity levels sustained above 80% Relative humidity for a period of 48 – 72 hours. We often refer to dry air as having 60% or less of its holding capacity, however this isn’t as severe as it sounds. To live comfortably we require the presence of moisture in the air to make the air comfortable and breathable, totally dry air would be intolerable.

Tipping the balance over 60% relative humidity towards 80% relative humidity isn’t however that difficult when we consider the number of wet or moisture producing activities we undertake within our homes. Irrespective of this we really shouldn’t be creating conditions which are conducive to mould growth in our homes and the levels of ventilation and thermal insulation should be sufficient it keep our homes warm and dry.

Moulds require three basic fundamentals for their survival food, oxygen and water, and the first two are present in all atmospheres therefore fundamental for their growth is always the availability of water. Moulds also favour cellulose materials such as paper, fabrics, leather and wood therefore ideally its best to avoid these in high moisture producing rooms. Wall papers is bathrooms is a definitive no, no!

If moulds do germinate many people treat them with a fungicide although in my opinion this is often a pointless exercise. When a surface is cleaned and treated for the eradication of mould, it is only clean for that moment in time. After the cleaning process the surface will inevitably only become dirty again. Mould spores within the atmosphere will land once again upon the surface regardless of its treatment of not and if conditions remain the same, over time these will germinate. As such, it is critical that if certain parts of the structure are problematic with mould that the cause is identified and corrected rather than the using cleaning as an expedient to the cure.

Not all moulds are bad, remember moulds and fungi contribute to the valuable chain of our eco system, breaking down complex organic substances into simple compounds and molecules to digest forming natures very own organic recycling process. Without moulds and fungi this process would simply end and the world would consume itself buried in a trail of matter.

The Homes Act 2018

The new Homes Fitness for Human Habitation Act 2018

The Homes Act 2018 came into force on the 20th March and amended the prior ‘Landlord and Tenant Act of 1985’ by extending the Acts obligations ensuring tenanted properties are ‘Fit for Human Habitation’.

The intention of the Act is to ensure that all tenanted properties both social and private are fit for human habitation by maintaining decent standards which do not place the health and safety of the tenants at risk. This applies both at the start of the tenancy and throughout its duration. The new homes Act however goes beyond the parameters of the prior ‘Landlord and Tenant’ Act and now provides the power for tenants to take legal action against landlords without having to wait for the Council or Environmental Health.

The vast majority of landlords already take their responsibilities and tenants’ risk to health seriously. The act merely means to impose upon those who do not. It is however important to mention that whilst landlords have a responsibility to ensure that properties are provided with adequate means of ventilation and are free from damp, it is the tenants responsibility to ensure that facilities provided are used, and that they themselves are not contributory to the cause.

So, what constitutes as ‘unfit’ for human habitation?

A property is classified as unfit for human habitation if it achieves a category one hazard in one or more of the following:

Repair – is in a poor state of repair

Damp – suffers from serious issues with damp penetration, rising damp, water ingress or condensation.

Ventilation – inefficient means of ventilation both natural and mechanical

Stability – has structural integrity / stability issues

Water, heating and sanitation – poor quality water supply or sanitation

Preparation Facilities – Poor quality food and cooking facilities 

Dangers – has defects considered dangerous to health. Section 4 of the Defective Premises Act 1974 places a duty on landlords who are contractually obliged to maintain or repair premises, to take reasonable care in the circumstances to ensure that anyone likely to be affected by defects in the premises are reasonably safe from personal injury, or from damage to the property caused by a relevant defect.

Under the new Homes Act, if one or more of the above are considered a serious category one defect landlords have a legal obligation to correct them. Even if some complaints do not meet a category one under the HHSRS risk assessment they may still be considered a ”Statutory Nuisance’ and can be reported to the local council for an abatement notice.


There are however some exceptions where a landlord’s responsibilities cease:

  • Problems caused by tenant behaviour – meaning that the tenant has behaved irresponsibly or illegally
  • Fires, storms and floods which are completely beyond the landlord’s control (sometimes called ‘acts of God’

Where repair works are the responsibility of a superior landlord such as, a management company or where third party consent is needed.

I hope this article has gone some way to aiding your understanding your understanding about condensation / mould related problems or at least refreshed your memory about the some of the problems / challenges we as surveyors will face over the forthcoming months.

Happy surveying


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