Mould and ventilation management
Ventilation systems
Is improved house ventilation needed to reduce condensation and combat mould?
Problems such as condensation and associated mould development do not always need to be passed on to specialist manufacturers and installers of ventilation systems. A house surveyor ought to be aware of the basic ventilation systems available on the market and the general advantages and disadvantages of each. He/she should also be able to recognise when quite simple and acceptable adjustments to lifestyle could be agreed and put into practice to minimise condensation in the home to an acceptable level and help reduce mould.
Ventilation systems are not always needed, and condensation can be simply reduced by adjustments to lifestyle. Mechanical ventilation systems act often simply to override much-needed lifestyle changes that occupiers are not able or indeed willing to make.
On finding mould development in a house, consider very carefully why this is happening in order for the correct remediation to be designed and implemented. Mould typically arises because of an event or because of longer-term moisture imbalance.
Longer-term moisture imbalance
Mould is often caused by condensation resulting from imbalances and deficiencies in the way a property is heated, insulated and ventilated. More than just a basic repair is needed to remediate condensation damp and mould.
There are 2 distinct routes that can be taken to remedy a longer-term condensation problem.
- Adjustments to 'lifestyle' may eliminate or sufficiently limit condensation and mould. (See Reducing condensation.) Sometimes it is possible to solve a condensation problem without investing in heating or ventilation systems or improving home insulation.
- Install improved heating, insulation or ventilation if condensation problems are deemed to be significant and cannot be simply addressed by a change in lifestyle. It is worth carrying out feasibility research to clarify the best value solution. This can be carried out at various different decision-making stages:
- Feasibility study A: compare options: lifestyle adjustment versus design change, assess the heating/insulation/ventilation issues.
- Feasibility study B: compare options: improved heating versus improved insulation versus improved ventilation.
- Feasibility study C (having decided, for example, on improved ventilation): compare options: e.g. local mechanical extractor fans versus background ventilation versus ventilation incorporated in a window replacement work package versus positive input ventilation versus whole house mechanical extraction.
Feasibility research should ideally include discussion and input from all interested parties, e.g. funding authorities, clients, occupiers, managing surveyors. The prime aim of the feasibility research is usually to establish value for money in terms of the costs of options versus how fully certain key criteria are met.
Even a one-off house condensation problem needs organised thinking behind any remediation strategies, ideally set out in the form of a written appraisal. When a surveyor prepares a feasibility report to evaluate options for damp remedy, for example to address a condensation problem, the surveyor's report may perhaps best conclude by identifying a preferred strategy, but should explain how the various remedy options meet key criteria. It may often be the client who has the greatest influence on remedy decision as they will be paying.
The surveyor only needs to include remedies with a good chance of success in the feasibility report.
Other reasons to install/improve house ventilation
The most common reason for installing additional ventilation may be to combat condensation-led mould, but there are other reasons too. Improvements to house ventilation can:
- provide fresher air;
- reduce air humidity to an acceptable level;
- control condensation;
- reduce mould colonisation on surfaces;
- reduce airborne mould spores and fragments; and
- remove pollutants generally, for example:
- allergens (e.g. house dust mites);
- volatile organic compounds (VOCs) such as formaldehyde;
- carbon dioxide;
- carbon monoxide;
- tobacco smoke;
- odours; and
- ensure safe operation of some combustion appliances.
A useful table of indoor air pollutants can be found in Environment and services (Table 3.3).
So if the original aim was purely to reduce mould colonisation, in improving ventilation you can also achieve other important benefits.
If all you do is install an extractor fan in a kitchen or bathroom, you may not significantly reduce air pollutants in the property as a whole. To significantly reduce the range of indoor air pollutants, a whole-house mechanical ventilation system would be needed, incorporating the appropriate filters.
There are other reasons why you should consider improving ventilation to reduce condensation and mould (rather than upgrading heating or installing additional insulation):
- upgrading ventilation is likely to be the most cost effective remedy;
- installing local or whole house ventilation systems would in most cases cause less disruption; and
- extractor fans and even whole-house ventilation systems are surprisingly cheap to run, using very little electricity and requiring minimal maintenance.
Ventilation and building airtightness
We need to draw a distinction here between 'leaky buildings' and 'breathing buildings'. Some buildings self-ventilate effectively due to gaps around window sashes or in floors, roof coverings, etc. that enable movement of air from inside to outside and vice-versa. Such a building may be leaky from air movement, but may not necessarily 'breathe'. Buildings do not 'breathe' primarily by virtue of gaps in building elements or components, but because the basic building fabric is porous and can take in and evaporate out moisture.
Are buildings becoming 'tighter'?
Successive revisions to Building regulations have increased thermal insulation requirements, but often at the expense of ventilation.
Many experts in the industry agree that over the past few decades, buildings have become more sealed up. For example, it is common for the installation of double glazing to have a very negative impact on house ventilation. The new windows usually incorporate extremely efficient seals, and rarely include a small 'night ventilator'. Any property improvement should therefore consider the knock-on effects on ventilation.
In estate improvement works, overcladding and indeed over-roofing are also usually installed with the aim of improving thermal insulation, weathertightness and building appearance. The down side may be less ventilation.
Table 3.2 of Environment and services lists factors that have reduced ventilation rates in buildings:
- abandonment between 1965 and 1990 of the requirement for flues or vents in habitable rooms;
- more tightly sealed windows;
- lack of night ventilators in windows;
- dense airtight construction in some cases;
- weather stripping;
- security dictating that windows must be kept closed; and
- flueless heating systems or balanced flues not drawing air from interior.
Laminate flooring could also be added to that list. Many older houses, perhaps with uneven boarded floors, are being covered by new wood or wood effect flooring systems, which are closely fitting and reduce the opportunity for any fortuitous ventilation through gaps in adjacent floorboards. You will also often find a run of sealant between new floor covering and the base of the perimeter skirting board, reducing ventilation by air leakage at that position. (They also make access to the floor void very difficult for surveyor or builder.)
No building yet built has been perfectly airtight. Movement of air can occur into and out of the building through cracks, gaps and holes. The sum total of the routes through the building envelope is usually expressed in terms of air changes per hour (ach) when the building is subject to a pressure test of 50 Pascals (Pa), achieved by a fan normally sited in a removable door panel.
The effect of airtightness on ventilation efficiency
It is usually considered that, for whole-house ventilation to be effective, a rate of 0.5 to 1 air change per hour (ach) should be achieved. The key issue is how much of this is achieved by controlled mechanical ventilation and how much by air leakage. Air leakage is not always predictable, as it depends on atmospheric conditions. Experts predict that by reducing air leakage to an acceptable level, ventilation becomes more predictable and energy efficient.
The Building Research Establishment informs us that houses typically have a natural infiltration rate of 0.7ach. Some tightly sealed homes have a natural infiltration rate of as low as 0.2ach, meaning that there are insufficient air changes to achieve good indoor air quality.
In Digest 398 the BRE state that 'the dwelling should be as airtight as practicable for economic operation of an MEV or MVHR system'.
Types of ventilation system available
A basic requirement of the Building Regulations is that 'there shall be adequate means of ventilation provided for people in the building'. The implication is that the primary purpose of ventilation is to create satisfactory living conditions for people rather than to promote good conditions for the wellbeing of the building fabric, finishes and components. But in most cases the level of humidity achieved by good ventilation to suit humans (i.e. around 50%) would be a suitable condition for many building materials and often suits the contents as well. High humidity causes materials to expand and distort, and moulds to develop. Excessive dryness causes some materials to damagingly shrink and warp.
The old Approved Document F was prepared in the light of changing perceptions of what ventilation needs to provide for occupiers and the wider national interests:
- good air circulation as buildings are becoming increasingly airtight;
- reduction in internal relative humidity to inhibit mould development;
- reduction in relative humidity to reduce colonisation by dust mites;
- improved air circulation and filtering to reduce build-up of VOCs (volatile organic compounds), pollens, smoke and other pollutants and to generally improve indoor air quality (IAQ);
- reduction in background ventilators to reduce noise ingress;
- increased energy efficiency of fan motors;
- reduction in wasted heat from air extraction without heat recovery; and
- increased efficiency rates of heat exchangers.
Clause 0.4 'performance' clearly underlines how seriously Building Regulations are now addressing energy issues in ventilation – with heat recovery featuring prominently.
The beauty of a mechanical ventilation system with heat recovery, according to industry experts, is that an effective ventilation solution can meet both the new AD F and AD L (energy efficiency) requirements. Another bonus of using this kind of system is that no window trickle ventilation is required, so noise ingress can be limited.
Under new Approved Document F, 1 of 4 systems is to be selected for consideration:
- System 1 – background ventilators and intermittent extractor fans;
- System 2 – passive stack ventilation (PSV);
- System 3 – continuous mechanical extraction (CME); or
- System 4 – continuous mechanical supply and extraction with heat recovery (MVHR).
The chosen product or installation should comply with the relevant Building Regulations. (Systems or products carrying a BBA certificate may be considered more reliable.)
System 1: background ventilators and intermittent extractor fans
This system is usually used in kitchens, bathrooms and WCs for removal of wet air and odours.
The new Approved Document F stipulates fan flow rates for various room types, e.g. 60 or 30 litres/second for kitchens and 15 litres/second for bathrooms (very much as per the existing Approved Document). The fans need to be used in conjunction with trickle (i.e. background) ventilation to achieve the required flow rates as set out in tables in AD F.
In the new AD F, a dedicated section on installation has been included in Appendix E: Good Practice Guide to the Installation of Fans for Dwellings, which contains good design guidance on the alternative types of fan (axial, centrifugal, in-line) and the terminals to be fitted. Much guidance is included on how to design and fit ducts from fans, which are often badly routed, kinked or too lengthy to enable efficient air extraction.
BRE advice
The BRE Good Repair Guide 21, Improving ventilation in housing highlights the popularity of mechanical extractor fans for removing moisture from wet rooms and points out that fans can be operated manually or mechanically. The guide also cites the Building Regulations fan rate requirements.
In the guide, BRE warns us that long ducts from ceiling fans reduce air flow rate. There are standard formulae for calculating the loss in efficiency. Ensure that the power of a ceiling-mounted fan matches the length and configuration of the associated duct. This is where manufacturer's advice is key. Noise from ceiling mounted fans is a common problem, as the ceiling can act as a sounding box and vibrations can cause noise. BRE advises to mount fans, if possible, on more solid construction, and away from bedrooms and living rooms. It also advises that circular ducts cause less noise than square or rectangular sections. Further advice is contained in BRE Good Repair Guide 21, Improving ventilation in housing.
Always consult a suitably qualified plumbing engineer when an extractor fan is used that could affect an open-flued combustion appliance. Dangerous 'spillage' can occur, i.e. where the extractor fan pulls flue gases back into the room.
Also bear in mind that the AD F sizing of fans for various house rooms is a minimum, not a maximum sizing. So consider a more powerful fan to reduce motor wear and save running the fan on maximum speed.
Electrical works, particularly in a bathroom, shower room or kitchen, must be carried out by a suitably qualified electrician and all work tested on completion.
Even though a fan is correctly installed, a house or flat may still suffer condensation and mould due to moisture generation from a bathroom or kitchen because occupiers have not obeyed key rules. For example, the fan may be operating during bathing, and still running after bathing, but the occupier dresses in the bedroom leaving the bathroom door ajar or open. Alternatively, trickle vents could cause draughts – and be closed. This defeats the efforts to remove moisture at source as it can now readily migrate to other parts of the accommodation. Bathroom and kitchen doors should remain closed until such time as moist air has been expelled to the outside.
Fans are also best fitted with a filter to help keep the motors clean and free from dirt and grease. Filters need to be removed and washed or replaced as necessary. For maintenance by consumers, fans should be designed for easy removal of filters, etc., ideally without the need to use screwdrivers to remove face covers. A reputable fan manufacturer will offer all the best advice on running maintenance.
How a humidistat-controlled fan can be fooled
We know from basic psychrometrics that, as air cools and is not subject to any change in the moisture it contains, relative humidity will increase. This is a very basic law of physics. So air at, say, 20°C and 50% RH when cooled at nightfall in an unheated bathroom could drop to 16.5°C. This drop in air temperature would activate the fan, as RH will have risen to 65% (the common threshold RH for the fan to activate).
The running of an extractor fan could be a source of annoyance for an occupier, who might resent any noise or vibration from it, especially at night, and might think that electricity was being wasted (no matter how efficient the motor). So a frustrated occupier may disconnect the offending extractor fan.
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Key points – System 1: background ventilators and intermittent extractor fans
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System 2: passive stack ventilation
Passive stack ventilation systems (PSV) can be designed and installed for both new and existing buildings. The system might comprise little more than 100mm PVCU soil stack pipework routed from a kitchen or bathroom to the ridge or slope of the roof. There may be a humidistat or infrared sensor fitted at the ceiling vent, opening the system when the room is entered. (Of course, old houses often already incorporate a type of PSV system – otherwise known as a chimney flue.)
You need a separate stack for each wet area, so the installation is likely to be expensive. Also, as air is pulled out of the dwelling, there needs to be an opportunity for replacement air to come in, usually by trickle vents – which can be a noise problem and may simply be closed or shut off.
A PSV can be used as a substitute for, or in addition to, mechanical extractors in rooms such as kitchens, utility rooms and bathrooms.
The system is incredibly simple and described as follows by the BRE in the Good Repair Guide 21:
'During the heating season, and under normal UK weather conditions, warm moist air from the room travels up the duct to the outside. When the indoor air temperature goes up, because of cooking, washing or bathing, the airflow rate in the duct increases. So the PSV system is more effective at the time it is most needed.'
BRE consider PSV systems to be adequate in removing moisture from dwellings, 'keeping condensation and mould growth under control', providing of course that they are properly designed and installed.
The BRE Information Paper IP 13/94, published several years before GBG 21, explains that a PSV system depends for operation on:
- a difference between inside and outside temperature; and
- the effect of wind passing over the roof of the dwelling.
This means that the system might not work effectively in certain weather conditions, such as outside the cold season or when there is insufficient wind.
The cost of a PSV installation includes not only the cost of ductwork, routing the ducts through the property, but also hiding ducts in casing. PSV can be difficult to install in a property of multiple tenure, with work being needed in several accommodation units. Costs can double or treble where several rooms need to be ventilated, as each room must be fitted with its own independent stack. BRE recommend a duct size of 125mm internal diameter for a kitchen, 100mm for a utility room or bathroom, and 80mm for a WC. BRE mention problems from external noise, and suggest that PSV cannot be installed close to a tall building.
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Key points – System 2: passive stack ventilation
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System 3: continuous mechanical extraction
There are quite a few advantages with continuous mechanical extraction (CME) systems. CME systems basically use a single energy-efficient fan with multi-speed control, located in a loft or airing cupboard, and with ducts to wet areas or WCs. Pollutants are removed at source from bathrooms, WCs and utility rooms. The fan runs continuously; being sited away from living areas, there will be little noise disturbance from their operation.
But as with other systems, there is a down side. There will be a need for replacement air via background ventilators, and therefore draughts and possible noise ingress. There is no heat recovery.
These systems may or may not have the ability to 'purge ventilate'. You can imagine that when a kitchen fills with smoke from cooking, there will be a need to get rid of the polluted air very quickly. It may often be possible to just open a window, but some kitchens, bathrooms or WCs are sited towards the inner zones of the building plan, with no openable window available for rapid air dilution. So a means of quickly and efficiently ejecting foul air would be needed. Mechanical ventilation systems are designed to lowish extraction rates, intended to run continuously rather than as a swift response to an urgent air ventilation requirement. In a kitchen a cooker extraction hood could be installed to provide additional extract ventilation at source.
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Key points – System 3: continuous mechanical extraction
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System 4: continuous mechanical supply and extraction with heat recovery systems (MVHR)
With MVHR systems, fresh air is drawn into a heat recovery unit, which also pushes stale air out of the building. The incoming fresh air is warmed from the heat of outgoing air.
Heat recovery fan units can be used to ventilate a whole house or an individual room or space.
Background ventilation is not needed, unless the units are used for individual rooms and are then deemed to need background ventilation as per standard extractor fans. Some current equipment claims to have a 95% heat recovery efficiency. There is little noise ingress with such systems.
Manufacturers make quite optimistic claims concerning the efficiency of the heat exchanger; energy efficiency is highest when external temperatures are lower.
DETR Good Practice Guide 268 refers to typical heat recovery of 60% of heat from outgoing air. (Remember that standard extractor fans lose all the heat from outgoing air.) A continually running fan will also change house air, ridding it of pollutants and unwanted humidity developed inside the building envelope.
Under the DETR Best Practice Programme, it was shown that heat recovery mechanical ventilation systems can successfully reduce indoor relative humidity.
Systems usually incorporate low speed and high speed or 'booster' settings for periods of high moisture generation. The fans can provide background as well as rapid extract ventilation under manual or humidistat controls.
Ducting will be required from extractor diffuser positions and the heat recovery/fan units, often sited on an external wall in an entrance hallway at high level.
Figure 1: The basic principle of the room heat recovery ventilator. Note how a good circulation of incoming fresh air replaces the outgoing stale and moist air. © Courtesy Kair Ventilation Ltd.
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Key points – System 4: continuous mechanical supply and extraction with heat recovery (MVHR)
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System 5: single room heat recovery ventilators (HRV)
These units are really just sophisticated extractor fans, and their flow rates need to be calculated using the same outputs.
If fans are sited in a bathroom and a kitchen, the fans together are classed as a system, so need to meet Building Regulations performance rates for continuous extraction. If just 1 heat recovery fan unit is used in a property, it must fulfil the ventilation rates for standard extractor fans, as it will be considered an intermittent ventilation unit.
When through-the-wall units are used, trickle vents are required in the dry rooms only – not where the heat recovery unit is actually mounted.
This type of fan can be extremely quick and easy to fit – its body may be circular and all that is needed is the correct sizing of masonry hole cutter. Fans are sometimes very cleverly designed so fitting can be from the inside of the property, making the unit ideal for high-rise applications. Some manufacturers have even developed units that can be fitted within specially cut double-glazed units, or small diameter ducts designed to be fitted through the actual window frame.
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Key points – System 5: single room heat recovery ventilators (HRV)
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General ventilation considerations
Rapid dilution
Where doors or windows are opened, there can be rapid dilution of the ventilation.
For comfort and ventilation efficiency, part of the ventilation opening should be 1.75m above floor level and of a total area to comply with the current Approved Document F.
Background ventilation
Here a minimal supply of fresh air is allowed in over a long time period. Care must be taken that openings do not compromise security or enable damp penetration. Draughts should not cause discomfort.
Under Building Regulations, background ventilation could be by means of an airbrick, as is traditionally used, but is more often by way of a trickle vent built into a window or door frame providing an average of 6,000mm2 area for each room, and an absolute minimum of 4,000mm2 for each room. Mention is made in Good Repair Guide 21 Improving ventilation in housing of the different methods of ventilation.
Trickle vents can be a problem if there are noise issues, for example, in a dwelling near an aircraft flight path or a busy road. Trickle vents can usually be closed by a simple slider and can also be compromised by curtains over windows.
Positive input ventilation
In a house it is common in a positive input ventilation system (PIV) for a ventilation fan fitted with a filter to be installed in the loft. Air in a loft during the winter is warmer than outside air, and would otherwise constitute completely wasted stored energy.
A fan operating in a loft is noisy for occupants. Lighting cable is easily available to make a fused spur connection for the fan unit. Some units are light in weight and can be simply suspended, meaning no costly support work is needed. All that is needed is a cut out in the ceiling board to house the supply air diffuser. Units supported on the ceiling structure need to be founded on acoustic pads to reduce vibration.
Filters can be fitted in the unit to filter out unwanted air contaminants. Many houses are ideally suited to such an installation, as the first floor landing is easily accessible and an ideal place for the filtered air to be ducted in, with no problems of air draughts for the occupier. Fans are cheap to run.
Looking at figure 2, you can appreciate there is not much to it – simply a fan unit with filters, inlets and connection to an air supply diffuser. This could be the reason why some in the industry are a little doubting of the system's efficiency. Some argue that modern buildings are too airtight for PIV to work effectively, but leading installers have discovered that even quite modest exfiltration opportunity is sufficient for it to work.
Figure 2: The kind of fan unit that may be suspended in a loft space above a top floor landing, to push filtered air down into the habitable space via a ceiling diffuser in order to create a positive indoor air pressure. The loft fan shown is a Nuaire fan.
The incoming air from the loft displaces old and contaminated air by continuously diluting it, replacing it with good-quality filtered air. The stale air exits via the thousands of air leakage points found in all homes, and as a positive air pressure is created, this will actually suppress unwanted infiltration (draughts). A PIV system replaces the air in a house 20 times a day. So adverse health effects from airborne contaminants can be reduced or eliminated.
Some PIV systems are sophisticated, and can draw warm air collecting under tiles or slates heated by the sun, or use solar panels to warm air before it is expelled into the top floor landing, and even use solar panels for domestic hot water heating.
Flats of course do not have an available loft, except perhaps top floor flats. For flats it is still possible to install a PIV system, which can take advantage of warmer air near the ceiling. There would be more visible ducting in such a system, as the inlet would be at a greater distance from the supply diffuser position. Incoming air could be heated in the colder months.
BRE carried out comprehensive research into PIV systems and published an Information Paper, IP 12/00 in April 2000. Two testing methods were used:
- the testing of a BRE unoccupied test house where steam generators produced moisture that would be created by typical household activities; and
- the monitoring of 8 houses in Merthyr Tydfil and 7 houses in Aldershot.
The main conclusions were as follows:
- PIV reduced RH levels by 10% in the test house;
- in the occupied houses PIV was not consistently effective in reducing RH – it was found to be most effective when houses were at high RH;
- in the test house and occupied houses there seemed to be a problem with moisture entering the roof space from the rooms below, to create a high RH;
- installing low-energy PIV may not directly save energy; and
- occupiers were very pleased with the effectiveness of the PIV system.
It is important to seal all gaps in the top floor ceiling, e.g. around pipes, and to make sure the loft trap is well fitted and sealed.
Again, it may be more effective to use a combination of approaches. So, for example, in a PIV system, where a kitchen could be far removed from the supply inlet diffuser, it might be necessary to install a continuous trickle ventilation unit to help change the kitchen air.
Remember that, in a whole house ventilation system that relies on pulling in air from a loft, you must be absolutely certain that there are no pollutants in the loft that cannot be reliably filtered out. So if a loft space has been treated by chemicals for timber rot or beetle infestation, proceed with caution.
Is there a best fit for all?
There has been all manner of advice, sometimes conflicting, concerning the best or most efficient ways to ventilate and combat condensation in homes. There is much debate in the industry and there seems to be a battle between companies that promote and install individual extractor fans, and those that manufacture and install whole house ventilation systems. And within the whole house ventilation sector we see strong debate concerning the effectiveness of positive input ventilation, passive stack ventilation or heat recovery mechanical ventilation.
The trend seems to be towards heat recovery mechanical ventilation to help solve condensation problems in mould-ridden dwellings. The energy efficiency of the various systems is hotly debated across the industry. Some doubt the claims of heat recovery fan manufacturers concerning the amount of heat actually recovered from the outgoing (and therefore usually warmer) air. Some argue that heat recovery fans only operate efficiently under certain external wind conditions. New products and developments continue to enter the market place, fuelling further debate.
There seems to have been a gradual move over perhaps the past 10 or so years towards whole house ventilation systems rather than the piecemeal installation of localised extraction fans. There are a number of reasons for this. First, whole house systems are becoming very competitively priced and therefore economically efficient – it now costs little more to install a system to ventilate a whole house than it would to install a couple of extractor fans. The units and associated ducts have become less obtrusive. Engineering advances have led to units that are quieter and use less energy to run.
A whole house system takes in fresh air from outside – so assuming this air is less polluted than the air inside the home, you can improve the inside air quality, which can then give occupiers a more healthy living environment. Ventilation systems incorporating heat recovery become very marketable when there is a national and world drive to save energy.
Installing a mechanical ventilation system is much more than just a remedy for damp and the problems associated with condensation and mould. The additional benefits of living in a healthier internal environment cannot be emphasised enough.
Of course, just how healthy internal air from efficient mechanical ventilation is, depends on how good the quality of air around the building is.