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(The science)
How to control Condensation.

"Warm air cools against a blade of grass to form drops of dew.
Inside your home, as the air cools at night, water drops condense on any cool surface and as a direct result black mould, streaming windows, dust mites and mildew all occur. This is why the bottom of cold corners are most prone to mould attack.”

dew on grass

How is condensation caused in the home? 
We all generate lots of water vapour which is invisible and odourless. Up to 2 gallons a day for the average family.

Steam is obvious but it becomes invisible water vapour almost instantly – steam from a kettle disappears but is still there as a gas. Unfortunately the water vapour moves around the home and condenses on cold windows,  cold walls, cold leather and cold clothes when the house cools down each night.
Every morning the steam from yesterday’s shower is streaming down your bedroom windows and growing mould on your wallpaper, curtains and your children's shoes.

FACT – Water vapour evens itself out throughout the home. Steam from a shower can condense in the corner of the spare bedroom 40 feet away. See Boyle's Law. if you want to find out why.

Why is condensation a problem?
Black spot moulds need food from wall surfaces and moisture. The more dampness the better they grow. Moulds can destroy wallpaper, leather, timber and fabrics. In worst cases this causes £1000’s of damage each year.
The drudgery of sponging windows each morning is a waste of time.
House dust mites need moisture in the air which they absorb. Millions of microscopic dust mites infest damp homes. Mites eat a variety of food around the home. Mites are responsible for many allergic responses – up to 80% of allergics are affected by mites.
Bed bugs do not drink - they absorb water from the air so reducing humidity will reduce bed bug population.
Damp conditions are associated with poor health especially asthma – correlation between poverty and allergic responses due to condensation.
Damp air needs more energy to heat. Dryer air ‘feels warmer’ at a lower temperature - control damp and reduce your energy bill.
Wet rot. Condensation can dampen timber allowing wet rot spores to germinate and wood weevils to thrive.
Wet insulation is ineffective and increases heat loss. Usually this is a hidden problem.
Musty smells make the home unpleasant to live in.

Self help to control condensation
Please read our Condensation Solutions for information on self help or click on the links below to find out more about Condensation Cures that work well

Systems proven to control condensation in the home
Positive Pressure Units in the roof
Cheap to install (competent DIY) – cheap to run -  automatic and extremely effective – solar gain to heat the home - dry the whole house with one unit in the roof. The best choices are available in our products section.

Heat Recovery Ventilation through walls   
Extremely effective - very cheap to run - fully automatic - ideal for flats with no roof void - perfect for bathrooms and kitchens where most of the damp is generated - very quite running - conform to FI Building Regs

Humidity Controlled Fans fitted in walls and ceilings

Cheap to buy and install, cheap to run, fully automatic. Easy replacement for existing switched or timed fans. The first option on limited budget.

Condensation Cures That Don't Work!

  • Dehumidifiers – Very expensive to run – noisy - need emptying – take up floor space – only reduce humidity in the immediate area - produce lots of distilled water. They can sometimes help with a minor problem in a remote room. Borrow one - there are lots about not being used because they don't work.
  • Draught proofing. It is lack of air that causes condensation..
  • Double glazing. Makes the home 'tight' and often increases condensation.
  • Cavity wall insulation, and loft insulation – will not combat humidity – some 'improvements' make condensation worse by making the house ‘tight’ - Insulation reduces heat loss only.
  • Anti-condensation paints. Essentially these are formulated to be poisonous to moulds and will do this job well. A cup of bleach with a gallon of water will also kill black mould just as well! However poison will not reduce the condensation: it will only kill black mould for a limited time. The windows will still get wet, moulds will grow on clothes/shoes and dust mites will thrive.
  • Air bricks in walls – do you want to live in a wind tunnel? – air bricks are only installed to ventilate roofs and timber sub floors.
  • Make the home hotter – self defeating - the air will then hold more water - it is a waste of your money – heat can make the problem worse. Damp air feels colder than dry air and costs more to heat.
  • Water proof the outside of the house – please re-read the page starting at the top. Paint will stop penetrating damp only.

Black Spot Moulds

Black Spot Mould occurs in cold corners, beside windows, inside wardrobes, on shoes and all sorts of damp surfaces. Moulds need damp - without it they die so don’t poison them with chemicals – dry them!

Grey and black mould types/genera include Pithomyces, Ulocladium, Alternaria, Memmoniella, Cladosporium, Stemphydium, Aureobasidium, and Stachybotrys.

Stachybotrys and Memnoniella are black mould types known to produce mycotoxins which are a potential health hazard. Stachybotrys and Memnoniella infestations typically occur on repeatedly wetted materials that contain cellulose. These include paper, cellulose insulation, cardboard, wood, etc. They are unlikely to grow where there is no cellulose.

Moulds growing on tiles and shower curtains are not likely to be infested with toxic Stachybotrys or Memnoniella as the surfaces don't normally contain cellulose.

The most common black mould is Cladosporium; it's not considered toxic and it's around all the time. There is a risk of allergy and asthma owing to exposure to Cladosporium but this risk has yet to be quantified. You're more likely to suffer from dust mite allergy due to high humidity than from Cladosporium

Fortunately the other moulds which cause black spots are not known to cause a mycotoxic hazard.

If black mould is growing on materials containing cellulose and it's prudent kill it with a dilute bleach spray but test a piece first before treating the whole lot. This is only temorprary...... the mould will come back!

 Things You Should Know About Black Mould

    Potential health effects and symptoms associated with mould exposure include allergic reactions, asthma, and other respiratory complaints. However, per the Centres for Disease Control "there is no causal link between mould and health issues in humans at this point in time". There is no practical way to eliminate all moulds and mould spores in the indoor environment as mould is ubiquitous. The way to control indoor mould growth is to control moisture. If mould is a problem in your home humidity must be reduced....... "

    HOW ? Reduce humidity to about 65% and condensation will stop....follow our condensation cures advice.

Dust Mites Control.


Dryhomes are not qualified to advise on mites and bugs but here are some useful notes.

….Roughly 80% of people suffering allergic responses are triggered by house dust mites…

Building Regulations part F 0.30  Ventilation Regulations     2006

House dust mite allergens can trigger allergic reactions in susceptible individuals. Measures for source control are provided in BRE Report BR417. Building Regulations health and Safety

      (For a detailed academic paper please click here)

November 16, 2007 by Thomas Schmidt 
Précis of a press release 
House dust mite allergy is caused by mites living in the house dust - the allergy is brought on by the excrement of the mites. The house dust mites are co-inhabitants of our domestic environment and have nothing to do with inadequate hygiene. Neither do they carry any diseases. After desiccation, the excremental capsules decay into very small particles and join up with the house dust. This allergy-containing dust can then be inhaled by breathing and lead to allergic complaints such as watering or itching of the eyes, coughing, congestion of the nasal mucosa, sneezing attacks, skin reactions and in serious cases to dyspnea and allergic bronchial asthma. If these symptoms occur all year round and if they are particularly severe in the evening or the early morning on rising, this would indicate a house dust mite allergy.</>
Details on the house dust mite
The two most common types of dust mite in our everyday environment are dermatophagoides pteronyssinus and dermatophagoides farinae. The dust mites belong to the arachnid group. They feed mainly on human and animal skin scale and mould. A human being loses about one to two grammes of skin scale every day, enough to feed 1.5 million house dust mites for a day. Apart from the availability of nourishment, the house dust mite population is encouraged by high atmospheric humidity.

 ( Click on the image

What are dust mites?

Dust mites are tiny organisms with eight legs, distantly related to spiders. You can't see them with the naked eye. Other than triggering allergy and asthma symptoms, they don't cause any harm. They like to live in areas that are warm and humid and can be found in many homes. Some people are allergic to dust mites' decayed bodies and fecal material, which become components of airborne household dust.

It's almost impossible to clear your house of dust, but isn't there something you can do?

There are two ways to try to control dust mite allergens in the home. One approach is to reduce dust mite proliferation by making the home environment as inhospitable to dust mites as possible. The other strategy is to reduce the amount of overall dust in the house.

How can you initiate the first strategy?

First, remember that environments that are moist — such as carpeted basements in warm climates — are perfect for dust mites. Mattresses and pillows also are excellent habitats for dust mites. If you're trying to reduce your dust mite exposure, keep indoor relative humidity low — definitely less than 50 percent. Eliminate any water leaks around the house, particularly in the basement. In addition, eliminate surfaces where dust mites can proliferate such as carpeting and upholstered furniture.

Is the bedroom of particular concern?

It is, mainly because most people spend about 8 of every 24 hours in the bedroom. As to the second approach — reducing the overall amount of dust in the house — techniques include putting allergy-proof encasements over the bedding, including the mattress, box spring and pillows. There are newer products that are somewhat more comfortable than plain plastic-bag encasements. Cotton encasements with high thread counts (260 or more threads per inch) can prevent dust mites from crawling in and out of the pillow's interior. They are available at a variety of outlets, such as furniture stores, mattress stores, department stores and medical supply stores. Wash the encasement and your pillow case in hot water (130° F.) at least once each week.
Avoid going to bed with wet hair. Moist hair and bedding attracts swarms of dust mites.

1. The black stuff round the window and on the walls is a mould growth (fungus) probably Aspergillus niger spp. 

2. The wall paper is peeling because the wall is DAMP! Condensation is probably the most common form of dampness in modern homes.

3. You can see the water on the window but on the wall it is absorbed into the plaster - although when severe you can sometimes see it running down the wall!

4. If you have any of the above then the humidity levels inside your house are high and the house dust mite is present in massive quantities.

5. It is proven that living in these conditions is bad for you and your children’s health - does anyone in your household suffer with chest complaints such as asthma?

6. The house dust mite is one of the major triggers of asthma in people who are allergic to it – that’s approx 80% of asthma sufferers!

7. Reducing condensation not only removes a source of damp and mould but can also lower the house dust mite population because both thrive from high relative humidity levels!
This is the house dust mite Dermotophogoides pteronyssinus. They are only about 0.3mm long so it’s virtually impossible to see them without a microscope. It is the faeces (dung) from the house dust mite which contains the allergen Der p1 which the vast majority of asthma sufferers are allergic to. It therefore makes sense to change the conditions within your home to reduce the numbers of dust mites.

Extract from web site
One of the most strongly allergenic materials found indoors is house dust, often heavily contaminated with the faecal pellets and cast skins of House Dust Mites. Estimates are that dust mites may be a factor in 50 to 80 percent of asthmatics, as well as in countless cases of eczema, hay fever and other allergic ailments. Common causes of allergy include house dust mites, cat dander, cockroach droppings and grass pollen. Symptoms are usually respiratory in nature (sneezing, itching, watery eyes, wheezing, etc.), usually NOT A RASH. However, there are reports of a red rash around the neck. Other allergic reactions may include headaches, fatigue and depression.
The wheeze-inducing proteins are digestive juices from the mite gut which are quite potent. An exposure to the mites in the first, crucial year of life can trigger a lifelong allergy. There is no cure, only prevention. One must control house dust mite levels.
Beds are a prime habitat (where 1/3 of life occurs). A typical used mattress may have anywhere from 100,000 to 10 million mites inside. (Ten percent of the weight of a two year old pillow can be composed of dead mites and their droppings.) Mites prefer warm, moist surroundings such as the inside of a mattress when someone is on it. A favourite food is dander (both human and animal skin flakes). Humans shed about 1/5 ounce of dander (dead skin) each week. About 80 percent of the material seen floating in a sunbeam is actually skin flakes. Also, bedroom carpeting and household upholstery support high mite populations.

Simple guide to relative humidity (RH)

Relative humidity (RH) is the proportion of water vapour the air can carry at a given temperature – it is expressed as a percentage. “The warmer the air the more water it can hold. “RH graph
(Click on the chart to see a larger version)

1000grams of air at  50C can hold about 5 grams of invisible water vapour.
1000grams of air at  250C can hold about 20 grams of water (4 times more)

Tracing the numbers on the  psychrometric chart please following this argument :-



Air at 00C, carrying 90% of its capacity in water vapour (90% RH) enters the home and is heated up to 200C.


At 200C the RH reduces to only 23% but it soon takes on the water from the damp air inside the house


Going up the chart the RH of the air rises to that which is in the home - 70%


On a cold surface, as at position 4, about 140C, this air becomes 100% humid and the water condenses out forming water droplets – CONDENSATION.

If air is slowly introduced into the dwelling the relative humidity gradually falls to below 60% and the damp problem goes away. This is because any air coming from outside, soaks up the moisture, and leaves the building with lots of water vapour.
The house dries and the living conditions improve immediately.

To control damp you need more heat or more air (or a combination of both – a cold house is harder to keep dry) – air costs very little to heat and air from a roof void is not only dry but often warmed by the sun.

Positive pressure ventilation works by pumping in “dry” air from the roof space.*

*Dutch research by professor Prof. Mollier
The relationship between RH (relative humidity), temperature and energy necessary to heat 1m3 of air.
At 20˚C and a RH of 80% 50 K joule is needed
At 20˚C and a RH of 55% 40K joule is needed
A saving of 10K Joule which is 20%.
In practice this does not lead to a 20% saving on your total energy because it will contain more than only energy consumption for heating. It proves however that your heating bill will go down when humidity levels in the house drop. (Source

From around March to Sept warm air from the roof enhances the heating within the house – free heating from the sun

So - in most homes only 20% of all the heating costs actually heats up the air – 80% heats up the walls, ceiling and floors. So only a very small percentage of the heating cost is lost when air is introduced and this is balanced by being able to turn down the thermostat and still feel warmer!

Heat recovery ventilation can reduce RH by removing “wet air” and pumping dry air in from outside at the same time. These units are particularly specified where there is no roof void, where the damp generation is focused as in the bathroom or kitchen, or when dwelling is small. There is no solar gain but about 80% of the heat is extracted and returned to the home making them very efficient.

A humidity controlled fan will automatically switch on when the humidity level is too high. This is a cheap and effective way of combating mild cases of condensation

Relative humidity(RH)

Large RH graph
Click on the chart to return to the Relative Humidity page

1000grams of air at  50C can hold about 5 grams of invisible water vapour.
1000grams of air at  250C can hold about 20 grams of water (4 times more)

Tracing the numbers on the  psychrometric chart please following this argument :-


Air at 00C, carrying 90% of its capacity in water vapour (90% RH) enters the home and is heated up to 200C.


At 200C the RH reduces to only 23% but it soon takes on the water from the damp air inside the house


Going up the chart the RH of the air rises to that which is in the home - 70%


On a cold surface, as at position 4, about 140C, this air becomes 100% humid and the water condenses out forming water droplets – CONDENSATION.


Air changes needed for healthy living

Minimum air changes per hour according to Building Regulations 1995 section F1.

2 changes -  bedrooms 3 changes – showers/bathrooms (F1 a fan capable of minimum extract capacity of 15 litres per second 54m³hr) lounge, hall, landing, toilets ( F1 plus a fan with 15 min over run) cellars 10 changes -  kitchens (F1 a fan  extracting 60 litres per second 216m³hr) laundry/utility room ( F1 a fan capable of extracting 30 litres per second 108m³hr)

Thus a 2 mtr x 4 mtr x 2.5 mtr bathroom needs 60 cubic metres per hour ventilation. That is a lot!

The table shows where most water vapour is created….little by sleeping but a lot by washing and cooking….regulations apply to new build but we should all be striving to upgrade our own homes….or live with must, black moulds and streaming windows every morning.




British Standard BS5250 Condensation

L19 Taken from British Standard 5250    Explanatory leaflet (paragraphs.2-7)

 2.  WHY CONDENSATION OCCURS. Condensation occurs when warm moist air meets a cold surface. The risk of condensation therefore depends upon how moist the air is and how cold the surfaces of rooms are. Both of these to some extent depend on how the building is used.

3.  WHEN CONDENSATION OCCURS. Condensation occurs usually in Winter, because the building structure is cold and because windows are opened less and moist air cannot escape.

4.WHERE CONDENSATION OCCURS. Condensation which you can see occurs often for short periods in bathrooms and kitchens because of the steamy atmosphere, and quite frequently for long periods in unheated bedrooms; also sometimes in cupboards or corners of rooms where ventilation and movement of air are restricted. Besides condensation on visible surfaces, damage can occur to materials which are out of sight, for example from condensation in roofs.

5.WHAT IS IMPORTANT. Three things are particularly important :-
 (a) To prevent very moist air spreading to other rooms from kitchens and bathrooms or from where clothes may be put  to dry.
(b) To provide some ventilation in all rooms so that moist air can escape.
(c) To use the heating reasonably.

(a)Good ventilation of kitchens when washing or drying  clothes or cooking is essential. If there is an electric extractor fan, use it when cooking, or washing clothes, and particularly whenever the windows show signs of any  misting. Leave the fan on until misting has cleared.
(b) If there is no extractor fan, open kitchen windows but  keep the door closed as much as possible.
(c) After bathing, keep the bathroom window open, and shut  the door for long enough to dry off the room.
(d) In other rooms provide some ventilation. In older houses a lot of ventilation occurs through flues and draughty windows. In modern flats and houses sufficient ventilation does not occur unless a window or ventilator is open for  a reasonable time each day and for nearly all the time in a room that is in use. Too much ventilation in cold  weather is uncomfortable and wastes heat. All that is needed is a very slightly opened window or ventilator.  Where there is a choice, such as a top hung window, open the upper part. About a 10mm opening will usually be  sufficient.
(e) Avoid the use of paraffin or flueless gas heaters as far as possible. Each litre of oil used produces the equivalent of about a litre of liquid water in the form of  water Vapour. If these heaters must be used, make sure  the room they are in is well ventilated.
(f) If condensation occurs in room which has a gas, oil or solid fuel heating appliance with a flue the installation should be checked, as the condensation may have appeared  because the appliance flue has become blocked.
(g) Do not use unventilated airing cupboards for clothes drying.
(h) If washing is put to dry, for example in a bathroom or kitchen, open a window or turn on the extractor fan enough  to ventilate the room. Do not leave the door open or moist  air will spread to other rooms where it may cause trouble.

(a) Try to make sure that all rooms are at least partially  heated. Condensation most often occurs in unheated rooms.
(b) To prevent condensation the heat has to keep the room  surfaces reasonably warm. It takes a long time for a cold building structure to warm up, so it is better to have a small amount of heat for a long period than a lot of  heat for a short time.
(c) Houses and flats left unheated during the day get very cold. Where ever possible, it is best to keep heating on  even at a low level.
(d) In houses, the rooms above a heated living room benefit to some extent from heat rising through the floor. In bungalows and in most flats this does not happen. Some  rooms are especially cold because they have a lot of  outside walls or loose heat through a roof as well as walls. Such rooms are most likely to have condensation  and some heating is therefore necessary. Even in a well  insulated and with reasonable ventilation it is likely to be necessary during cold weather to maintain all rooms at  10C in order to avoid condensation. When living rooms are in use their temperature should be raised to about 20C.

Boyles Law

Many clients ask – “Why bother with the bathroom? – the problem’s in the back bedroom - here’s why….

Boyle's Law 1676,  potted version - pressure exerted by a gas varies inversely with the volume (at the same temperature)….

Let’s assume the bathroom, hall and remote bedroom are all the same size!
I have a shower and make 60 grammes water vapour – this is a gas that creates a vapour pressure in the room.
If I open the  bathroom window for an hour (and keep the door shut) the vapour is forced outside by pressure – the vapour pressure will eventually be equal to the pressure outside.
But if I open the bathroom door into the hall the vapour is evened out between the two rooms. Now the hall and the bathroom have 30 grammes in each room.
If the bedroom door is open the vapour evens out further and 20 grammes end up in each room.
At 4am when all is cold the vapour condenses in the far corner of the bedroom and mould starts growing on wallpaper and paint.

So the water has migrated from the bathroom through the hall onto the bedroom wall….

Stated as a formula, Boyle's Law  V1/V2=P2/P1 (at constant temperature)

where V1 equals the original volume, V2 equals the new volume, P1 the original pressure, and P2 the new pressure



Humidity and dust mites and asthma. Technical information

Click here to return to Dust Mites page

Academic paper relating increases in humidity to dust mite allergy. More reading is available at the end of the paper…..

Summary This paper is concerned with historical changes in domestic ventilation
and vapour dissipation rates and the associated risk of dust mite colonisation. A
controlled trial evaluated allergen and water vapour control measures on the level of
house dust mite (HDM) Der pI allergen and indoor humidity, concurrently with changes
in lung function in 54 subjects who completed the protocol. MHRV units reduced water
moisture content in the active group by an average of 12% while HDM allergen
reservoirs in carpets and beds were reduced by over 96%. Self reported health status
confirmed a significant clinical improvement in the active group. The study can form the
basis for minimum winter ventilation rates that are likely to inhibit dust mite colonisation
and activity in maritime/temperate climatic regions.
Domestic ventilation rates, indoor humidity and dust mite allergens
- Are our homes causing the asthma pandemic?
SG Howieson1 BArch DipArch MPhil A Lawson1 B.Eng, C McSharry2 BSc PhD G
Morris3 BSc PhD E McKenzie4 BSc MSc PhDand J Jackson1 BEng
1Dept. of Architecture and Building Science, University of Strathclyde, Glasgow, UK
2Dept. of Immunology, Western Infirmary, Glasgow, UK
3Scottish Centre for Infection and Environmental Health, Glasgow, UK
4Dept. of Statistics, University of Strathclyde, Glasgow, UK
List of symbols
ach-1 air change rate per hour
HDM house dust mite
MHRV mechanical heat recovery ventilation
Der pI HDM allergenic protein
RH relative humidity
ESPr computer based integrated hygro-thermal modelling programme
Tao intake air temp
Tei extract air temp
1 Introduction
Concurrent with the dramatic rise in the incidence of asthma during the latter part of
the twentieth century, has been a fundamental change in the design, construction and
use patterns of domestic buildings in the UK. In addition to legislative changes - such
as increasing the thermal resistivity of the building fabric - a variety of trends have
combined to make dwellings warmer and more humid; conditions in which the dust
mite species Dermatophagoides pteronnysinus can thrive:
• The move away from solid fuel and the associated sealing of open fireplaces has
reduced internal air change rates.
• The proliferation of timber frame dwellings incorporating polythene vapour barriers
and little thermal mass, has lead to increased diurnal temperature fluctuations and
reduced vapour dissipation rates.
• The use of ‘hard’ internal surface finishes such as gypsum plaster - a material that
is relatively impermeable to water vapour absorption/desorption - will have reduced
water vapour transmission through the construction layers.
• The rising incidence of central heating systems is likely to have increased average
whole house temperatures.
• Improving hygiene standards have increased water vapour production with
showers in particular contributing to aerosol migration.
• Internal clothes drying (possibly due to changing female employment patterns) will
have increased the internal moisture burden.
• Retrofit double-glazing replacing notoriously draughty steel and timber units will
also have reduced fortuitous background ventilation (between 1991 and 1996 the
percentage of dwellings in Scotland that had double-glazing increased from 36% to
62% (1)).
• Fitted carpets and ‘soft toys’ also provide suitable micro-climates for the
photophobic dust mite.
2 Research questions
• What is the scale of the historical reduction in domestic ventilation rates?
• What is the level of occupant exposure to dust mite allergens in dwelling types
where ventilation rates are likely to be low?
• What allergen avoidance measures and winter ventilation rates are required to
ensure allergen burdens are maintained below sensitisation thresholds?
• How can this best be achieved in relatively small modern dwellings?
3 Methods – Simulating changes in domestic ventilation rates
ESP-r is an integrated modelling tool for the simulation of thermal, visual and acoustic
performance of buildings. Five traditional Scottish house-types were modelled to norm
reference air change rates in a typical living room (volume, materials, window type,
flues and vents) for a 48 hour winter period. By inputting a set amount of water vapour
into the system, a mass flow calculation can estimate the rate of moisture diffusion.
4 House type
The Scottish House Condition survey(1) defined housing in Scotland by type, style and
construction date - each representing the respective percentage of the total housing
stock (2,232,000). The five main generic types viewed as most common to an epoch
were taken to be:
19th century tenement 1930’s semi-detached villa 1950’s 3 storey tenement
1970’s multi storey 2000’s timber frame
The main construction characteristics (volume, materials, heating system, flues,
window type, crack length and trickle vents) were input as boundary conditions to the
appropriate living room area. Simulations were then run to estimate air change rates
during a 48 hr mid-winter period (Jan 21st- 22nd – composite climate file).
5 Results
Max (ach-1) Min. (ach-1) Ave. (ach-1) Vol. (m3) (m3/h)
1890 model 2.15 0.78 1.66 83 138
1936 model 2.00 1.10 1.63 42 69
1950 model 1.03 0.63 0.835 37 31
1970 model 0.95 0.05 0.74 41 31
2000 model 0.63 0.23 0.45 33 15
Table 1: ach-1 for five living room models
6 Discussion
The most apparent difference between the 2 earlier and the 3 later models is the
impact of the ‘Clean Air Acts’ on the presence and use of open flues. The air change
rates for the 1890 and 1936 models at the chimney node within the airflow network
were 12ach-1 and 10ach-1 respectively, providing evidence of the impact a flue has on
the total room air change rate. An open fire would drive even greater air change rates,
however, it was not possible to model such a scenario. In terms of volumetric air-flow,
the late Victorian tenement has over nine times the rate of a contemporary timber
frame model. Although the simulations are not necessarily attempting to criterion
reference the models to produce an accurate facsimile of reality, the method can be
defended as a technique to benchmark longitudinal trends and identify the influential
factors (wind speed and direction) driving the system. The CIBSE(2) recommendation
of 8 litres/s per person for odour control equates to an air volume approaching 29m3
per hour - almost twice the background air change rate of the twenty first century
model. Such low rates in tight modern dwellings will cause odour and smoke to build
during the winter -when windows are likely to remain closed - particularly when
occupied by more than one person. The implications for vapour dissipation rates are of
equal concern.
7 Moisture dissipation rates
Ventilation rates will have an important impact on water vapour diffusion. The following
calculations are based on the mass flow rate of moisture, relative to the specific air
change rates for each generic house type. It is assumed that the external air entering
the living room through one inlet is mixing perfectly with the ‘moist’ air in the zone. The
calculation is based solely on the living room as the test bed, with notional input and
output boundary conditions for a typical winter day (incoming air temperature 4oC,
relative humidity 80%, mixing ratio 4g/kg, specific volume 0.7898 m3/kg). The internal
conditions are determined using the resultant data from the simulations: temperature
21oC, air change rate, volume and flow rate specific to each model.
The outgoing air is set to a temperature of 21oC and a relative humidity of 70% (as this
is the boundary condition for mould growth) and 50% RH, as this is the boundary
condition for dust mite viability and colony activity.
Input A: Tao = 4oC, RH = 80%, g1= 4.024g/kg, v= 0.7898m3/kg
Input B: 3 litres of water vapour
Output X: Tei = 21oC, RH = 70%, g2= 11g/kg
Output Y: Tei = 21oC, RH = 50%, g2= 7.857g/kg
The volume flow rate is calculated for each model using the volume of the model and
the associated air change rate. This is then used to calculate the mass flow rate of air,
which when multiplied by the moisture difference between the internal and external air
provides the mass flow rate of moisture. Moisture dissipation rates can then be
determined assuming that all the moisture leaves the room. Three litres of water
vapour was introduced into the model which equates to a pro rata (m2) proportion of
the average daily household production(3).
moisture difference = g1 –g2 [kg/kg day]
volume flow rate = volume×air change rate [m3/s]
Inputs A B
Volume Y
mass flow rate of air = volume flow rate [kg/s]
specific volume
mass flow of moisture = moisture difference×mass flow rate [kg/s]
time to dissipate 3 litres of moisture = 3 / mass flow rate of moisture [hrs]
Time (hrs)
@70% RH @ 50% RH
1890 model 2.47 4.53
1936 model 4.93 8.98
1950 model 11.02 20.11
1970 model 11.12 20.25
2000 model 22.4 41.28
Given that the air input has a relatively low mixing ratio it is predicted to take over 40
hours to reach the 50%RH boundary condition in the 2000 model. It is thus more likely
that relatively tight, energy efficient, modern dwellings will be subject to progressive
and cumulative moisture build-ups during the winter months if windows remain closed.
They are also likely to have water vapour burdens that maintain RH above the crude
50% threshold, allowing the establishment and proliferation of dust mite colonies.
Furthermore, diurnal temperature variations – particularly in lightweight construction
that has little thermal capacitance – will be greater, increasing relative humidity and
condensation rates, which can be absorbed by carpets, bedding and soft furnishings.
As ambient air in the warmer months - particularly along the western seaboard – is in
the main higher than 7g/kg of dry air, there is little scope for reducing the internal
humidity below dust mite viability levels. Winter ventilation rates are thus the crucial
variable in inhibiting colony size and activity.
8 Indoor humidity and the dust mite hypothesis
Several epidemiological studies(4-6) have identified the health implications of living in
damp homes. Dampness has been strongly associated with respiratory disease and
higher mite numbers. The 1996 Scottish House Condition Survey(1) established that
25% of all dwellings suffer from problems of dampness and/or condensation. It is
estimated(7) that around 80% of allergic asthmatics react to extracts of mite allergens.
This indoor allergen exposure is important as contemporary leisure pursuits, provided
by television and computers, have increased time spent indoors(8). Evidence
implicating the house dust mite as the prime causal factor, came from an investigation
into the rise in asthma symptoms associated with mite colonisation in Papua New
Guinea(9) where other causal factors associated with Western lifestyles were not
The ideal conditions for mites to proliferate is at a temperature of 25oC and a relative
humidity of 80%(10&11). A high humidity is very important to the survival of these
creatures as most of their water is gained from the atmosphere by osmosis (10). Under
ideal conditions the life span of a mite is approximately two to three months(11). Mite
numbers vary seasonally, rising and falling in accordance with humidity cycles within
the house (12&13). Where ventilation rates are low and average internal temperatures are
above 20OC, ambient humidity may play a reduced role, as domestic activities such as
showering and internal clothes drying, could significantly increase the internal water
vapour pressures and reduce the influence of external circadian variations. As the
main food of mites is human skin, heavily used soft furnishings, provide a suitable
environment for the development of mite colonies``(14&15). Skin scales absorb moisture
from the atmosphere and are colonised by the fungus Aspergillus Sp..(16). The yeast
causes the scales of skin to swell moistening and softening them and reducing the fat
content as an aid to digestion (17). This adds further importance to the role of humidity in
the lives of the house dust mite, as moulds generally require a relative humidity of 65%
or greater(18&19). Such conditions can however exist in specific micro-climates even
when the surrounding humidity is relatively low and dust mites have a range of survival
techniques which can be deployed during periods of low humidity eg hibernation and
10 Dust mite allergens sensitisation thresholds
Tovey(20) observed that individual mite faecal pellets contain very high concentrations
of allergens, which can trigger reactions in susceptible individuals. During their active
reproductive life, females were observed to produce (under laboratory conditions) 200
to 300 eggs (21). As each dust mite can produce up to 60 times its own body weight in
faecal pellets during its life-span(20) it is thus possible for relatively small colonies to
generate large reservoirs of allergenic micro-particles. As these allergens have been
shown to be stable for at least 4 years (22), the number of live mites at any given period
may bear little correlation with the quantity of allergen present. The assay which
identifies the allergen protein Der pI, can thus be used as a longitudinal marker to
determine the history of dust mite activity at any given location. Any study designed to
investigate the role of HDM on asthmatic patients must include a strategy to attack the
existing reservoir of HDM allergenic proteins contained in the dust reservoirs.
11 Methods – An interventionist, double blind, placebo controlled protocol
68 volunteer asthmatics (43 public sector dwellings) in North Lanarkshire (UK) were
recruited via the local schools (50 < 15 years at the outset) and a range of remedial
measures were applied to their dwellings to reduce allergen reservoirs and dust mite
activity. The dwellings were typically four apartment semi-detached properties with gas
fired central heating, urea-formaldehyde cavity insulation and PVCu double glazing
(average NHER rating of 5.2). The dwellings were monitored for six months to
establish baseline profiles before retro-fitting MHRV cartridge units (Baxi Clean Air
Systems E100) in bedrooms and living rooms. The cohort was split into two active
groups: AG2 (n=32), (steam cleaning of carpets, new bedding and active fans) and
AG1 (n=17), (steam cleaning of carpets and new bedding with placebo fans). A control
group, (n=19) received placebo steam cleaning and placebo fans. Dust samples were
taken from living room carpets, bedroom carpets and beds on a 3 month cycle;
temperature in living rooms and temperature and relative humidity in bedrooms were
continuously monitored at 90 minute intervals; self-recorded peak flow readings were
taken morning and night, and a face to face health questionnaire was completed every
three months.
12 Results
12.1 Immunological assays of dust reservoirs (0.5m2 sampled in the living room and
asthmatic’s bedroom – one minute vacuum time) were undertaken and this initial
monitoring cycle confirmed that 59% of living room carpet dust samples, 75% of
bedroom carpets and 78% of beds contained Der pI ratios greater than the WHO
sensitisation threshold of 2ug/g of house dust.(12) In addition to this, 25% of living room
carpets, 50% of the bedroom carpets and 56% of the beds, were found to contain
concentrations greater than the upper threshold of 10ug/g of house dust (see fig. 1)
Samples n=139
Fig. 1: Cycle I Log Ratio of Der pI/g of dust (carpets & beds) - WHO levels 2 & 10 ug/g
When the initial dust sampling cycle was compared with the final cycle (1 & 8), the
percentage above the WHO thresholds (2 & 10ug/g dust) for all dust samples (carpets
and beds) fell from 80% to 21 % and 65% to 4% (n= 78) respectively in active group II,
from 61% to 41% and 34% to 4% in active group I (n= 31) and 65% to 15% in the
control group (n=39). The total reduction for active group II was 96% (see fig 2).
Fig. 2: Log ratio Der p1/g house dust AG2 (all samples: cycles 1(♦) & 8(Ä))
12.2 Relative humidity (bedrooms) – Readings taken during the first six winter
months at 90 minute intervals in all bedrooms prior to intervention produced a mean
RH of 55% (see fig 3). Even when the external moisture content is low - due to cold
winter air temperatures – most of the bedrooms remained suitable for dust mite
1 11 21 31 41 51
Mean RH
Max RH
Min RH
Figure 3 Mean RH and ranges (Oct 1998 – Apr 1999)
12.3 Hygro-thermal monitoring - Humidity and temperature readings taken in
bedrooms (every 90 minutes over a 22 month period), demonstrated a reduction in
internal absolute water vapour pressures of 12% in the active group in comparison with
the control groups (measured at the same time of year Jan-May 99 vs Jan-May 00).
Figure 4 illustrates the hygro-thermal profile output for a dwelling in active group 2. A
comparison of the 4 month winter periods before and after intervention (where ambient
conditions were almost identical) shows a significant reduction in bedroom water
vapour pressures. Mean internal dry bulb temperatures did not vary significantly before
and after the intervention in the living rooms (20.2 to 20.6OC), however, the bedroom
temperatures did fall by an average of 1.4OC (19.2 to 17.8OC). This is a function of the
increased background ventilation rates.
Nov Dec 1999 Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 2000 Feb Mar Apr May Jun Jul Aug
Time (starting 27/10/1998)
Humidity %
Temperature °C
Fig. 4: Comparative before and after hygro-thermal profile of AG2 bedroom
12.4 Changes in Health Status - Peak flow readings taken before and after the
interventions at the same time of year show some lung function improvements in all
groups (see fig 5), however these figures have to be correlated with measured
reductions in drug use, before any trends could be identified. Some of the increase will
be due simply to child growth.
1 2 3 4 5 6
AG2 AG1 Control
Fig. 5: Mean daily peak flow per group per cycle (litres/min mean am and pm readings)
12.5 Health status questionnaires - Face to face interviews using health questionnaires
(shortened version of MacMaster(23)) were completed on a 3 monthly cycle by a
contract researcher who was blind to the cohort groupings. The following are results
from the two key questions introduced for the sixth data collection cycle (9 months after
Q1. Has the indoor air quality been affected since remedial measure
Improved Same Worse
Control (n=7 houses) 1 5 1
AG1 (n=7 houses) 6 1 0
AG2 (n=20 houses) 18 2 0
Q2. How has your asthma been affected since remedial measure
Control (n=10 people) 1 7 2
AG1 (n=12 people) 5 7 0
AG2 (n=32 people) 26 6 0
From these figures we can conclude that there was a highly significant association
between the extent of the intervention and perceived improvement in both air quality
(Fisher’s exact test p=0.001) and asthma (Fisher’s exact test p<0.0001). Testing at the
5% level of significance, Q1 demonstrates that the three groups do not have the same
probabilities for improvement. The active groups cannot be separated (at the 5% level)
but have higher probability than the control group. The estimated probabilities are: 14%
& 89%. Again, for Q2 the three groups are not the same. The Control group cannot be
separated from AG1, with an estimated common probability of improvement of 27%,
whereas AG2 are 81%.
13 Discussion
The level of Der pI identified in the existing dust reservoirs was considerable, with 59%
of the living room carpet dust samples, 75% of the bedroom carpet dust samples and
78% of the bed surfaces over the WHO threshold for allergen sensitisation. Indeed
50% of the bedroom carpets and 56% of the bed surfaces contained more than 10ug/g
– the upper threshold known to be associated with severe allergic reactions. Since the
amount of dust mite allergen seems to have an associated clinical risk, it is noteworthy
that some exceptionally high concentrations were found with 18 samples above 100ug
Der pI/g of dust, 9 of which were above 200ug Der pI/g of dust and 3 of which were
above 400ug Der pI/g of dust. This pilot study has identified that seven out of ten
cohort subjects were exposed to house dust mite allergen burdens above the WHO
sensitisation thresholds. Although the cohort was not randomly selected - being
volunteers - the house-types studied are typical of the Scottish public sector stock as a
On average Der pI levels in AG II - which had the additional MHRV intervention - were
reduced by over 96% (comparing cycles 1 & 8 - absolute weight of Der pI). The MHRV
units also reduced the absolute humidity in the bedrooms, which may explain the
reduction in re-colonisation rates. Although the participants were unaware that some
fans were placebo units, which simply re-circulated the internal air, the health
questionnaires demonstrated significant improvement in air quality and asthma.
13.1 Confounding variables - There were however several confounding variables that
may have influenced the overall result:
• As no pressure tests were undertaken the background air infiltration rates were
unknown. Although the fan units provide circa 12 air changes per day to the
bedrooms/living rooms, this has to be compared with the air infiltration
characteristics of the dwelling.
• The placebo units were imperfect having filters that scoured the internal air of a
significant amount of air-borne particulates.
• The age profile was heavily skewed (65% of the cohort were under 16 at the outset
of the study). Childhood asthma appears to be more capricious than the adult
condition and their growth over the period would result in some increase in lung
• As no skin prick tests were undertaken to assess the cohort’s sensitisation to
HDM, the project could not differentiate between the health effects influenced by a
reduction in airborne Der pI and/or the overall improvement in indoor air quality by
simple dilution of internal pollutants.
14 Conclusions
Changes in the design and use of the domestic environment over the latter part of the
20th century are likely to have led to a significant increase in house dust mite
concentrations in temperate/maritime influenced climatic regions. This in turn may be
the prime causal factor influencing the rising incidence of asthmatic symptoms in
children. Effective allergen denaturing/avoidance techniques such as steam cleaning
and mattress encapsulation can significantly reduce the reservoir of allergenic protein
available for inhalation. Mechanical heat recovery ventilation can reduce mixing ratios
particularly during the winter months to a level below 7g/kg of dry air and/or 50%RH,
which will inhibit dust mite re-colonisation rates. This study has demonstrated that such
a strategy is likely to result in an improvement in air quality and lung function. In
addition to clinical improvement, significant cost savings from a reduction in drug use
and primary/acute care services can accrue, rendering such an approach cost-effective
both as a treatment and a preventative strategy. The implications for house design
and ventilation rates are clear. As room volume drops and air tightness increases,
complementary ventilation regimes will have to be incorporated if dust mites allergens
are to be kept below sensitisation thresholds. From the simulations a rate of circa
0.9ach-1 appears to be the ‘ball-park’ figure (the simulated background rate with an
additional 0.5ach-1 induced by MHRV). MHRV can also reduce the effective air change
rate - in terms of energy losses – to around 0.3ach-1. Warm ‘dry’ healthy homes are
now relatively easy to achieve.
1. Scottish Homes The 1996 Scottish House Conditions Survey
Edinburgh Scotland (1997)
2. CIBSE Guide Section B2 London (1986)
3. BRE Tackling condensation Garston Watford (1991)
4. Burr ML St Leger AS Yarnell JWG Wheezing, dampness & coal fires
Com.Med: 203 (1981)
5. Burr ML Miskelly FG Butland BK et al. Environmental factors and symptoms
in infants at high risk of allergy J Epidemiology 108:99-101 (1989)
6. Platt SD Martin CJ Hunt SM et al. Damp Housing, mould growth, and
symptomatic health state BMJ 298 1673-1678 (1989)
7. Morrison-Smith Clinical significance of skin reactions to mite extracts in
children with asthma BMJ 2:723-726 (1969)
8. Esmen NA. The status of indoor pollution. Environ Health Perspective
62:259-265 (1985)
9. Dowse GK, Turner KJ, Stewart GA, et al. The association between
Dermatophagoides mites and the increasing prevalence of asthma in village
communities within the Papau New Gineau Highlands. J Allergy Clin Immunol
75:75-83 (1985)
10. Hallas TE. The biology of mites. Allergy 11:6-9 (1990)
11. Wharton GW. House dust mites. J Med Ent 12 : 577-621 (1976)
12. Spieksma F Spieksma-Boezeman M The mite fauna of house dust with
reference to the house-dust mite Dermataphogoides pteronyssinus.
Acarologia, 11:226-241 (1967)
13. Platts-Mills TAE de Weck AL Dust mite allergens and asthma - a worldwide
problem J. Allergy and Clinical Immunology 83:416-427 (1989)
14. Sesay HR Dobson RM Studies on the mite fauna of house dust in Scotland
with special reference to that of bedding Acarologia 14:384 (1972)
15. Van Bronswijk JEMH Dermataphogoides pteronyssinus (Troussart 1897) in
mattress and floor dust in a temperate climate (Acari: Pyroglyphidae). J Med
Ent 1 63-70 (1973)
16. Douglas AE Hart BJ The significance of the fungus Aspergillus
penicilloides to the house dust mite Dermatophagoides pteronyssinus.
Symbiosis 7:105-117 (1989)
17. Whitrow D Pycock Rhouse Dust Mites: How they affect asthma, eczema
and other allergies. Elliot Right Way England (1993)
18. Gravesen S Fungi as a cause of allergic disease Allergy 34 135-154
19. Hart BJ Whitehead L Ecology of house dust mites in Oxfordshire Clin Exp
Allergy 20:203-209 (1990)
20. Tovey ER Chapman MD Wells CW et al. The distribution of dust mite allergen
in the houses of patients with asthma. Am Rev Repsir Dis 124:630-635 (1981)
21. Furumizo RT The biology and ecology of the house dust mite
Dermatophagoides farinae PhD Dissertation Univ. of California (1973)
22. Kort HSM Kneist FM Four-year stability of Der pI in house dust under
simulated domestic conditions in vitro Allergy 49 131-133 (1994)
23. McMaster University Asthma quality of life questionnaire. Dept of Clinical
Epidemiology and Biostatistics, McMaster University, Ontario, Canada (1992)
The authors would like to acknowledge the financial support of the University of
Strathclyde, Energy Action Scotland, EAGA Charitable Trust, North Lanarkshire
Council, Scottish Power and Baxi Clean Air Systems Ltd.