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2.2
Mould Index
Tadj Oreszczyn, University College London
Stephen Pretlove, South Bank University
“then he who owns the house shall come and tell the priest, ‘There seems to
me to be some sort of disease in my house.’ Then the priest shall command
that they empty the house before the priest goes to examine the disease, least
all that is in the house be declared unclean; and afterward the priest shall go
in to see the house. And he shall examine the disease; and if the disease is in
the walls of the house with greenish or reddish spots, and if it appears to be
deeper than the surface, then the priest shall go out of the house to the door
of the house, and shut up the house seven days. And the priest shall come
again on the seventh day, and look; and if the disease has spread in the walls
of the house, then the priest shall command that they take out the stones in
which is the disease and throw them into an unclean place outside the city;
and he shall cause the inside of the house to be scraped round about, and the
plaster that they scrape off they shall pour into an unclean place outside the
city; then they shall take other stones and put them in the place of those
stones, and he shall take other plaster and plaster the house.”
LEVITICUS 14 35-43
Mould is not a new phenomenon in buildings as the above quote from the
Bible indicates. One of the main health issues often related to affordable
warmth is that of mould growth and dampness in dwellings. This chapter
reviews the current results of research into the causes of mould growth,
proposes a mould index for dwellings and investigates the impact of fuel
poverty on the occurrence of mould growth.
1. The problem
Approximately 20% of all dwellings in England suffer from mould growth
and dampness to some degree. This number of incidences of mould growth
and dampness in private rented dwellings, so often occupied by the fuel poor,
is significantly higher than in any other sector (DOE 1996).
There have been many studies that suggest a link between health and
dampness and mould (IEA 1991). In fact one could argue that the causal link
is perhaps stronger than between low internal temperatures or fuel poverty
and health. However, it is always difficult to identify a causal relationship
because of the many confounding factors.
Mould growth is not the only potential health hazard resulting from
dampness and high internal relative humidities. The House-Dust Mite (HDM)
which thrives in high relative humidities is considered to be an important
causal agent not only for asthma, but also for other allergic disease, such as
atopic dermatitis, rhinitis and keratoconjunctivitis (Cunningham 1996). The
MRC Institute for Environment and Health conclude that "There is clear
evidence that antigen derived largely from mite faeces is one of the major
causes of allergic sensitisation in the UK" (Humfrey 1996).
2. Conditions for mould growth and dust mites
There are many different types of mould species, the most common in UK
dwellings include the genera Aspergillus, Penecillium and Cladosporium.
There are other genera commonly found in dwellings and they all thrive
under slightly different environmental conditions. However, all moulds
require the following to grow.
• Mould spores: these normally enter the building with the air from outside
which normally contains several hundred spores per cubic meter although
the concentration increases in summer when external concentrations are
considerably higher than internal concentrations, even if the property has
mould growth.
• Nutrients: the slightest layer of grease found on all building surfaces is
adequate for mould to grow.
• Temperature: different mould species thrive under different temperatures.
Most moulds will survive in the range 0°C to 40°C, found in most
buildings. Moulds will normally thrive better in warmer conditions.
However, over the range of temperatures normally found in dwellings,
10°C to 20°C, there is little change in activity.
• Moisture: The availability of moisture is normally the critical factor
determining whether mould will grow in a dwelling. Moulds extract
moisture from the substrate they are growing on which in turn extracts
moisture from the surrounding air. Therefore, both the surface properties
and local relative humidity are important factors. Mould can grow on
hygroscopic materials such as wood and leather at a relative humidity as
low as 70% whereas on surfaces which do not absorb moisture such as
glass and ceramic tiles require water to condense on the surface which
will occur at a relative humidity of 100%. For common wall coverings
such as painted plaster or wallpaper the critical surface relative humidity
for mould growth is 80%. If a relative humidity of 80% is maintained for
a period of several weeks then mould will grow. However, conditions
within dwellings are rarely stable, either because the external climate is
varying, the heating system is being turned on and off or moisture is
being introduced into the dwelling through the occupants breathing,
cooking or washing. Few detailed studies have been undertaken into the
impact of transient conditions on mould growth and those that have,
indicate that mould will grow if the relative humidity is above 80% for
50% of the time (Adan 1994). Analysis of monitored data from dwelling
living and bedrooms indicate that this corresponds to a mean relative
humidity of 80% (Oreszczyn & Pretlove 1998).
Proliferation of the house dust mite (HDM) can be reduced by removing the
source of food (human skin scale) by microporous barriers, killing mites with
acaricide sprays, heat treatments, freezing or dehydrating mites. Controlling
the relative humidity and temperature is the only long term cost effective
method to control dust mite proliferation prior to sensitisation. The dominant
HDM in the UK, Dermatophagoides pteronyssinus, rapidly increases its
feeding, defecating, mating and egg production above 73% relative humidity
at temperatures above 25°C. The HDM depends on mould to breakdown skin
scales and make them digestible. However, if the relative humidity is above
85% the mould activity results in scales so decomposed as to make them
inedible to the mites. Adult mites dehydrate and can survive no longer than 6-
11 days at a relative humidity less than 50%. As temperature falls, egg output
slows and egg-to-adult development time rapidly extends from 34 days at
23°C to 123 days at 16°C (at 75% relative humidity). This is one of the
reasons why central heating has been suggested as a possible cause of
asthma. However, the evidence for this is inconclusive and whereas the
temperature associated with central heating may encourage mite populations
to expand, central heating should substantially reduce the relative humidity
therefore reducing mite numbers. Temperatures have to remain below
freezing for some time in order to kill adult mites. Although many texts refer
to maintaining an absolute humidity below 7g/kg, as proposed by Korsgaard
(1983), this rule of thumb only applies to well-heated dwellings. For room
temperatures below 20°C the relative humidity of surfaces with soft
furnishings should be kept below 75% with occasional periods of a week or
two below 50%. This is most easily achievable in dwellings during cold
spells when the external air is very dry.
The above clearly identifies relative humidity as the key parameter for
determining if mould or dust mites will survive. As relative humidity is a
function of both the moisture and the temperature of the air the next sections
identify the key factors that impact on these, both in general terms and
specifically at the locations where mould and mites live.
3. Micro environmental conditions.
Moulds and dust mites live on surfaces and within soft furnishings
respectively and hence it is the microenvironments that surround these
locations that are of importance and not just the average airspace conditions
within a dwelling. This is particularly the case in older uninsulated properties
so often occupied by the fuel poor.
A relative humidity of above 70% is often quoted (BSI 1989) as the critical
condition for mould growth. This figure refers to the average conditions of
the air that is typically found in dwellings where mould occurs. However,
mould and house dust mites grow on surfaces and the local surface conditions
can be significantly different from the air temperature. Since relative
humidity is a function of both temperature and the moisture content of the air,
the local relative humidity close to the surface of a colder wall will be higher
than the relative humidity in the bulk of the room. This is particularly the
case in dwellings that are only partially heated, uninsulated, or where thermal
bridges occur, such as where insulation is missing or where there is two or
three-dimensional heat loss (Oreszczyn 1992). In order to keep the surface
relative humidity below 80%, and therefore avoid mould growth, on an
uninsulated brick wall the critical airspace relative humidity can be as low as
65%. For an well-insulated cavity wall the critical airspace relative humidity
can be 75% and on a typical cold bridge it can be as low as 60%. For the
above reasons there may be cases when the average room conditions are
below the critical levels when either mould or mites should flourish but
where there are specific microenvironments where moulds or mites can and
do thrive.
4. Moisture production and movement in dwellings
The amount of moisture in the air within a room is dependent on the moisture
produced within a space and the moisture transported into and out of the
dwelling predominately by ventilation.
Moisture production
The amount of moisture produced and released into the air within a dwelling
can range between 1 and more than 20 litres per day, depending on the
number of occupants and their moisture production activities. Figure 1 shows
a typical moisture production profile for a family of four. The largest
moisture source is due to the perspiration and exhaled air of the occupants,
followed by cooking and then bathing. However, personal habits can
radically affect this. For example, microwave cooking of ready-prepared
meals can substantially reduce the moisture associated with cooking. Also,
occupants who spend all day inside the dwelling, such as retired couples, will
result in increased occupant moisture production when compared to working
couples. Dwellings should be designed to cope with a typical range of
moisture production in a dwelling of a given size. However, any dwelling can
experience mould growth if excessive moisture is generated. Dwellings
should be designed to cope with some internal drying of clothes and if the
dwelling is a flat without a secure garden or laundry facility it is likely that a
substantial proportion of drying will be carried out indoors on radiators.
Figure 1 - Typical moisture production for a family of four (kg/day)
Moisture transport
The predominant mechanism for moisture transfer in dwellings is ventilation.
Figure 2 summarises the typical daily moisture flows in a dwelling. The
incoming air has moisture associated with it and additional moisture is added
to this air due to the moisture production detailed above. This moisture is
then transported through the fabric of the dwelling by diffusion and by the
extract air. This air has greater moisture content than the incoming air so
increasing the ventilation rate reduces the internal moisture content. The
moisture transfer through the fabric by diffusion is negligible in terms of the
total quantity directly carried by the air. If this moisture condenses as it
diffuses through the fabric then interstitial condensation may arise and the
fabric may be damaged. Such interstitial condensation can occur if a wall is
drylined with insulation but no vapour barrier is introduced.
3.95
2.20
0.50
0.88
0.45
1.25
1.00
0.20
Occupants
Cooking
Washing clothes
Washing floors & surfaces
Washing dishes
Drying clothes
Bathing / washing
Plants
Total = 10.43 kg/day
Figure 2 - Typical daily moisture flows in a dwelling
Up to 200 litres of moisture may be stored in the fabric of a typical dwelling.
As moisture is released into the space some will be absorbed into the fabric
and then when the moisture production is stopped the moisture may then be
re-emitted into the internal air. The extent of moisture absorption and
desorption depends on the surface materials inside the building. An example
of a highly absorbent environment would be a wood panelled room with very
hygroscopic materials within such as a library. The other extreme could be a
fully tiled bathroom, which contains minimal hygroscopic material.
However, although moisture storage in the fabric plays an important role in
short term fluctuations, what is absorbed is generally re-emitted and so this
phenomena can be ignored if one is concerned with long term averages, as is
the case with mould growth. The one main exception to this is where the
fabric of a new dwelling is still drying out following construction. Evidence
collected from a variety of new dwellings suggests that significantly elevated
internal relative humidity is experienced for about 6 months to a year after
construction is completed which can lead to mould growth.
5. Internal temperature
The internal relative humidity is not only dependent on the internal moisture
but also on the internal temperature. The lower the temperature for given
moisture content the higher the relative humidity. Therefore as dwellings
become hard to heat and occupants cannot afford to heat them the relative
humidity rises within the dwellings. What’s more although ventilation may
reduce the moisture content of the air it will also reduce the temperature in a
dwelling occupied by the fuel poor. In these situations, when the ventilation
rate increases beyond a certain level the temperature drops and so the relative
humidity rises. These two factors often compete against each other. Therefore
200kg stored in building fabric
30 to 60 kg/day
in incoming air
6 kg/day swing
with the building
fabric
Occupants 3
to 20 kg/day
0.5 kg/day in
diffusion losses
30 to 80 kg/day
in exhausted
ventilatio n air
whereas a strategy of increased ventilation to eliminate mould growth may be
appropriate in a fuel rich property it may be less so for a fuel poor dwelling.
This is indicated in Figure 3 below. In addition to the ventilation rate, the
temperature will be affected by the type of heating system, its control, cost of
operation and the levels of fabric insulation.
Figure 3 - The impact of increased ventilation on internal relative humidity
6. Relative humidity modelling
Because the internal surface relative humidity is dependent on both the
moisture and the temperature, the relative humidity is dependent on many
inter-related factors including the following:
• Fabric Insulation
• Occupants ability to pay fuel bills
• Occupants moisture generating activities
• Hours of occupation
• Number of occupants
• Heating system, efficiency, control and distribution
• Ventilation
• External climate
In determining the internal surface relative humidity, many of the above
factors interact in a relatively complex manner. As a result of this complexity
a computer model called Condensation Targeter II (Oreszczyn & Pretlove
1998) has been created in order to gain an insight into the potential risk of
40
50
60
70
80
90
100
00.5 11.5 22.5 3
Air ch an ges p er h our
Internal RH
"Fuel poor"
"Fuel rich"
Crit ic al Inte rn al RH
for Mould Growth = 70%
mould growth in a particular dwelling with a standard occupant. This model
utilises the thermal model BREDEM-8 to predict the internal temperature.
Added to this monthly temperature calculation is a simple algorithm for
calculating the surface temperature and a steady state moisture model
(Loudon 1971). Predictions of internal relative humidities for 36 dwellings,
mostly modern, have been compared with measured relative humidities and
shown to agree to within +/- 10% for 95% of cases as indicated in Figure 4.
Figure 4 - Condensation Targeter II monthly modelled versus monitored
relative humidity (Oct-May) for 36 dwellings
7. Sensitivity
In order to determine the impact that different factors have on the risk of
mould growth, a sensitivity study using Condensation Targeter II has been
undertaken. Figure 5 shows the impact of changing fabric, services and
occupancy factors on the average air relative humidity within an uninsulated
top storey flat.
0.00
20.00
40.00
60.00
80.00
100.00
0.00 20.00 40.00 60.00 80.00 100.00
Monitored RH (%)
Modelled RH (%)
+10% RH
-10% R H
Figure 5 - Sensitivity study for uninsulated top-storey flat with two exposed
walls and roof where fuel cost is no object
This flat is modelled as having an occupant who can afford to heat the
property to 21°C for an average 10 hours a day and the standard case has a
predicted relative humidity of 71%. The first bar shows the impact of
changing the heating pattern. If heating is reduced to only 5 hours a day then
the relative humidity increases to 92%. Alternatively if the flat is heated for a
total of 16 hours a day then the relative humidity drops to 58%. The different
interventions are ordered from left to right as roughly having less
significance. Figure 6 shows the same dwelling, but this time the tenants are
assumed to have a limited fuel budget of £ 60 per week. In this case
insulation has as significant an impact as ventilation.
Figure 6 - Sensitivity study for uninsulated top-storey flat with two exposed
walls and roof where fuel cost limited to £60/week
50.00
55.00
60.00
65.00
70.00
75.00
80.00
85.00
90.00
95.00
100.00
Heating pattern
Occupancy
Ventilation rate
Demand
temperature
Geographical
location
Insulation
Clothes drying
indoors
Unvented gas
heating
Number of
chimneys
Window type
Location of
kitchen
Input parameter
Airspace Relative Humidity (%)
Worst
Best
Never
5 hours/day
Never
7 loads/week
South West
(January)
East Scotland
(January)
Zone 1
Zone 2
Unopenable
All very loose
None
2 Chimneys
19 C
23 C
Highest
1.15 ach
Lowest
0.56 ach
Two
Six
Extended
16 hours/day
Evenings only
5 hours/day
Standard dwelling airspace Relative Humidity = 71%
50.00
55.00
60.00
65.00
70.00
75.00
80.00
85.00
90.00
95.00
100.00
Heating pattern
Occupancy
Ventilation rate
Demand
temperature
Geographical
location
Insulation
Clothes drying
indoors
Unvented gas
heating
Number of
chimneys
Window type
Location of
kitchen
Input parameter
Airspace Relative Humidity (%)
Worst
Best
Never
5 hours/day
Never
7 loads/week
South West
(January)
East Scotland
(January)
Zone 1
Zone 2
Unopenable
All very loose
None
2 Chimneys
Highest
1.15 ach
Lowest
0.56 ach
Two
Six
Extended
16 hours/day
Evenings only
5 hours/day
Standard dwelling airspace Relative Humidity = 75%
8. Mould Index
Chapter (?) proposes the introduction of an Affordable Warmth Index (AWI).
A similar index has been developed called the Mould Index (MI). The Mould
Index indicates the risk of mould occurring on the coldest surfaces in a
dwelling. If the average surface relative humidity of the coldest surface
equals 80% in any month then the MI will equal zero, whilst if the average
surface relative humidity equals 50% in any month then the MI is given a
value of 100. Between these two criteria the MI is assumed to be linear and
can extend below and beyond the 0 and 100 MI ratings. A high value on the
AWI scale indicates affordable warmth in a dwelling and a high value on the
MI scale indicates avoidance of mould growth.
Using the computer model described earlier it is possible to examine the
impact that different interventions have on the AWI and MI. It is also
possible to model dwellings where fuel cost is limited and where it is
unlimited which has an impact on the MI values. As the level of insulation is
improved in a poorly insulated dwelling the AWI and the MI increases.
These are indicated in Figure 7 below.
Figure 7 - Effect of insulation on AWI and MI
As expected, the fuel poor MI values, for situations where there is limited
fuel expenditure, are lower than the fuel rich MI values where fuel cost is
unlimited. Although the trend is similar, as the level of insulation in a
dwelling reduces mould growth is far more likely in dwellings of the fuel
poor.
The impact of ventilation rate has also been modelled as shown in Figure 8.
This indicates that if the ventilation rate is increased then the AWI gets
-20
-10
0
10
20
30
40
50
60
70
80
90
100
Pre 1900 1900 - 1965 1966 - 1976 1977 - 1981 1982 - 1990 Post 1990
Index
AWI
Fuel Rich MI
Fuel Poor MI
Linear (AW I)
Linear (Fuel Rich
MI)
Linear (Fuel Poor
MI)
worse. As the ventilation rate is increased the general trend of the MI is
positive. However, in dwellings occupied by the fuel poor the MI gets better
until the air change rate reaches 1 air change per hour. Beyond this point the
risk of mould growth increases significantly. This is shown in Figure 8
below.
Figure 8 - Effect of ventilation rate on AWI and MI
References
Department of the Environment "English Housing Condition Survey 1991:
Energy Report" (HMSO) (1996)
International Energy Agency (IEA) "Annex 14: Condensation and Energy:
Volume 1: Sourcebook" (IEA) (1991)
Adan O. C. G, "On the fungal defacement of interior finishes", PhD thesis,
Netherlands Organisation for Applied Scientific Research (TNO), (1994)
Arlian L, "Water balance and humidity requirements of house dust mites",
Exp. & Applied Acarology, 16, 15-35, (1992)
Ashmore I, "Asthma, housing and environmental health", Environ Health,
January 1998, 17-23
Bordass W & Oreszczyn T, "Internal Environments in Historic Buildings:
Monitoring, Diagnosis and Modelling", Report to English Heritage, October
(1998)
Boyd D, Cooper P, Oreszczyn T, "Condensation risk prediction: Addition of
a condensation model to BREDEM", Building Serv Eng Res & Technol,
9(3), 117-125, (1988)
BS5250, British Standard Code of practice for control of condensation in
buildings, British Standard Institute, (1989)
Bronswijk J, "House Dust Biology", Zoelmond, Netherlands, (1981)
-20
-10
0
10
20
30
40
50
60
70
80
90
100
0.4 0.6 0.8 11.2 1.4 1.6 1.8
Ventilation Rate (ach)
Inde x
Affordable Warmth Index
Fuel Rich MI
Fuel Poor MI
Linear (Affordable Warmth
Index)
Poly. (Fuel Rich MI)
Poly. (Fuel Poor MI)
Cunningham M J, "Controlling Dust Mites Psychrometrically - A Review for
Building Scientists", Indoor Air, 249-258, (1996)
Humfrey C et al, "Indoor Air Quality in the Home", MRC Institute for
Environmental Health Assessment A2, (1996)
Korsgaard J, "House-dust mites and absolute indoor humidity", Allergy, 38,
86-96, (1983)
Oreszczyn T, “Insulating the existing housing stock: mould growth and cold
bridging”, in Energy Efficient Building (Eds: Roaf S & Hancock M),
Blackwell Scientific Press, (1992)
Oreszczyn T and Pretlove S, "Condensation Targeter II", Building Serv Eng
Res & Technol, 20 (4) (1999)
