WHY VENTILATE A BUILDING?
For each room of a house you should ask: why is ventilation needed in this
space? Three possible reasons are:
• for fresh air supply
• for direct comfort ventilation to cool or heat the occupants of the space
by convection
• for indirect comfort ventilation, for heating and cooling the actual structure
of a building to indirectly enhance the comfort of the room’s occupants
and to use ‘free energy’ more efficiently. In this way, day-time solar
warmth can be stored in the structure and used at night, or coolth from
night air can be stored to cool the people indoors during the day.
FRESH AIR
The need for fresh air ranges from the nominal amounts needed for breathing
(2 litres s1) to the much higher ventilation rates necessary to control odours
(up to 16–32 litre s1 is a commonly quoted figure for fresh air needed to mitigate
the effects of smoking smells). In houses, care can be taken to provide
zones for smokers where the smell of a cigarette can be dealt with by opening
a window. This may be more easily said than done. Studies have shown that
homes with smokers use significantly more energy to heat or cool because the
windows are continually opened to get rid of the smell. This should be taken
into account and the problem designed out by sensible zoning of spaces.
How the room is furnished will affect lingering odours, with smells collecting
more easily in the soft fabrics of curtains and carpets. Different activities
in the house may be associated with different smell levels, so finishes can
be chosen appropriately. The need for fresh air can be influenced very much
by the chosen room finishes. It should be noted that building regulations are
typically not designed to mitigate against the impact of smells or the transmission
of diseases. It is not possible to regulate against air-borne diseases,
although many indoor air quality regulations have evolved to deal with condensation
risks in buildings that do have indirect health implications.
Fresh air is also needed to prevent the build up of moisture in a room. This
is obvious for kitchens, bathrooms and utility rooms but there can be a real
build-up of moisture in bedrooms as well. There are six ways to design out
moisture as a problem in housing.
1 Provide wet zones in the house outside the main envelope of living rooms.
Build a front porch/lobby/air lock in which the temperature of the outside
air can be modified to be warmer or cooler before it enters the house. All
wet clothing, coats and shoes can be left there, so keeping a great deal of
moisture outside the house on wet days.
2 Build an outdoor drying space where clothes and bath towels can be left
outside the main body of the house in winter. Wet clothes are a main
source of excessive moisture in winter.
3 Build a high-level vent window above the kitchen stove that can be easily
opened to immediately remove the hot, wet air generated by cooking. A
good, non-cold-bridging window is much better than a mechanical vent
above the stove as metal vents across a wall act as one large cold bridge
and will cause condensation to collect in the wall at one of the wettest
points in the house.
4 Bathrooms should either have a window that can be opened after a bath or
shower or a very good passive stack outlet that will carry the moist air out
of the room, which may, or may not, have a small fan to assist its draw.
5 Design out cold bridges from the walls of the house.
6 The wall-surface finishes of rooms should be chosen to be capable of
absorbing some moisture. Where possible use an organic water-based
paint on walls. These can range from the traditional white-washed walls
and natural wood products to the use of modern, sophisticated, waterbased
paint products. The room finishes can play a significant part in controlling
moisture build-up in the room as well as smell.
A common-sense approach is needed to estimate the best ventilation strategy
for a particular building. For example, a high-volume low-occupancy house
with all the above features and wet-plastered high-mass walls or exposed
timber walls with a natural finish could, on some days, get all the fresh air
necessary from people intermittently opening doors.
However, a low-mass, small-volume, high-occupancy room with hard finishes
may need significantly more. For a well-designed house, even with low mass,
hard finishes and normal occupancy but where the moisture problem has
been removed from the interior of the house to a wet zone, most problems
of air quality will disappear when the air change (ac) rate is 0.2 ac h1. That
means that one-fifth of the air of a room is changed every hour. Humidity
control can be achieved with a rate of 0.3 ac h1 or more (Marshall and Argue,
1981). Actually, this level of air change can be achieved almost with dooropening
air intake only. In many houses the air leakage rate through the
structure will be of this order. Robert and Brenda Vale recommend an air
change rate of around 0.45 ac h1 as acceptable but, again, there is little reason
not to design to the lower levels (Vale and Vale, 2000). Attention should
be paid to houses where radon or carbon monoxide problems from open
fires may exist, see Chapter 6.
DIRECT COMFORT VENTILATION
The comfort and thermal delight of the occupant is what makes a great
house (Herschong, 1979). Issues of comfort should certainly dictate how to
ventilate a building. If you think about times when you have been blissfully
comfortable in a house, the feeling is probably either associated in winter
with being near a warm radiant heat source, perhaps a fire, or in summer
being in a cooling breeze. Sue Roaf remembers a summer’s evening in
Baghdad, coming out onto the freshly watered veranda at around 7 p.m. with
an iced drink, wearing a cool cotton dress, sitting chatting in the early evening
breeze and thinking that ‘this is bliss’. Only on looking at the thermometer
was it seen to be 42°C! It had been almost 50°C all that day.
People acclimatise to ambient temperatures. How warm or cold they feel
depends on what the temperature has been over the last three or four days.
It can take two to three weeks to adapt to a whole new climate.
If it is too hot or cold people do something about it. They may put on
or take off clothes, they may change places within a room or move from
one room to another. They may open a window, close a door or take a cold
or hot drink. In extremes they may change buildings or even move to a
different region with a more pleasing climate. They adapt their circumstances.
It is only at the very extremes that people die of heat or cold. One
of the key strategies they adopt in adapting the building to improve the
indoor climate is to open a window to let in warm or cool air, another is to
go to sleep.
Passive building design is driven by the relationship between the outside
and the inside air temperatures. Michael Humphreys demonstrated this over
20 years ago with his classic diagram (Figure 5.6) showing that people wholive in hotter climates are comfortable at higher temperatures. In this book Lim).
we adopt his simple, but effective, equation to show, very roughly, at what
temperatures locally adapted people are comfortable:
Tc 0.534 (Tmean) 11.9
Where Tmean (Tmax Tmin)/2 and is monthly mean outdoor temperature; Tc is
comfort temperature; Tmax is monthly mean daily outdoor maximum temperature;
and Tmin is monthly mean daily outdoor minimum temperature. Tmin and
Tmax are usually available from Meteorological Office data.
This equation, strictly speaking, applies to summer conditions in freerunning
buildings (not air-conditioned ones) but gives a general idea of the
comfort conditions required indoors by locally adapted populations.
Where the comfort temperature lies somewhere between Tmax and Tmin in
a well-designed passive building with good levels of thermal mass and no
excessive solar gain it should be possible to open windows for comfort cooling
where Tmax is 35°C or below. Comfort warming from the window breeze
should be achievable in a good passive building when Tmax is over 22°C and
Tmin is over 15°C.
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