a.aa MMC M11 52pp Web

Article
Managing
heat, air &
moisture
movement
in modular
buildings
For modular buildings to be energy efficient, healthy, and
moisture free what is required is a holistic approach to a total
system which manages the balance of Heat, Air, Moisture
Movement (HAMM), considering an integrated approach to
airtightness, insulation and condensation control.
Buildings with very low rates of air leakage
require correspondingly higher levels of
ventilation as part of a balanced, design
approach. It’s important to bear in mind that
ventilation is controllable, and therefore can be
accounted for within the overall design, whereas
uncontrolled air leakage is not.
The incorrect specification or installation of
effective thermal barriers will lead to unmanaged
heat loss, impacting directly on the energy
efficiency of the building and its systems.
Airtightness and vapour permeability
Air movement is important in the building
envelope both infiltration and escape. We need to
control interior conditioned air (whether heated or
cooled) escaping and exterior air infiltrating that
puts more pressure on heating or cooling
mechanisms internally. Airtight membranes are
key in this area whether vapour and air
open/closed or variable.
Moisture vapour will pass through the various
layers of any construction by both convection and
diffusion. The objective is to ensure, by design, that
the moisture vapour can disperse to the outside
atmosphere without being cooled to below
dewpoint temperature, thus eliminating
condensation and associated problems such as
mould growth.
To avoid the occurrence of excess condensation,
which can result in mould growth and damage to
the building fabric, designers should assess the
amount of water vapour likely to be generated
within the building and determine the resultant
increase in internal vapour pressure above that of
external air. They should then consider the
physical properties of the construction separating
inside from outside.
Why airtightness is crucial to modular building
design
There is absolutely no question that an integral
part of modern building design is influenced by
energy efficiency. In the EU it is estimated that
buildings account for approximately 40% of
energy consumption and are responsible for some
36% of CO2 emissions. Closer to home, around
45% of UK CO2 emissions come from the built
environment, (27% from domestic dwellings and
18% from non-domestic).
Airtightness improves energy efficiency
As thermal insulation requirements have
increased over the last few years, the proportion of
energy lost through air leakage has become more
evident. The ever-increasing thermal insulation
required will, however, be rendered largely
ineffective unless the airtightness of the structure
itself is addressed. Air leakage greatly reduces the
effect of thermal insulation; therefore if energy
efficiency is to be improved within buildings, this is
the most critical area to focus on.
In addition to improved insulation, energy efficient
heating systems will also be ineffective if warm air
can escape the building and cold air can seep in.
This is reflected in the fact that total space heating
costs in an airtight construction may be
considerably less than in a leaky one.
Air leakage through cracks, gaps, holes and
improperly sealed elements such as doors and
windows can cause a significant reduction in the
performance of even thermally insulated
envelopes. Architects are increasingly turning to
air barrier membranes as an essential part of the
design process in achieving the most effective
means of controlling and reducing air leaks.
In terms of the energy efficiency of a building,
uncontrolled air flow will almost certainly have a
major impact. Initial heat load calculations for
heating and cooling equipment will usually make
an allowance for a level of natural infiltration or
uncontrolled air flow. The higher the infiltration
rate, the lower the energy efficiency of the building.
Efficiency levels can be affected by both natural
and mechanical air movements. The forces of wind
and stack effects will lead to a level of air
infiltration and subsequent efficiency loss. Sealing
the shell of the building and any un-designed
holes can reduce the impact of wind and stack
effects and improve the overall energy efficiency.
Airtightness protects building fabric and reduces
maintenance costs
Unmanaged or uncontrolled air flow will act as a
carrier for moist air, drawing it from outside in, or
pulling it from inside out, into walls, ceilings, and
roofs. The impact of uncontrolled moist air
movement can have a long-term detrimental
effect on the durability and life of the building. This,
in turn, can lead to:-
• Decay of organic materials such as timber
frames.
• Saturation of insulating materials, thus reducing
their insulative effect (further increasing heat
loss).
• Corrosion of metal components.
• Frost damage where moisture has accumulated
on the cold side of the insulation
32 Feb 2019 M11