Mechanisms of Heat Transfer in Human Body and Heat Loss in Uninsulated Brick Veneer House

This article discusses the mechanisms of heat transfer in the human body, the main paths of heat loss in an uninsulated brick veneer house in Sydney, the calculation of thermal conductance across a cavity wall, and five strategies for passive solar cooling in hot dry climates. It also describes how angled louvers blades can be designed to manage the entry of direct sunlight into a building in a location like Sydney.

SOLUTIONS TO THE QUESTIONS
QN 1: The ways that the human body gains or losses heat to its environment and explanation
how these mechanisms work:
ANSWER:
The following are the ways in which the human body either gains or losses heat (depending on
the environment)
Normally if too cold, the body will want to gain heat and more heat from the surrounding and
when too hot, the body will want to lose more heat so as to cool the body.
(i) Radiation
This is the mode of heat transfer in which the body either gains or losses heat to the
surrounding.
(ii) Evaporation
This is facilitated by sweating in hot and humid weather such that the liquid (sweat)
on the skin turns to vapor hence causing a cooling effect on the body.
(iii) Convection
This is the process in which air currents flow over the skin and with it takes away
some heat from the body surface. By moving around, the rate of heat loss through
convection increases.
(iv) Conduction
This is such that whatever the body is in direct contact, it either loses or gains heat.
For instance, in the figure below, the person is seated on a stone hence heat from
his body is lost to the stone via conduction. Sitting establishes direct contact of the
body with the stone.

QN 2: For an uninsulated brick veneer house in Sydney in winter, description of the main paths
of heat loss to the outside air
ANSWER:
The house loses heat from different paths from the interior. Mainly the major losses are
realized via the openings such as doors, windows and roofs as shown in the diagram. The
arrows show the direction of heat loss.

QN 3: Calculation of the thermal conductance (U value) across a cavity wall consisting of:
 Outer skin 110mm brickwork (South facing, exposed aspect
 30mm cavity
 Inner skin of 90mm studwork with 10mm plasterboard
ANSWER:
.
Discuss whether or not cavity brickwork is a thermally efficient wall for housing in Australia.
Inner skin
Studwork material
c
a
v
i
t
y
Outer skin
Brickwork
material

90mm 30mm 110mm
Given the following:
Kbrickwork= 0.7W/m-k
K cavity= 0.07W/m-k
Kstudwork= 0.375W/m-K
Assuming the surface area A= 0.04m2 for all material layer
Firstly, the resistance is calculated for each material:
For inner skin, Ri= L/RA= 0.09/0.375x0.04= 6.00
For cavity, Rc= 0.03/0.07x0.04= 10.714
For outer skin, Ro = 0.11/0.7x0.04= 3.929
Total resistance Rt= 6+10.714+3.929= 20.643 (since they are in series)
Hence the overall conductivity U= 1/Rt= 1/20.643= 0.04844m/m2-K
Based on the U-value obtained above and in comparison with the example, it can safely be said that
brickwork is a thermally efficient wall for housing in Australia especially in Sidney.
QN 4: Description of five strategies for passive solar cooling in hot dry climates
ANSWER:
The following are the major strategies that are used in passive solar cooling:
(i) Insulation
This is done for both wall and roof insulations. They provide a building with thermal
insulation envelope against the outside heat. This boosts the indoor air quality and
thermal comfort. The use of insulation materials such as polymeric membranes (with
low conductivity value) would often resist UV irradiation.
(ii) Double glazed windows

This provides heat barrier against which too much heat gain or loss from the indoor
is regulated. The cavity is often filled with argon gas to further provide heat
encapsulation.
(iii) Natural Ventilation
This is facilitated by the wind catcher which ensures healthy indoor environment is
established. It cools and freshens air hence encouraging cross ventilation especially
in the wet areas that may require sufficient aeration (free natural air circulation)
(iv) Use of horizontal louvers
The horizontal louvers prevent direct sunlight in summer hence reducing solar heat
gain in the room. They will only allow light to penetrate but prevent unnecessary
high amount of heat from being transferred.
(v) Use of reflective materials
Radiation losses are minimized using reflective materials which may basically
compose high reflective colors coating for walls and roof. Additionally, green roofing
can also substantially lower the amount of heat loss and gain.

QN 5: Description of how angled louvers blades can be designed to manage the entry of direct
sunlight into a building in a location like Sydney
ANSWER:
The angled Louvre blades are designed by having them being adjustable based on the
orientation of the direct sunlight rays. As the sun rises in the morning, the rays tend to
penetrate the building with changing angle of attack. The louvers would mimic the movement
of the sun such that their angles are automatically adjusted as the direct sun gradually moves to
the west as shown in the figure. However, the design must not seriously affect the natural
ventilation and air exchanges between the interior and exterior of the building. The following
parameters are often critical in designing the blade Louvre:
(i) Blade height
Mostly designed in the range of 100 to 150mm
(ii) Blade length
Any length as long as performance is optimized
(iii) Blade thickness
6mm is the optimum thickness for glass material.
(iv) Blade strength
Different materials would exhibit different strength characteristics. For glass, it may
be fragile. However, as thickness is increased so is the strength especially against
horizontal pressure and impact loading.

Reference
(Used class lecture notes and modules provided)