2. Some basic physics
• All about heat:
– There is no such thing as cold, just
differences in levels of heat
– Heat energy moves from bodies with
higher heat levels to bodies with
lower heat levels (which we inevitably
describe as “cold”)
=
3. Heat in the house
• Heat in buildings
– If our environment is at a lower
temperature than our bodies, heat flows
from us to it and we feel “cold”
– We heat the environment to restore a
comfortable balance
– The whole building now becomes warmer
than the external environment and heat
starts to flow out of the building
– Modern buildings are designed to minimise
this heat flow.
4. Two crucial considerations
• When considering heating and
insulating buildings, two points
must be addressed:
1. How does heat flow from one body
to another?
2. What is thermal comfort?
5. Heat flow
• Heat flows from one body to
another through three physical
processes
– Conduction
– Convection
– Radiation
• Insulation addresses conduction
and convection
• Radiation is more difficult to
control
6. Conduction
• Heat conduction occurs when a hot
body is in direct contact with a cold
body
• Energy is transferred until both are at
the same level of heat (temperature)
• Materials with high conductivity
– Metal, glass, ceramics, stone=bad insulants
• Materials with low conductivity
– Still air, wood, fabrics= good insulants
7. Insulating buildings materials
• Special materials are incorporated
in the building enclosure which are
good insulators (bad conductors of
heat energy)
• Invariably the fundamental
insulating material is stationary air
• Fibreglass quilts, expanded
plastics, foamed paints all rely on
the air contained in them to
provide the insulation
9. Measuring heat loss: the “U” value
Inside temp
10 OC
For a difference in
1m2 of wall temperature between
inside and outside of 1OC
The amount of heat
travelling through one
m2 of wall is the U value
Outside temp
9 OC Acceptable U values:
Walls - 0.35 W/m2 OC
Roofs - 0.16 W/m2 OC
10. Convection
• Fluids (liquids and gases) can
move internally within the body of
the fluid.
• Heating fluids makes them expand
and their density lowers
• Under influence of gravity, denser
fluid falls and less dense rises. This
movement is convection
• Air is a fluid…
11. Convection inside a wall cavity
Warm inner leaf causes
air to rise
Cold air drawn in from
below
Warm air drawn over to
outer leaf at top and
cools as it descends
Heat is transferred from
the warm inner leaf to
the cold external leaf
12. Convection inside a wall cavity
Warm inner leaf causes
air to rise
Cold air drawn in from
below
Warm air drawn over to
outer leaf at top and
cools as it descends
Heat is transferred from
the warm inner leaf to
the cold external leaf
Cavity insulation
stops convection
currents as well as
reducing conduction
13. Radiation
• Heat energy is carried away from a
hot body by infra red (IR) radiation
• All bodies above 0oK emit IR
• Smooth, light coloured surfaces
emit, and absorb, least IR
• Dark, matt surfaces emit, and
absorb, most IR
• Smooth, shiny surfaces may reflect
IR
14. Aluminium foil radiation control layers
• Foil layer on
warm side of
insulation acts as
reflector and
vapour barrier
15. Thermal comfort
Air temp (conduction)
Drafts (convection)
Wall, ceiling and floor temp (radiation)
Body
temp 37oC
Environmental temp slightly
below body temp most
comfortable. Why?
16. References
• Hall, F E. ; Greeno, R. (2010). Building
services handbook. Butterworth-
Heinemann
• Roaf, S. ; Fuentes, M. ; Thomas, S.
(2007). Ecohouse : a design guide. 3rd
ed. Architectural Press.
