Building Layout & Design: Passive Solar Design: Thermal Mass.
A material acts as thermal mass when it is capable of absorbing, storing
and emitting solar heat. The mass will absorb heat from the air during
the day and when the air temperature cools during the evening, emits
heat back into the room. During the cool months the thermal mass shall
be exposed to the sun, whilst in the hot months it shall be shaded from
the sun. Deciduous trees and awnings designed for the local climate
can provide shading for this.
Thermal mass requirement is a relationship between the internal air
volume and the surface area and heat storage capacity of the mass that
is accessible to the surrounding air.
Thermal Mass of Materials
|
|
|
Thermal Mass or Storage Capacity (kJ/m3K)
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Ideally thermal mass should be heavy, dense and dark. If a material
has no thermal mass, then the heat will be lost almost instantaneously.
Note that concrete and stone have a similar storage capacity, but as
concrete also has a high-embodied energy, local stone has been selected
for the Newton House.
An extensive list of raw element properties is found at http://fridge.arch.uwa.edu.au/materials/index.html.
Thermal mass assumes that the mass of a material is great enough to
dampen out daily temperature fluctuations. To assess this it is necessary
to understand time lag of thermal mass. This is the delay between the
external maximum or minimum temperature, and the internal maximum or
minimum temperature. High thermal mass walls can significantly out perform
low mass walls of the same R-value when outdoors temperature fluctuates
significantly on a daily basis. Insulation further reduces the daily
fluctuations significantly.
Time Lag for various materials.
|
Material and Thickness of Material
|
Time Lag (hours)
|
|
Concrete 250mm
|
6.9
|
|
Double brick 220mm
|
6.2
|
|
AAC 200mm
|
7.0
|
|
Adobe 250mm
|
9.2
|
|
Rammed Earth 250mm
|
10.3
|
|
Compressed earth
blocks 250mm
|
10.5
|
|
Sandy Loam 1000mm
|
30 days
|
|
Clay Earth covered
building 2000 - 5000mm
|
65 – 165 days
|
A bed of stone on the floor in the bedroom follows the line in which
the sun penetrates during winter. There will be no sun reaching this
mass from the 22 September till 23 March due to the selected awning
width of 1.2meters.
Links
http://www.greenbuilder.com/sourcebook/PassSolGuide1-2.html
http://www.greenhouse.gov.au/yourhome/technical/fs17.htm
