[time-nuts] Thermal insulation choice?

[Neville Michie]

"Insulation" of an object to reduce heat transfer has three main  
components.
Convection, the movement of air which carries heat from place to  
place, this is easily reduced by
small cell (less than 5mm) structures, below this size thermal  
convective circulation does not occur.
Conduction, this is reduced by making the cross-section of any solid  
material very small, the length great
and by choosing a low conductivity material.
Thermal Radiation is very significant, and as many materials are  
transparent to long wave (10 micron)
radiation it is important to design for it. A low emmissivity (very  
shiny) surface reduces radiative transfer,
but shiny surfaces usually tarnish with time. A non transparent  
barrier, like metal foil, will stop radiation,
but the foil will heat up and re-radiate. If you have a setup with  
two parallel metal plates, a certain
amount of heat is transferred which does not change with increasing  
distance. If you add an intermediate
plate, it will heat to half the temperature difference,and as each  
plate sees half the temperature difference
only half the heat is transmitted. So on to 10 layers where only one  
tenth of the heat is transmitted.
So if you want to use polymer foam, make it in thin layers with a  
very thin layer of foil between each layer.
Balsa wood sounds as if it could be good, because it will have  
distributed absorbers of radiation
throughout it, equivalent to many layers of foil, and its conduction  
is low because it is mainly air.
I wonder if TVB knows what the brown foam in 10811 is? Does it have a  
radiation absorbing powder in it?

cheers, Neville Michie