Why is air not the best insulation for homes?

[velociraptor]

So I've been curious about thermal conductivity lately, and came across a good reference that showed the conductivity of various materials.

http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

It says that air has very poor thermal conductivity (in other words, it acts as an insulator), and is actually so bad that it is the 6th worst on the list. Also on the list were various 'insulators'. Fiberglass, wool insulation, kapok insulation, etc. Now for my question. If air were truly such a great insulator, then why do home builders blow insulation into the walls and in the attic? The whole purpose of insulation is to completely decouple the home temperature from the outside temperature, correct? According to this list, air would do a better job than anything they blow into the walls. What gives?

 

[russ_watters]

Air is a good insulator, but with one caveat: it has to be stationary to be a good insulator. If you have an empty gap between an inside wall and an outside wall, instead of heat flowing via conduction out from the inside wall to the outside wall, the air flows up the inside wall and back down the outside wall in a convection loop.

The purpose, then, of all types of insulating media is to stabilize air while providing as little of their own matter as possible to conduct heat.

 

[velociraptor]

Thanks for the quick response. I guess the same can be said for water too then, right? It's only a decent insulator if it's stationary. Is there any way to measure what the effective 'thermal conductivity' would be for non-stationary air? How much heat can moving air/water transfer?

I'm interested in this for another idea that's been floating around my head. If you apply a very cold plate to a closed container of liquid (say a can of soda), will the cold plate form convection currents in the liquid inside, thus allowing the liquid to cool evenly? Would there be a better way to transmit the cold from the plate into the center of the can?

 

[GT1]

Thanks for the quick response. I guess the same can be said for water too then, right? It's only a decent insulator if it's stationary. Is there any way to measure what the effective 'thermal conductivity' would be for non-stationary air? How much heat can moving air/water transfer?

I'm interested in this for another idea that's been floating around my head. If you apply a very cold plate to a closed container of liquid (say a can of soda), will the cold plate form convection currents in the liquid inside, thus allowing the liquid to cool evenly? Would there be a better way to transmit the cold from the plate into the center of the can?

The "thermal conductivity of non-stationay air" is called the convection heat transfer coefficient. It depends on a few factors such as : the air velocity,the object geometry, the air properties etc.

 

[russ_watters]

Thanks for the quick response. I guess the same can be said for water too then, right? It's only a decent insulator if it's stationary.

No, the thermal conductivity of water is much greater than for air. It is never a good insulator. Density really is a big factor.

Is there any way to measure what the effective 'thermal conductivity' would be for non-stationary air? How much heat can moving air/water transfer?

Sure, the measuring is easy. Trying to calculate it from scratch is what is hard. In my job (heating and air conditioning engineer), I do the measurements all the time.

I'm interested in this for another idea that's been floating around my head. If you apply a very cold plate to a closed container of liquid (say a can of soda), will the cold plate form convection currents in the liquid inside, thus allowing the liquid to cool evenly?

There will be convection and that will help make the transfer even, but it won't be completely even.

Would there be a better way to transmit the cold from the plate into the center of the can?

Well - a heat sink in the middle of the container or a wrap-around heat sink.

 

[adambeazley]

When your talking about insulation, you have to think about the three different types of heat transfer: conductive, convective and radiant. Although air is a poor conductor, the moisture in the air can make it move convective heat very efficiently. Then you have radiant heat which is the most efficient type of heat transfer, which is more of a magnetic ray that move through air regardless of the motion.

A http://www.raflect.com" is very effective at stopping radiant heat transfer as is a r39 insulation.
http://www.ornl.gov/sci/roofs+walls/insulation/ins_01.html" are used to stop conductive heat transfer.

 

[pallidin]

Humidity can play a significant role in the quality of insulation. The drier the air within and surrounding the insulation, the better.
Home construction techniques to reduce humidity from reaching high levels within and around insulation is often employed, so browsing web-sites involving do-it-yourself home insulation can be a valuable resource.