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Guru

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Insulation Material

11/26/2011 7:14 PM

Why people need to use rockwool or similar material for heat insulation, why not just use air ? Isn't the insulation property of rockwool due to the air gap trapped inside ? If so, just simply using a few inches or air gap is far cheaper and easier in air-con ducts and wall.

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#1

Re: Insulation Material

11/26/2011 7:32 PM

You can't trap air in the walls of a house, or it would already be the standard. Insulation slows it down.

Do you think your idea is revolutionary? Look around you.

Al you have to do is build a vacuum bottle around your house.

Do a little research first.

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#2

Re: Insulation Material

11/26/2011 8:16 PM

"Open air" allows convection currents to occur; rockwool or other cellular material inhibits this. It isn't just the air trapped inside; it is the disruption of convection within that air.

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Guru

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#3
In reply to #2

Re: Insulation Material

11/26/2011 8:49 PM

GA, unlike Lyn, most of the time you provide good answer minus the the sand and dirt. LYn always sounds as if someone always step on her toe or tail.

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#5
In reply to #3

Re: Insulation Material

11/26/2011 11:29 PM

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Guru

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#11
In reply to #5

Re: Insulation Material

11/28/2011 5:13 AM

"LYn always sounds as if someone always step on her toe or tail."

Is it correct? are you lady?.Lady Engineer?.

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Anonymous Poster #1
#19
In reply to #3

Toe/Tail Material

11/30/2011 5:48 PM
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Commentator

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#10
In reply to #2

Re: Insulation Material

11/28/2011 4:44 AM

To chip in on the answer from Tornado which already covers the specific case and generalize a bit: There are three ways of heat transfer: - conduction (two objects in contact exchange heat until reaching the same temperature)

- convection (a fluid changes density with temperature and creates a draft, transporting the heat faster than pure conduction)

- radiation (any body at above absolute zero emits energy by (infrared) radiation)

In order to optimize insulation properties the sum of these three phenomena has to be minimized.

you are correct, that the conduction in air is poor. However, that is only oe of the three terms of heat loss. Tornado already points out correctly that in the hollow wall case you would increase convection considerably.

An interesting case study of these phenomena are modern windows: If you check a modern window with two or three panes you will see that they are spaced somewhere around 1-2 cm apart. A distance around that gives a good conductive resistance but still limits convection due to the limited space for a draft to form. You also see that in very well insulated windows, 3 panes are used. That way the conductive resistance through the double air layer is doubled without giving the air more possibility to move around.

Finally, there is the radiation term which is often overseen. A double pane window would traditionally let pass around 3W/m2/K. However, most of this heat loss is due to the infrared radiation through the window. Modern windows have low enissivity coatings that reflect the infrared radiation back to the inside of the building. That way these windows come down to about 1W/m2/K.

I hope this example serves you to understand the importance of all three contributions to heat loss a bit better.

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Guru

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#18
In reply to #10

Re: Insulation Material

11/28/2011 2:37 PM

GA, thx

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#4

Re: Insulation Material

11/26/2011 11:23 PM

It's not just the trapping of air that makes a good insulator. It is also the average size of each trapped cavity and the combined lightness and thermal conductivity of the material creating the trapped cavity pockets. This combination of lightness and tiny pocket size is why aerogel materials are both known as solid smoke and such an incredible themal insulator.

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#6
In reply to #4

Re: Insulation Material

11/27/2011 11:06 AM

I had this strange image come to mind... Would a stack of parallel plates have different insulating characteristics, depending on whether the plates are vertical or horizontal?

Question:

If there is a difference, can I exploit it to make a perpetual motion machine ;^)

{sorry}

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Guru

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#12
In reply to #4

Re: Insulation Material

11/28/2011 5:48 AM

Aerogel web site is interesting. Is it popular in U.S? How it compares in price and quality compared to other insulation materials?.Thanks for information.

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#16
In reply to #12

Re: Insulation Material

11/28/2011 7:44 AM

It depends on the application. Aerogel is popular in many scientific instruments where extreme temperature differences are found; cryogenics, kilns, submarines, etc. There are companies here that are developing aerogel insulation for domestic and industrial use. But I would not hazard a guess if this is considered popular.

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Guru

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#7

Re: Insulation Material

11/27/2011 11:16 PM

Insulating material, through its structural arrangement, is providing very many, points of "random" refraction and reflection, that any radiated infra red energy must encounter and pass through before complete penetration. The refraction occurs at every air/material different density junction and reflections occur wherever the junction is met at critical incident angles.

This effectively lengthens the propagation distance and the IR path is attenuated accordingly

That takes care of the radiated heat.

Insulation is made of poor thermally conductive material. Thicker has more thermal resistance than thinner. The "random" structure and connections of the insulating material also increase the conduction length.

Steel wool would not be a good choice as an insulating material.

If an aluminium sarking is fitted as well, this will reflect much of the radiated heat. The insertion loss of aluminium sarking is quite high. It will warm up and become a pre-attenuated source of radiation but then the insulation's properties take care of that as before.

That's how I reckon it works and you won't have me using an air blanket anytime soon.

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#23
In reply to #7

Re: Insulation Material

02/04/2013 5:55 AM

"Insulation is made of poor thermally conductive material".. That is a misconception!!!

Rockwool is made of rock fibers which is purely conductive and the insulating property is just because of it's air trapping ability by using the bonding resins.

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#25
In reply to #23

Re: Insulation Material

02/04/2013 10:06 AM

Air is the poorly conductive material. It is important to have the air pockets small enough to not induce circulation currents of convection.

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#8

Re: Insulation Material

11/27/2011 11:38 PM

well! You are forgetting about the problem of convection, which will also transfer heat eventually. Materials such as rock wool, etc., prevent the convection currents from developing and make the air a better insulator. Moving air causes heat transfer, which one does not want in insulation.

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Guru

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#9

Re: Insulation Material

11/28/2011 4:28 AM

If bulk air is such a good insulator why does the room air cool down in the first place?

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#13
In reply to #9

Re: Insulation Material

11/28/2011 6:19 AM

Hello Bioramani, The first part to your question is replied to by all the answers stating convection as an important source of heat transfer. The second part of the answer is that you must not forget that air has a low specific heat and since its density is low there is very little energy trapped in a volume of air. Thus draining even little energy out of a volume of air will affect its temperature quickly. The volumetric heat capacity of air at sea level is about 1.3kJ/m3/K only. A stone of the same size would probably require about 2000 times that energy to change its temperature by a degree. Therefore air is a poor medium for storing heat and consequently cools down quite quickly.

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#14

Re: Insulation Material

11/28/2011 6:58 AM

The gorilla in the room is Radiation. At room temperature, more heat is transferred by radiation than by conduction. The above discussion of radiation is limited to mostly radiation and reflection. It has left out the effects of absorption and re-radiation with concomitant conductive thermal equilibration with the surrounding mass of air. Putting all this together is complex but effective. Fine steel wool has good insulation characteristics but poor building characteristics, namely it oxidizes, both burning and rusting in contact with moisture.

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#15

Re: Insulation Material

11/28/2011 7:16 AM

In addition, the air blanket around any solid object is created by the energy equilibration between the molecules of air and the surface of the solid object. The air within the distance of the mean free path of the air is slowed down by sharing its momentum with the solid surface. Since the solid object is not moving, the air is "stuck" to the surface. This effect is good for approximately one to two centimeters. Thus the air inside a blanket of glass wool can be blown out by a pressure differential, but remains essentially motionless inside a container, so sealing the wall is important.

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#17

Re: Insulation Material

11/28/2011 10:30 AM

In looking at insulation values for IGUs(Insulate Glass Unit). The pane of glass to the exterior of the home cools. This cools the air close to the glass inside the seal unit. Cold air drops. The interior pane warmer heats the air the warm air rises. You start to get circular motion. The greater the circular motion(convection) the faster the the heat transfer. This motion is greater the farther the the panes are away from each other. As in a wall the distance is great in compared to an IGU. The insulation retards the convection. In an IGU we use gases heaver the air which retards the convection. Little hard to see through insulation.

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Anonymous Poster #2
#20

Re: Insulation Material

12/01/2011 11:17 PM

sheet rock is sometimes used as a fire barrier

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Anonymous Poster #2
#21
In reply to #20

Re: Insulation Material

12/01/2011 11:18 PM

please ignore

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Anonymous Poster #2
#22

Re: Insulation Material

12/05/2011 6:40 AM

air will move in or out of wall cavities as baromatic pressures change. this will produce moisture intrution that can only be stopped if the walls are absolutely air tight. this is not a practical solution.

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#24

Re: Insulation Material

02/04/2013 9:54 AM

Greetings.

The reason that I use rock wool is that it starts to disintegrate at 1,800 Degrees F, if I remember correctly, and does not add to the fire or flame.

I have been told that fiberglass insulation starts to burn at 900 Degrees F and then adds to the flame.

Had I known this before the Lord and I built my house I would have put Rock Wool in it instead of fiberglass.

In 1999 I put in LP Gas Parlor Stove and a LP gas in line on demand hot water heater and wrapped the exhaust pipes through the walls with rock wool. It was passed by the building department.

I have been told on numerous occasions that is why when a house fire gets to a certain point that the fire fighters will pull back because the insulation in the attic will flash and explode because the attic has hit the 900 Degree F temperature on all of that open surface. Maybe that is why it seems that they spend a lot of effort cooling the roof. Don't know. Not a fire fighter never wanted to be one.

Have a Great Day,

Oly

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