R value?

Frank_3

There are three ways that heat can be transferred: conduction, convection, and radiation.

Conduction is the transfer of heat *through* a medium (gas, liquid, or solid). For example, if you have an electric stove and place a pot on the heating element, heat is *conducted* from the element to the surface of the pot because they are in close contact, and then again conducted from the outside of the pot to the inside.

Convection is the transfer of heat by the flow of a gas or liquid. For example, if you hold a piece of aluminum over a lit candle the heat is convected to the aluminum (carried by the flame).

Radiation is the transfer of heat from one object to another across an open space. For example, if you put a slice of bread in the toaster, the electric coils inside will radiate heat to the bread.

U-value is a measure of thermal conductivity -- that is, the ability of a medium to transfer heat through itself. R-value is the inverse of U-value. Think of it as a measure of thermal non-conductivity, or the inability of a medium to conduct heat.

You need different kinds of insulation to prevent the different kinds of heat transfer.

Fiberglass insulation is designed to limit the effects of *conducted* heat. It does this by creating thousands of air pockets seperated by strands of glass.

Reflectix is a product that is designed to limit the effects of *radiated* heat. The R-values that the manufacturer provides are not "real" R-values because it doesn't block the *conduction* of heat. The disclaimer is that it is an "effective R-value" when installed according to the manufacturers instructions and dependent on orientation (horizontal, vertical, or inclined).

If you take a jar of boiling water and wrap it with fiberglass insulation you can measure the surface temperature of the insulation and compare it to the surface temperature of the uninsulated jar. That will show you the effects of R-value -- resistance to heat conduction.

If you take that same jar and wrap it with Reflectix, then measure the surface temperature of the Reflectix you won't see as much of a difference because a) the only *conductive* insulation is the small air gap between layers of aluminum foil, and b) the aluminum foil does a great job of holding heat and you're measuring the surface temperature of the aluminum.

So, what it comes down to is that you're comparing apples to oranges -- fiberglass insulation (a conductive barrier) .vs. Reflectix insulation (a radiant barrier).

If you don't mind telling us, Danielle, what is your science project attempting to prove?

Danielle

R- value

Hi I am a student doing a science project. What does R value mean? I used r4.2 and r5.6 to insulate a jar of water. The r4.2 did better. The r5.6 was refectix foil. Does that need 1/2 inch of air space to be truely R5.6?

thanks

Eric

R value

Not sure of your level but, here are basics.

R- Value stands for resistance. Resistance of heat transfer from cold to hot. Higher R the better the insulator.

R-Value has units of hr-ft^2-F/Btu.

TFor conduction, the sum of 1/R1 +R2+ R3)of a system is call U or the overall heat transfer coefficient. Q=UAdelta T: Q is heat rate in Btu's/h: A is area: and delta T is temperture difference.

Remember there are 3 main types of heat transfer.
Conduction (contact of surfaces (uses R value)
Convection (moving fluids)
Radiant ( the sun/hot surfaces)

Bill_3

Cold to Hot

I was taught Hot always goes to cold. I think it has something to do with 2nd law of thermodynamics

Frank_3

Short essay on heat transfer...

There are three ways (or modes) that heat can be transferred: convection, conduction, and radiation.

Convection is the transfer of heat by the flow of a gas or liquid.

Conduction is the transfer of heat *through* a medium (gas, liquid, or solid).

Radiation is the transfer of heat from one object to another across an open space.

For example, take a blow torch and apply heat to one corner of a 1 foot square piece of copper. The heat starts at the blow torch and is *convected* by the gas flame to the copper sheet. It is then *conducted* from the corner throughout the sheet. If you hold your hand near the copper -- without touching it -- the heat is *radiated* to your skin.

U-value is called the "heat transfer coefficient", or a measure of how well heat can move from one place to another. Higher U-values indicate a greater ability to transfer heat. R-value is the inverse of U-value. That is, it's a measure of how well a medium can resist the transfer of heat. Higher R-values indicate a greater ability to resist transfer of heat.

Some insulations are better at resisting certain modes of heat transfer than others.

Aluminum is a wonderful *conductor* of heat. If you apply heat to an aluminum sheet the heat will move quickly throughout that sheet. (Say that 5 times fast.) However, aluminum is miserable at *radiating* heat. It has a low "emissivity", a measure of heat radiation (or emission).

Reflectix is designed to resist the *radiant* transfer of heat. Since radiant transfer occurs across an open space, Reflectix needs to be placed in that space -- between objects that are transferring heat radiantly -- and not in contact with the heated objects.

So, the short answer is yes, you need an air gap for the Reflectix to be effective.

Hope that helps. Let us know what the science project is trying to demonstrate and maybe some of us here can give you other ideas that you could find helpful.