# K (thermal conductivity) value question.

1. Jun 12, 2006

### lawtonfogle

Does the k value have any relationship with the temperature of the wall (use of the word wall becuase that is what the k value is mostly used with).

2. Jun 12, 2006

### Mech_Engineer

Almost all materials' thermal conductivity coefficients are temperature dependent (I say almost to avoid absolutes, although I don't think I have heard of any materials that aren't). The thermal conductivity you can look up on www.matweb.com is usually at room temperature (25 degrees C), but there are exceptions (Some stainless steels, for example, may have a few tempertuare values listed). Heat transfer textbooks will tend to have temperature dependent properties of solids listed in their appendices.

Usually, a simple problem will approximate the conductivity coefficient as constant, but several highly complex problems I have solved in FEA must take this into account, especially with thermal conductivity, and electrical resistivity. Is there any particular material you are inquiring about?

As a point of fact, almost all material properties are to some extent temperature dependent.

3. Jun 12, 2006

### lawtonfogle

Concrete, and how does it change when heated. I need the information for a simulation program, but all I have is the normal k value.

4. Jun 12, 2006

### Staff: Mentor

If you are just interested in calculating building heating/ac loads, you can consider it constant.

5. Jun 12, 2006

### Gokul43201

Staff Emeritus
What kind of accuracy do you need to specify k to (allowed % error) and what temperature range do you expect the wall to experience?

Yes, in general there is a dependence, but it may be small enough that you don't have to worry about it (but not if you are doing a very precise calculation and your tolerances are 0.01% or somesuch).

6. Jun 13, 2006

### lawtonfogle

The wall will be experencing a temperature range (in F) from no less than 55 to no more than 110. Mainly, I need to calculate is the difference enough to try to heat the concrete as an effective way to keep warmth in a house, and so I am looking for the equation to see if there is.

7. Jun 13, 2006

### Mech_Engineer

Those are pretty small temperature changes in engineering terms. You should be able to use just one value for the conductivity.

8. Jun 13, 2006

### Staff: Mentor

AFAIK, ASHRAE doesn't even list multiple k/r/u values for HVAC purposes, and the software I use doesn't calculate it. You may want to try it.... www.elitesoft.com