# Formula for R-Value?

| Posted in General Questions on

This is probably a really stupid question, but is there a way to calculate the temperature of air after it has passed through insulation?

For instance, if it is 20 degrees outside, and I have a R-30 wall, what would be the temperature of the air that is on the other side of the insulation?

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### Replies

1. | | #1

Well first off, air should not be moving through your walls, seal them up tight. Second the temperature of the air on the inside will be whatever your thermostat is set at, +/- a couple of degrees on the wall surface.

Insulation does not heat or cool the air, that is the job of the furnace and A/C unit. Insulation only slows the transfer of energy. So if you were to loose power it will take much longer for the temperature to drop to an uncomfortable level in your home.

2. GBA Editor
| | #2

J.S.
Q. "If it is 20 degrees outside, and I have a R-30 wall, what would be the temperature of the air that is on the other side of the insulation?"

A. Under steady-state conditions -- for example, if the weather stays at 20 degrees (and cloudy) for a week -- the temperature on the other side of the insulation will be 20 degrees.

The temperature on the other side of your wall will only change if there is a source of heat (solar gain through windows, a television, or a furnace, for example), and a fairly airtight enclosure to hold the heat indoors.

3. | | #3

Yes, there are formulas you can use... try something like this http://www.thermalcalconline.com/

4. | | #4

Thank you all!

5. | | #5

I'd be much more concerned about the air temperature on the inside of the window areas, presuming the space has some. R30 will be pretty cosy by comparison.

6. | | #6

Q= - KA(T2-T1) where Q=heat rate btu/hr
K=conductivity btu/hr*ft^2*F (1/R)
A=area ft^2
T2= outside temp F
T1=inside temp F

note the negative sign in front of the K. If the answer is positive the the heat direction goes from 1 to two, if negative,heat moves from 2 to 1

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