Message Board Thread - "Calculating Insulation U Values"

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Calculating Insulation U Values Bob Berry 11/23/2004
I am trying to calculate overall U Values for a building based on internal/ external temperatures.I know this should be fairly straightforward provided i take all variables into account. I need a formula to help with this, I have tried several, but none of them give me great confidence.
 
Re:Calculating Insulation U Values Gary Orlove 11/23/2004
Calculating U or R values based on temperature measurements can be difficult. I am not surprised the results you have obtained thus far have not given you a lot of confidence.

Please see my more detailed discussion of this at http://www.infraredtraining.com/community/boards/thread/1652/

Gary Orlove
Infrared Training Center

 
Re:Calculating Insulation U Values JKEngineer 11/24/2004
This is my third try at posting this.

In order to calculate the U for the wall assembly using a surface temperature, you have to break out the heat transfer from the wall surface to the surrounding air. It will follow the equation: q=h*A*deltaT, where h is the local heat transfer coefficient for the surface to air. It is dependent on air and wall temperatures, air velocity, humidity, wall orientation (if it is not vertical, e.g.). It can be obtained from literature correlations. Using it will allow calculation of q. For steady state, (see Gary's comments) q is constant through all parts of the heat transfer path. Using this derived q and the ambient conditions on each side of the wall will allow you to calculate the U for the wall.

I think you will find that the calculation is very sensitive to small variations in the measured temperature, especially at "reasonable" values for U.

HTH. If you would like to hire this work out -- see my contact info below.

I had tried including an image from this year's Inframation showing the thermal profile through a wall exposed to 24 hours of environmental variation, but I think it may be the reason I could not post, so I am leaving it out this time.

Jack
Jack M. Kleinfeld, P.E.
Kleinfeld Technical Services, Inc.
Infrared Thermography, Finite Element Analysis, Process Engineering

Bronx, NY 10463

718-884-6644
866-884-6644 toll free
212-214-0919 fax and voice mail

JKEngineer@aol.com or JKEngineer@KleinfeldTechnical.com
come see what we can do for you: http://www.KleinfeldTechnical.com
 


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