Hi There,

 i was wondering if anyone had any experience with in-situ u-value measurment of wall constructions.

I have recently moved into studying in this field, and have read that you can measure the u-value of walls by employing a heat flux sensor, as well as gauging the internal and external temperatures.

i understand this theory, but cannot find any "conversion?" as to how to derive the u-value once the heat flux and Temperatures are known?

would anybody be able to help?

MA

It's found in the very definitions. U-value is the heat-per-unit-area per degree-difference. Heat flux is defined as heat-per-unit-area, which you've measured, you've measured the difference in temperatures...

Hi MA, A couple of years ago I went looking for a simple version of what you are describing.  Most of what I found might have made sense 40 years ago, but was deeper than I wanted to go.  I settled for a chart I found with inside/outside temps leading to an estimated r-value.  As it has turned out in the field, that has been all I have needed, although I rarely use it. 

Here is a link to a paper from ORNL that may help or provide a reference that helps.
http://www.ornl.gov/sci/roofs+walls/whole_wall/rvalue.html
New to this board, so not sure if I posted that link properly

Good luck
Bud

Bud- it seems he's more looking at measuring existing structures (of possibly unknown or degraded construction), not in estimating U-values on new buildings. ( One good measurement trumps all calculations & estimates, eh? ;-) ) This can be useful for determining where retrofit money is best spent, and as a means of verifying construction performance without internal inspection after it's complete. Building retrofit is going to be key if total carbon emissions are going to be reduced worldwide- it makes sense that this sort of field survey is an up & coming field of endeavor. There's only so much you can get out of blower door testing and infra-red imaging.

Whether & when such a measurement is necessary or desirable is a bit of a puzzle though- the biggest & most-obvious heat loss issues are easily determined by other means.

When we have a building with custom curtainwall based on a perfrormance specification, the curtainwall manufacturer is often required to do thermal testing in a full size mockup that will typically be 1 or 1.5 stories tall and 20 to 30 feet wide.  They have big insulated chambers to do this as well as to test static and dynamic wind load and water leakage.

It has been a few years since I have been involved with one of these thermal mock up tests, but I think there is a defined ASTM procedure for it.....I will see if I can find the ASTM test number(s).  I do not remember any temp instrumentation other than thermocouples. 

We often undertake air and water leakage tests in the field, but I do not think we have ever tried an inplace thermal test or instrumented exterior/interior surface temps.  We typically do thermographic tests at the conculsion of a project.  The performance killers are usually details....especially atypical ones.

This heat transfer stuff gave me a headache in school 40 years ago.......and still does;-) 

Bruce

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I find it is a lot like the circus barker who can guess everyone's weight.  Understanding bridging and material r-values, then a quick inspection and plug in some numbers.  Get to that first run on the software and see how far off you are.  Make some adjustments and you're done.  After the first hundred audits, you find you are usually pretty close.

 

Of course that approach doesn't make RESNET happy, but it does tell the customer what they want to know.

 

If you are familiar with the conversion from CFM50 to natural air leakage, the Lawrence Berkeley Laboratory went to the effort to establish a more accurate conversion chart.  Looks great until you read that the Lab gives their own number a minus 50% plus 100% accuracy rating. 

 

I don't look forward to being forced into RATINGS.

 

Bud