[BLDG-SIM] On-site U-value measurement

[D. Charlie Curcija]

Radiation on the inner side of the wall is more than 50% of the overall heat
transfer, so it certainly can not be ignored.  It can be reasonably
estimated by using standard black body radiation equation, but it can not be
ignored.  Also, convective surface heat transfer coefficient of the wall is
not really known.  You would need to know if the air flow is within natural
or forced convection (i.e., measure velocity) to be able to get even close
to the real h.

 

For in-situ measurement of the U-factor, which is in reality heat flux
through the wall divided by air to air temperature difference:

 

U = q/(T_air-in - T_air-out)

 

The best and most reliable approach is direct measurement of q, which can be
accomplished by surface heat flux transducers.  These are available from
Omega and probably some other companies.  Their price is in $300 range.
They connect to standard DAS such as HP 34970A.  One thing to consider is
correction of outside air temperature if there is significant difference
between air temperature and radiation temperature (e.g., night sky
radiation).  There is standard equation for mean radiant temperature
calculation in that case, so T_air-out should be really T_mr_out.  

 

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Gunes Inan
Sent: Wednesday, May 09, 2007 10:00 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] On-site U-value measurement

 

Dear All,

 

Thanks for your replies.

 

We currently have a thermal camera but I never used it to record
quantitative results. Brandon, you mentioned about "tuning" the heat
transfer equation. If we write a steady-state surface heat balance equation
for the inner side of the wall:

Q_convection=Q_conduction

h*(T_air - T_wall_in)=k*(T_wall_in - T_wall_out)/x_wall 

where radiation is negligible and h is known, than we should be able to
calculate k and u-value, if I am not wrong. Is this what you mean by tuning?


 

The u-values that I obtain either by calculation or measurement need not to
be accurate in the academic level, instead it should be practical so as to
analyze more houses and reach more people. We actually intend to improve the
energy efficiency of houses for which neither we nor the inhabitant have
information about the constituents of the wall.

 

Prof. Sibilio, thanks for referring me the Hukseflux. As I called the
company, they told me that each heat flux measurement takes about 24 hours.
Is this the way you do it or do you know any tricks to get reasonable
results in a shorter time.

 

Regards,

 

Gunes Inan

 

  _____  

Kimden: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] Yerine Brandon Nichols
Tarih: 08 Mayıs 2007 Salı 20:58
Kime: BLDG-SIM at gard.com
Konu: [BLDG-SIM] On-site U-value measurement

 

Hello,

 

You may find an infrared thermal imager to be a powerful tool, useful for
the application you have indicated and many others.  Here's a link to Fluke
Corp. thermal imaging notes for building systems, there are many other
manufacturers...

 

http://us.fluke.com/usen/support/appnotes/default?category=AP_TH_BM(FlukePro
ducts)
<http://us.fluke.com/usen/support/appnotes/default?category=AP_TH_BM(FlukePr
oducts)&parent=AP_THERMO(FlukeProducts> &parent=AP_THERMO(FlukeProducts)

 

For building systems, the thermal imager is useful for finding voids in
insulation, infiltration losses, calculating the overall heat loss
correction due to framing framing elements, etc..

 

Using the thermal imager, you will have to "tune" heat loss equations based
on observed temperature differential data to derive U-values. These
instruments typically have the ability to scan large areas and quickly
calculate a weighted average and other statistical factors for each image.  

 

Unfortunately they are not cheap, and probably overkill for just measuring
the U-values of specific materials.  Personally I don't know any way to
measure U-values directly with a hand-held instrument, but it seems such a
device wouldn't be hard to put together with a data acquisition unit and a
couple of thermocouples.

 

 

Regards

 

Brandon Nichols, PE
Mechanical
HARGIS ENGINEERS
600 Stewart St
Suite 1000
Seattle, WA 98101
d | 206.436.0400 c | 206.228.8707
o | 206.448.3376 f | 206.448.4450
www.hargis.biz

 

  _____  

From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Gunes Inan
Sent: Tuesday, May 08, 2007 7:46 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] On-site U-value measurement

Dear All,

 

We are looking for a hand-held instrument for on-site measurement of the
U-value or thermal conductivity of walls. Any experience about the
measurement techniques and recommendations about instruments and brands are
greatly appreciated. 

 

Thank You,

 

Gunes Inan

 

R&D Engineer 

Betek Paint and Chemical Corp., Inc. 

Kavakpınar Mah., Tandoğan Cad. No: 175, 

Kaynarca, Pendik 

Istanbul, Turkey 

Tel: +90.216.396.2070 / Ext: 177 

Fax: +90.216.396.6989

 

 
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Another commenter mentioned accounting for non steady-state conditions.
This is also important, but if you can catch couple of hours of relatively
unchanging conditions, you should be OK. 

 

Regards,

-------------------------------------------------------------------

D. Charlie Curcija

DesignBuilder Software

18 Tanglewood Rd.

Amherst, MA 01002

 

Tel: (413) 256-4647

Fax: (413) 256-4823

cell: (413) 575-3487

email:  <mailto:curcija at designbuildersoftware.com>
curcija at designbuildersoftware.com

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