Re: [EnergyPlus_Support] Does U-value change with temperature?

On Wed, Jan 18, 2017 at 11:02 AM, Jean Marais jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

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Joe and Edd have good points, however for most estimates we assume the effect is negligable for the "surface to surface" U-value. What does change are the film resistances which are convection dependant...and e+ calculates those dynamically. So the overall u-values are changing all the time. This is not to be confused with RATED u-values, which are the manufacturer specs at given rating test conditions.
2017-01-18 15:56 GMT+01:00 'Edward G. Lyon' eglyon@xxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@yahoogroups.com>:
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A lot of that work is now part of the WUFI program.Â WUFI materials can have variable R-Values with temperature and moisture content and WUFI will calculate instantaneous total assembly R-Values.Â This is a likely coming standard that will need to be integrated into E+.

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Ned Lyon, P.E. (MA, WV)
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From: EnergyPlus_Support@yahoogroups.com [mailto:EnergyPlus_Support@yahoogroups.com]
Sent: Wednesday, January 18, 2017 12:05 AM
To: EnergyPlus_Support@yahoogroups.com
Subject: Re: [EnergyPlus_Support] Does U-value change with temperature?

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I remember seeing a report by Jeff Christian of ORNL decades ago that looked into the change in conductivity for insulation products as a function of temperature.Â I recall he found something like a 10% change over the range of ambient temperatures.Â If you're interested, you should do a Google Search of the technical literature (or ORNL reports) and see if you find something.

Joe

Joe Huang

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On 1/11/2017 10:42 PM, Amrita Ghosh amritaghosh1986@xxxxxxxxx [EnergyPlus_Support] wrote:

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Hi,

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Can anyone tell me if U-value of a material changes with temperature?Â

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While evaluating the U-value, if the convective heat transfer coefficients (h) change, the U-value changes. But does it also change when the temperature, indoor or outdoor, changes? Since the equation states: U=Q/(A. deltaT)

where deltaT= temperature difference between indoor and outdoor.

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Thank you,

Amrita

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James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
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