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

Amrita,

You might be too quick to draw this conclusion. The rated U-value does help, it's even critical.

In a New Construction project, the rated U-value given by the manufacturer is usually all you can get and more than enough. It's up to you to understand the test conditions of the rated U-value given by the manufacturer (they might vary depending on the geographical location of manufacturer and therefore the standard they are submitted to) and more importantly your simulation engine: does the simulation engine add the interior and/or exterior films itself? (meaning does the U-value you enter in the software include these already?)
Once you understand that, you'll avoid double counting stuff.

If you really want to, you can model temperature dependent conductivity and moisture diffusion directly in E+ (see below)

There's some TL;DR below in case you have nothing better to do (I promise I didn't plan to write this much, sorry!)

Cheers,

Julien

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As far as the temperature dependence of the conductivity of a given material: it varies with the temperature of the body (material) and not its surrounding, thus lowering the range of variation, especially considering most walls are assemblies of materials. Most materials are not going to experience a drastic change in conductivity at the temperatures they will be subject to in a typical building [citation needed :)]
You can find curves online (eg for PU, Mineral Wool, Fiberglass) or experience yourself with WUFI.
eg: If you take a (dummy) surface made only of insulation and approximate the temperature of the insulation as the average between the inside and outside temperatures (and exclude the film effects too, they would only help), it's rare that you'll see the insulation temperature outside of the 0-40Â°C / 32Â°F-100Â°F range (-20Â°C outside, 20Â°C inside in winter, 60Â°C outside (could happen in an attic for example) and 20 inside in summer). For PU it's hardly a difference especially across the range, for Mineral Wool roughly from 0.25 to 0.27 BTU.in/h.ft2.Â°F, fiberglass 0.22 to 0.24. These are the extreme conditions... I feel pretty ok with these numbers myself.

I am not worried that this is the primary cause of the differences between a theoretical building (a perfect energy model) and a real, imperfect, building.

Also worth noting that E+ has different options for the HeatBalanceAlgorithm that can include moisture in the mix, but require more input and a (significant) slow down in calculation time.

If you set the HeatBalanceAlgorithm to ConductionFiniteDifference, you can also use MaterialProperty:VariableThermalConductivity which allow you to input temperature dependent conductivity. You'll note that the documentation is pretty clearly labeling this as "Advanced/Research Usage".

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Side point:

As far as what happens in real life and over time that's another story. There are indeed many many many reasons the effective U-value at a given point in time might be different: PU off-gazing like Jim mentioned, but it could just be bad installation like dense packing batts, or batts that feel inside the drywall over time, etc... And I'm not even mentioning the way-overlooked-in-North-America thermal bridging effects, etc.
But that's true of any building component. Take a boiler and its near piping: you've got scaling, sooting, badly commissioned controls, excessive bleeding, bad tuning of the burner across the modulation range, uninsulated piping and valves, poorly weighted barometric damper, ineffective (rope) gaskets, etc.

What point I'm trying to make? I'm not sure, I'll let you pick a few:

* It's important not to deliberately introduce bad quantities such as U-value: go with what makes sense.

* In NC jobs, you're clearly evaluating the "ideal" (theoretical) performance of a building (it's a rated building, with rated conditions, etc). So use the rated U-value (plus or minus the films)

* Would it be interesting to compute uncertainties then? Maybe be assigning a probability distribution to each variable and propagate them through the simulation engine, so that you'll get a probability distribution of your EUI too? You betcha.

* When you're modeling an existing building, thankfully you have bills (and meters) that are going to help you calibrate your energy model, to fine tune all the little things that went wrong since the construction of the building... You'll be able to estimate a starting point for the U-value given what you observed in the wall from generic tables, or if you have architectural plans, the manufacturer's rated U-value.

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) | (fr) :

2017-01-30 6:59 GMT+01:00 Amrita Ghosh amritaghosh1986@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

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Thank you everyone. So it seems the rated U-Value does not help much as the actual U-value changes every time with the ambient condition.Â

On Thursday, 12 January 2017 12:12 PM, Amrita Ghosh <amritaghosh1986@xxxxxxxxx> wrote:

Hi,

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

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.

Thank you,
Amrita