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How do I see what my overall u value is when I build constructions using materials in OpenStudio?

asked 2014-10-24 15:28:59 -0500

Mel's avatar

updated 2015-07-10 13:29:34 -0500

I'm trying to build an exterior wall construction in open studio using the materials in the library to match an existing building. When I drag my materials into the layer template, I don't see any resulting u-value. Also, where are the air layers for outside surface resistance and inside surface resistance?

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answered 2014-10-25 10:17:01 -0500

updated 2014-10-25 19:08:36 -0500

EnergyPlus calculates the inside/outside air film resistances during simulation so you don't need to specify these (see Construction).

If you want to enter the thermal performance yourself you can use the simplified Material:NoMass and/or WindowMaterial:SimpleGlazingSystem objects. However, the EnergyPlus documentation cautions against these because the effects of thermal mass are not considered - i.e. it assumes steady state heat transfer - and the results may differ significantly from the more detailed Material and WindowMaterial:Glazing objects.

Otherwise if you want to know the thermal performance of a Construction assembly pre-simulation you'll need to calculate it from each layer's properties (thickness, conductivity, etc), and then change one or more of these fields to match your target U-factor.

To confirm/view thermal performance post-simulation you can look in the Envelope Summary section of the eplustbl.htm file. Additionally, you can use/write an EnergyPlus Measure for OpenStudio (see the Measure Writing Guide) to add additional output file formats and change the unit conversion from SI to IP (see the OutputControl:Table:Style object in the Input Output Reference PDF - it's not in Big Ladder's online version).

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@MattSteen The Measure Writing Guide has moved here. We will be taking it off of the Wiki soon. Perhaps you could make your answer more future-proof if you changed the link.

aparker's avatar aparker  ( 2014-10-25 17:08:00 -0500 )edit

Updated and good to know, thanks.

MatthewSteen's avatar MatthewSteen  ( 2014-10-25 19:06:33 -0500 )edit

@MatthewSteen, Thank you for your answer,may I ask a question, you said that "the EnergyPlus documentation cautions against these because the effects of thermal mass are not considered ..." Then may I know what is your suggestion?Thanks in advance for your reply.

4Designer's avatar 4Designer  ( 2016-05-21 12:29:36 -0500 )edit

@OS_4design I would use the more detailed layer-by-layer method in which you build a construction assembly from the Material, Construction, and BuildingSurface:Detailed objects.

MatthewSteen's avatar MatthewSteen  ( 2016-05-21 16:06:01 -0500 )edit

answered 2014-10-24 21:14:25 -0500

keb's avatar

updated 2014-10-24 21:38:21 -0500

I don't believe a running-total U-value can be displayed in the Constructions>Contructions tab currently as you assemble your construction. The only place I've found a construction's U-Value is in the E+ results after a simulation run. If it is available pre-simulation, I would like to know also. :) You could use something like THERM in tandem as your testbed, then assemble the same in OS once you get what you want.

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This seems to be a cumbersome way to get the U-value for the constructions. With DOE-2, when the input processor calculates the response factors, it also displays the U-value of the surface with and without the air film resistances. I understand that EnergyPlus doesn't distinguish the input processing from the simulation, but if you want to know the U-values pre-simulation, why can't you just run EnergyPlus for a short time period, like one day, which seems a lot easier and dependable than running THERM in tandem.

Joe Huang's avatar Joe Huang  ( 2014-10-27 02:41:10 -0500 )edit

Agreed, I like that. Will try that out.

keb's avatar keb  ( 2014-10-27 06:57:40 -0500 )edit

I tried that out and it was a bit too slow; spooling up, running, post processing takes a few minutes. I will try Benjamin's suggestion below, and, look into a measure that David posted above which is probably quicker. In IES the U-values are displayed but I've been learning OS and E+ so there are differences/voids in workflow to adjust.

keb's avatar keb  ( 2014-10-28 16:29:45 -0500 )edit

That's too bad it's slow even when you minimize the run period. Not meaning to gloat by any means, but with DOE-2 running BDL is basically instantaneous.

Joe Huang's avatar Joe Huang  ( 2014-10-28 16:57:56 -0500 )edit

answered 2014-10-26 02:14:36 -0500

@Kent Beason if you are are interested in writing a measure to calculate construction properties pre-simulation, here is a code snippit that loops through an alphabetically sorted list of opaque constructions that are used on at least one surface in the model and lists the thermal conductance in IP units. You could of course show any units you want.

The construction methods I used are from this page. There is also for example a method for construction.uFactor you could use on non-opaque constructions.

# loop through constructions
model.getConstructions.sort.each do |construction|
  next if construction.getNetArea == 0
  next if construction.isFenestration
  conductance_si = construction.thermalConductance     
  if conductance_si.is_initialized
    conductance_ip = OpenStudio::convert(conductance_si.get,"m^2*K/W", "ft^2*h*R/Btu").get
    conductance_ip_neat = "#{OpenStudio::toNeatString(conductance_ip,2,true)} (ft^2*h*R/Btu)"
    runner.registerInfo("#{} has a thermal conductance of #{conductance_ip_neat}.")
    runner.registerInfo("Can't determine thermal conductance for #{}.")

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OpenStudio: there's a measure for that! (credit: @Ralph Muehleisen)

__AmirRoth__'s avatar __AmirRoth__  ( 2014-10-26 09:24:08 -0500 )edit

@__AmirRoth__ May I know which measure?I could not find in the BCL!

4Designer's avatar 4Designer  ( 2016-05-21 22:23:47 -0500 )edit

answered 2014-10-27 16:24:08 -0500

We created a spreadsheet to sum the conductivities of each material multiplied by the thickness to give a nominal total for the construction. The conductivity is shown in the material properties of each material used in the construction, as is the thickness.

While you should have surface resistances for the exterior and interior for the sake of accuracy, in the grand scheme of your building they will make very little difference to the resulting energy use if they aren't there. You could tweak the conductivity of one of the materials to account for it.

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Asked: 2014-10-24 15:28:59 -0500

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Last updated: Nov 08 '14