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

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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).

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.

@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("#{construction.name} has a thermal conductance of #{conductance_ip_neat}.")
  else
    runner.registerInfo("Can't determine thermal conductance for #{construction.name}.")
  end

end

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.