[EnergyPlus_Support] Adiabatic walls, timestep dependence

["eh11125" <erinhult@xxxxxxxxx>]

I have been running some test cases to look at heat storage in thermal mass in EnergyPlus and I have been seeing some strange behavior, specifically a substantial dependence on timestep when there are walls with adiabatic outside boundary conditions in the zone.
I have attached a file (TestBox.zip, containing TestBox.idf, TestBox.epw, TestBox_p1.pdf, TestBox_1.pdf, TestBox_10.pdf) with the following simple setup:
- 1m cube box
- All 6 walls are: 1cm thick, concrete with adiabatic outside bc
- No internal loads for 2 days, then a constant internal load for 2 days, then no internal loads for 2 days.
The figures (TestBox_p1.pdf, TestBox_1.pdf, TestBox_10.pdf) show the temperature evolution for the air (solid line) and the inside surface (dashed) for internal loads of 0.1W, 1W and 10W. The dotted line shows the predicted final temp, based on the thermal properties of the materials.
If you run the same case without any adiabatic walls, the results are independent of timestep.
These results raised the following questions:
1) Why is there such a dependence on timestep, particularly for the 0.1W case (up to 40% of dT)?
2) Why is the equilibrium temperature higher than expected based on the thermal properties of the materials? For such thin walls, the time constant for the wall interior to become isothermal would be short, so at long times, the air and walls should reach a constant temp throughout.  Is there assumed to be water vapor in the wall material? Is energy not conserved due to the modeling of heat transfer within the wall? 
Thank you for your help,

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