[EnergyPlus_Support] Problem with very thin high conductivity construction

["smporritt" <mail@xxxxxxxxxxxxxxxxxx>]

I have come across a problem with modelling thin metal ceiling tiles and wonder if anyone can shed any light on the issue.

I am modelling retrofit options for an airport terminal building, which consists of a large low zone (30m x 20m, ceiling height 2.7m), glazed along one long wall, with metal ceiling tiles (steel: thickness 0.001m, conductivity  50 W/mK, density 7800 Kg/m3, specific heat 450 J/KgK). Above the ceiling tiles is a cavity to an insulated metal deck roof.

I have tried two modelling approaches, the first being to build a single zone with a flat roof construction consisting of the metal tiles, an air gap and the insulated metal deck (called my integrated roof version). I have also built a version with a separate semi-exterior unconditioned zone above the occupied zone to represent the roof void to allow control over ventilation to the roof cavity, which I need to do later in my modelling (separate roof version). In both cases the zone volumes were kept the same.

The results for annual heating and cooling energy use using the two construction methods are very different, with the separate roof construction method using over 40% less cooling energy. However, if I replace the metal ceiling tiles with other lower conductivity materials, such as conventional ceiling tiles or plasterboard, the differences between the modelling methods are small (around 1%). Even if I reduce the thickness of the plasterboard to 0.001m the difference in cooling energy is less than 3% when comparing the integrated roof version to the separate roof version.

I have tried many tests, including building new models from scratch, EnergyPlus v6 compared to v7, CTF vs CFD, different timesteps and algorithms and reducing the roof air cavity to a very small depth to minimise differences between the modelling approaches. In all cases I am not getting any errors (e.g. convergence) reported by EnergyPlus. Using CFD I have also looked at node temperatures to see if anything odd is happening (oscillating at timestep), but apart from the temperatures being very different there is nothing odd happening.

If I increase the thickness of the metal ceiling tiles to 0.008m or more the results from the two methods converge, likewise if I reduce the conductivity of the metal tiles to around 10 W/mK or less the results converge. I also tried splitting the metal tiles into 2 x 0.0005m layers and adding a very low resistance no mass material layer between them and in that case the results were very similar for the two modelling approaches.

Is anyone aware of any issues with EnergyPlus modelling very thin high conductivity constructions? I have searched the forums and EnergyPlus documentation and can find no mention of this being an issue.  Any help or thoughts gratefully received, thanks.




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