OpenStudio - If I change the wall construction to be thicker on a base model, do I have to also change the geometry to match this?

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So am I right in thinking that the thickness essentially is only used to calculate a u-value, which is then assigned to an infinitely 2D surface? If so, how does this work?

If I draw my geometry based on outside measurements, so that I get a correct amount of material in contact with the outdoor air, then my internal quantity of air will be wrong, because it will not take into account the thickness of the walls that will reduce the volume within the space?

If I reverse this, and draw geometry by internal measurements then the quantity of material exposed to the outdoors is incorrect?

Also - If you internally insulate the walls of a building, presumably that results in a reduction in the volume of air in the building that requires heating. If E+ assumes the walls to be infinitely thin, but now with a lower u-value when internal insulation is modelled - the model does not take account of the reduced volume of air that requires heating?

Is this the case, or how does E+ deal with this?

Normally, OpenStudio auto-calculates the zone volume based on the geometry. As you've noticed, this geometry is planar, not 3D. In E+, you can manually set the zone volume to account for wall thickness or any other zone volume things (like maybe a big piece of machinery or something). Based on experience, I expect any effects of changing this number to be negligible for something as small as wall thickness. However, there's no substitute for experience, so I'd recommend doing your own sensitivity analysis.

To do this, I'd recommend writing an OpenStudio Measure, then using the ThermalZone.setVolume() API method to vary the volume stepwise by some % increment and see the impact on energy consumption. If you do this, I'd advocate posting a graph of the results back here for future people who look at this question.

Thanks, this has really helped me understand better how E+ conducts its simulations. I'm new (as you might have realised) to OS and am not used to coding, so I've not even ventured into using APIs. I'm not even sure what they are! I'm just using OpenStudio Application at the moment so I don't know how to use these commands that people keep referring to. If you know of a simple way to set thermal zone volumes in the OS Application then I may be able to run sensitivity analysis and return the results here.

Hang on. Sorry I thought this answered all of my confusion but I'm not sure if it has.

I understand now that geometry in E+ is planar. However in the book on modelling in OpenStudio by Larry Brackney (here) it says that you should draw surfaces as the face that is contact with the exterior environment. So when I build my base model, I use external measurements.

However, the external measurements will obviously change if I am installing external insulation all over my building envelope. Surely something here is incorrect? Either I shouldn't be drawing the surfaces as external measurements, or I should be and I would have to re-draw them to represent the new external measurements of the building which has external wall insulation installed?