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# 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?

If I draw a base model - say a 215mm thick solid brick wall terraced house, and then I want to run a simulation of the building with a hypothetical retrofit of 100mm of external solid wall insulation, will I have to then re-draw the geometry to make the sketchup drawing match?

I say this because the surfaces in OpenStudio are 2D and should represent the external face of the building - not the internal face. This would therefore mean that if I simply changed the construction set to reflect an addition of 100mm of external insulation, OpenStudio would 'think' that this was inside the initial external measurement - essentially modelling moving the brick wall 100mm inwards and putting the insulation where the brick wall used to be.

Am I missing something here, or will I really have to change the geometry for each different external wall insulation thickness that I want to simulate?

Thanks,

Jack

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## 1 Answer

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You do not. In EnergyPlus/OpenStudio, "thickness" is not a geometrical property, it is a property of materials/constructions used to calculate thermal characteristics.

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## Comments

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?

( 2020-09-16 03:06:13 -0500 )edit

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?

( 2020-09-16 03:12:36 -0500 )edit

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.

( 2020-09-16 07:48:37 -0500 )edit

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.

( 2020-09-16 07:50:14 -0500 )edit

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.

( 2020-09-16 08:00:36 -0500 )edit

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Asked: 2020-09-13 09:44:18 -0500

Seen: 627 times

Last updated: Sep 13 '20