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The issue of model surface position relative to floor slabs isn't unique to plenums, but I do see your point that with such a short height the percentage change in the volume is much more apparent. While general practice for interior surfaces is to draw the heat transfer surface at the center-line of the construction assembly, maybe in this case you should draw it at the bottom of the slab. This would maintain your plenum volume, but it would mis-represent by 1' how windows interact with the slab, unless you shift your windows by 1' so their position relative to the top of slab is maintained.
For reference I don't think most people follow the exterior surfaces to outside face and interior surfaces to center-line approach. I think this is generally used for walls but not for floors. I generally see the bottom floor drawn at top of slab vs. bottom, and other floor surfaces also at top of slab. So if you measure any floor you get the floor to floor height, and the bldg. height is equal to top of first floor/basement slab to top of roof. This approach in your example would produce a 4' plenum vs. 3'. I'm interested to see what others do. Another plenum related issue is WWR calc.
The issue of model surface position relative to floor slabs isn't unique to plenums, but I do see your point that with such a short height the percentage change in the volume is much more apparent. While general practice for interior surfaces is to draw the heat transfer surface at the center-line of the construction assembly, maybe in this case you should draw it at the bottom of the slab. This would maintain your plenum volume, but it would mis-represent by 1' how windows interact with the slab, unless you shift your windows by 1' so their position relative to the top of slab is maintained.maintained
For reference I don't think most people follow the exterior surfaces to outside face and interior surfaces to center-line approach. I think this is generally used for walls but not for floors. I generally see the bottom floor drawn at top of slab vs. bottom, and other floor surfaces also at top of slab. So if you measure any floor you get the floor to floor height, and the bldg. height is equal to top of first floor/basement slab to top of roof. This approach in your example would produce a 4' plenum vs. 3'. I'm interested to see what others do. Another plenum related issue is WWR calc.
The issue of model surface position relative to floor slabs isn't unique to plenums, but I do see your point that with such a short height the percentage change in the volume is much more apparent. While general practice for interior surfaces is to draw the heat transfer surface at the center-line of the construction assembly, maybe in this case you should draw it at the bottom of the slab. This would maintain your plenum volume, but it would mis-represent by 1' how windows interact with the slab, unless you shift your windows by 1' a foot so their position relative to the top of slab floor is maintainedmaintained.