I'm going to try a bit of a new design for my small container, organic SIPs since I'm finding that I'm having better results when I let them dry out a bit in between waterings than I do by keeping the reservoir full all the time.
I think at least part of the issue is that with smaller containers the perched water table sits at a much higher percentage of the total container height, leading to much less of a moisture gradient overall and leading to more sluggish, overwatered looking plants.
I've been pretty amazed that root rot is not an issue in SIPs even with the bottom inch or two of soil staying submerged in the reservoir water, and I think that is directly attributable to the air gap. So, I'm going to try extending that air chamber up higher into the soil column and see if that helps. I'll add a small, and well vented, cone-shaped container on top of my normal dome-style reservoir to bring the air chamber right up under the main root nexus and see what I learn.
The downside of course is that I'll lose even more soil volume in the process which is not ideal in an organic grow since the soil is the nutrient bank for the grow. But I figure that my small containers are too small to pack with enough minerals and nutrients to last the entire grow anyway so I already have to supplement them along the way.
My theory is that by extending the air chamber higher up in the pot, the upper, inner roots will have access to the oxygen that helps the lowers and allow for a wetter environment up there than would normally be ideal.
In addition, in an organic grow at least, it is generally advised to feed from the top which also means watering the feed from above which messes up the moisture gradient. But maybe with extra air up there, that factor will matter less than it does in a normally designed SIP.