I've wondered what can be done about this. Convection needs height to really get going so I wonder if some less permeable layer laid horizontally between the layers of bales might reduce it some.
My guess is it makes it worse, in the wall. It is not a pleasant thought for me, as I have insulated this way (using cardboard or paper) one wall already. It is better to let the air flow along the wall, as it makes no harm. The more convection cells spanning the full width of the insulation layer, the worse.
The airflow/convection barrier should be perpendicular to the heat flux, like panes in the window. This is easy in roofspace insulation (two layers of small bales or many sheets of straw from jumbo bales). I'm doing it on walls as well, using one decimetre thick sheets from large bales put into a light wooden framework.
Some maths concerning that is deep within the list archive... March and April 2005. Something more on that is in Czech only,
http://amper.ped.muni.cz/pasiv/slama/how_work.pdf
yours moderator and list almost-no-reader
Jenik