Pete I've assembled many a greenhouse(all in Maine) - the big thing with snow load is keeping it clear from the sides, basically plowing roadways down the edge every snow. The reason most collapse is because they are single ply covered, which allows the plastic to luff and stretch thus collecting pockets and with no clearing down the sides snow builds and collapses the structure. If you double the plastic and add a small blower fan to pump an air pocket between the sections it keeps the poly tight and the air pocket creates an insulation space. Adding a rollup side would take care of all the ventilation needs - all of this can be done with wood lathe and EMT conduit
