Eliot Coleman is a big proponent of hoop houses - he says they shift
you 2 zones south.

My experience in zone 5 bears this out. Cool weather crops should be
planted late in the season, but not too late. They should get close to
mature before your really cold weather hits. I had radish, chinese
cabbage, lettuce, beets, onions and 1 pea plant before we got nailed
with sub-zero temps. Now I have to wait until late feb/early march to
get things going again.

In general, daytime 30-40 degrees can produce temps in the hoophouse
50-70, depending on sunlight of course