Funny you posted this,
I was getting worried because the temp last couple of nights have been -12*C before windchill. I went out yesterday around noon to check things out in the greenhouse.
I insulate my "Costco" greenhouse with bubble wrap and create a form of "drop ceiling" using wires strung across the house. Then I cover the plants with agriculture fleece to hold in the radiant heat from the earth. Lastly, a small space heater on a timer. Anyway, the rosemary, thyme, tarragon, mint and oregano are unharmed and the soil in their pots is unfrozen. The guava and citrus plants have a some leaf curl from the cold, but they seem OK. The citrus and guava can apparently handle -5*C.
I did less this year insulating the greenhouse than last year, but its way colder this year. If I can make it through this year without deaths, I gonna feel alot better in the coming winters and probably wont stress as much.
One last thing: I use a minmax thermometer, but I dont think its that accurate. It showed a low of -10 and a high of 20! Would have loved to be in the greenhouse on that 20 degree day. Downright pleasant!