Greg, I have a 4 tier (shelf) mini greenhouse, it has a light weight clear plastic cover. These are about $35 +/- on Amazon. I have a pair of 22" T5HO lights for each shelf. It is under the house eaves against a brick wall with morning sun.
I open the front of the cover each morning and with the lights on, it runs 10*F above OAT. I think this cover would keep temps above freezing with air temps down to 28-30* with lights off. With lights on I think it would protect them down to 20-22F outside air temps. I have a small 200 watt ceramic heater set for 35* ON and 45* OFF as backup. I'm just going to wait and see
I have semps, echivira and crassula mostly. I have moved my Kalanchoes indoors for the winter. I've read that they do not like going below 40*F. I have over a dozen varieties now and would hate to lose any.
The past few years winter has been very mild here, mid 20's at the coldest, but just like Seattle a Arctic front can move down and it can get very cold. Back in the 70's I saw several SEA winters near 0*F, -5F in Lake Stevens.
I think keeping your semps dry certainly helps them deal with the cold. This is my first winter with Crasulla and Echivira's, it well be a learning experience for sure.
