Protecting tropicals in a Greenhouse

I'd like to protect a few tropical type plants, including a hibiscus and a palm. I'm in Atlanta, 7b. I don't want to bring them in, if possible.
I built a small (south facing) green house (I call it the solar cabana) out of 2*2's. It's about 8'*8' and has an arched roof of pvc pipe. Front is higher than the back. The back half of the top has an inside layer of bubble wrap. The outer walls (non south facing) are double plastic (poly on both sides of the 2*2's) and the back is a polisocyanurate sheet. The floor is salvaged brick. A rough guess (not including the floor) is a thermal conductance of 165 BTU's F. As far as cost, since it's made out of ripped 2*4's, painted with leftover Benny Moore, I have more money in scotch consumed than building materials. It's also way cool looking.
It's fabulous as a sun room, on an cloudy/overcast day it's 15F warmer inside during the day (no sunny days yet). But I wonder what I can do to protect those plants (without added external heat).
There's maybe a 1000 lbs or so of brick in the floor (in contact with the ground). I have more brick available.
So are these plants more sensitive to cold on the roots (they are potted) or the leaves?
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