Starting with the information that I have an unheated greenhouse: I’m in zone 8a/7b (we’re super borderline) and I’ve been told it would be good for the productivity of my pepper plants to overwinter them. Is this really viable in this zone? Do I need to cut them back hard like I’ve seen people do, or should I leave them to keep growing out in the protection of the greenhouse over the winter? In terms of how cold the greenhouse gets, my hydroponics out there never froze last winter, and lettuce, spinach and watercress all grew well without any cold damage in a channel system. I’ve since installed a thermometer that records the daily highs and lows, so I’ll have accurate data after this winter. I’m hoping someone else on here has tried this and can advise.
Peppers are killed in even the lightest frost. 9a is the coldest you can expect peppers to be safe most years.
Ok, then 
… The Winter Harvest Handbook by Eliot Coleman claims that each layer of protection (such as a tunnel or cold frame) effectively raises the growing zone of the space beneath it by one zone. So, if I tuck the plants under a little tent inside the greenhouse, they should be considered to be in zone 9. I’ll try a few plants this winter and see how it goes, both with some that are cut back and some that are left tall… Thanks for the input, @poncirusguy 
My peppers didnt have any frost, but they took nights down in the low 30s and even 1 night in the upper 20s. So it seems that frost might be the biggest you need to protect from.
I have problems in 9b even. Lost quite a few unless semi protected. They do hate the cold. Also seem to have pretty good results not cutting too much off the plants.
I overwintered a pepper in my kitchen. Its already produced ripe fruit outside this summer.
I think the one zone per layer is really more applicable to lengthening shoulder seasons and over wintering hardy vegetables. I don’t think two layers increases the yearly minimum by 20 degrees/2 zones – it does increase the annual minimum modestly but the bigger impact is reducing the time spent in the frozen/dessicated temp range and increasing the time with soil/air temps high enough to grow, reducing wind bite, etc.
In short – if your mean annual minimum is 10 or 15, I’d be surprised if a low tunnel + unheated high tunnel didn’t get down into the mid or high 20s and kill the peppers.
Zone 8… the temperature hit a high of 33F cloudy with wind and rain. No sun GH temp hits a high of 34 degrees. Night fall the temperature drops to 10F and the GH drops from 34 down to 25. Pepper plants die. My Greenhouse is 4 layers of glass thick and my GH gives me a 10F rise and then I have to add heat.
Might also depend on what kind of peppers you’re talking about. I’ve had good luck with keeping Capsicum pubescens happy looking all winter in my very cool greenhouse. I’ve had trouble getting mine to set any peppers, though, it flowers plenty but no fruit set. Maybe that species needs a second plant?
I tend to agree with this, and for most plants. If you can keep frost off of most everything, I’d argue that sheer temperature doesn’t affect anything nearly as bad as direct frost. Obviously this isn’t going to stop temps in the teens damaging something super cold sensitive, rather given the choice of cold or frost I opt to stop the frost 10/10 times
I’m in 9b as well and try to overwinter a variety of peppers in my unheated greenhouse. Even then, I lose a few every year. I agree with not cutting back too much in my conditions. I get significant dieback on annuum and chinense varieties over the winter, so cutting back before winter would mean that they die back to the roots. Waiting until spring ensures there are still buds for the plants to grow from. For overwintering indoors under lights, I think cutting back would keep the plants from taking up too much space as well as compensate for any roots lost if the plants are being dug up and potted.
C. pubescens and baccatum are much easier to overwinter and need cutting back in spring just to keep them from getting too lanky. C. baccatum goes into stasis and doesn’t grow but doesn’t die back either. C. pubescens will keep growing and flowering, but I don’t get any fruit set over winter either. I don’t think my light levels are high enough.