Some clarity is needed.
Normally, when camping, my girlfriend and I zip the two bags together to sleep. Up to -2 or 3 ish this has been toasty. I recognise that the best insulation is when you are in your own bag, but this was not necessary on that occassion. We sleep on Thermarests, and bike kit.
On ths night in question, I was camping alone, apart from our dog who came with me. I slept in my bag, atop some bike kit, and a well inflated thermarest. The tent was pitched on snow, on a footprint. It snbowed throughout the night and the tent was covered. I had though that might insulate too... The dog, luckily for her, was just curled up along side me on her sheepskin. I did not try to stuff her into my sleeping bag, although I considered at about 4 am!!
Since then I have also bought a closed cell foam mattress to go under the Thermarest, also... Thankfully it is now about 20-25 degrees!
I am thinking of buying a bvi bag too, but this is not a goretex one. I simply don't have the funds to pay £150, when a PVC one should be as protective and about a fifth of the price... For when I need it, Goretex is OTT.
Adventure: it's an experience, not a style!
(so ride what you like, but ride it somewhere new!)