@OttawanZ5 : I'm referring to "dormant" brebas . I'm not speaking of brebas that are already actively growing; and I'm speaking of -15°c for a short time at night , and not -15°C for two weeks in a row at daytime.
In that case yes, they do survive. At least mine do .
@fitzski : you could use the 80 liters trashcan technique inside your GH . See my post, on winterizing fig trees.
you could use fresh wet manure inside the GH, as decaying manure produces heat .
if your GH is near the house you could use hot water containers to warm the GH a bit during the night
Or Christmas lights around the branches of your trees .
Check your GH for any opened hole you could clog to avoid cold air. Throw tarps over the GH . Figtrees don't need light while dormant.