Here is some interesting data, from a field trial overwintering low tunnels in Massechussetts with different covers and combinations. The best figures are a combination of greenhouse plastic and row cover. I'm not sure if the row cover is inside or outside.
From this pdf:
https://ag.umass.edu/.../extending_v...shey_final.pdf
sorry that link doesn't work, the title is: Extending Vegetable Harvest and Sales Using Tunnels, Row Covers and Winter Storage
UMass 2014