Here is my technical take on where the insulation line is required. It's all about energy requirements (in cluster or in cluster with assistance from some type of cold protection). At some point the energy need will be too high (bees will starve) or will be stuck in one location unable to move to the honey (too cold for too long).
Insulation acts to reduce the CDH8 energy equivalent. The attached image is the nearest weather station I could find to Paul H's location. 5min exercise to generate.
Read up on a similar concept HDD - https://en.wikipedia.org/wiki/Heating_degree_day
HDD map of US https://en.wikipedia.org/wiki/Heating_degree_day#/media/File:United_States_Heating_Degree_Day_map,_1961-1990.jpg
I have just used the same logic and made my base temperature 8C and used hourly data to provide me with more granular daily information.
***********************************************
The BEE-L mailing list is powered by L-Soft's renowned
LISTSERV(R) list management software. For more information, go to:
http://www.lsoft.com/LISTSERV-powered.html