Peter > Preliminary results – the average temperature of the exterior hive bodies (upper deeps) of the bee cozy hives was ~6 degrees C colder that the roofing felt hives.  ~ 6 deg C versus ~12 deg C.

The Bee Cosy product by NOD and felt wrap are not comparable in terms of thermal performance. The Bee Cozy has an R-value of 8 while felt paper or tar paper is .06 -  almost zero. What you are measuring, with the results above, is the ability of the Bee Cozy to resist the transfer of heat. In your case from the outside surface of the cozy to the box surface.  The differential between the products means the cozy is working to resist heat loss/gain which is what you want it to do. 

What you are measuring with the felt paper is solar radiation or infrared heat because the surface is black. The infrared heat one feels on felt paper, with virtually no mass to store that heat, and without an insulated backup is soon lost to conduction and convection.  When the sun goes in, the insulation product works to resist heat loss coming from the colony while the felt paper is doing nothing.  

If you are trying to find out how heat flows in a live colony and if felt paper does anything to influence thermal efficiency, or when bees will fly, or how the cluster will react when wrapped that would be a different experiment (see Owens below). 


R-values: 

>http://www.newpaltz.k12.ny.us/cms/lib/NY01000611/Centricity/Domain/95/R-ValueDensitiesChart.doc


>The Thermology of Wintering Honey Bee Colonies
By CHARLES D. OWENS, Agricultural Engineering Research Division, Agricultural Research Service

2016 Bee Culture article:

>https://www.beeculture.com/winter-management/


Bill Hesbach

Cheshire CT

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