The density of EPS (extruded polystyrene) hives are measured in grams 
per liter volume. There is a limit around 100 grams where the material 
gets too hard and brittle. Insulation value decrease with weight as 
there is more material and less air the heavier (and harder) they get.

There has been hives made of 80g to save cost, but general consensus 
is that 95g is a good compromise between strength and production cost. 
If made too light (soft) bees will chew them.

My oldest boxes are from 1982, still working.

Some old pictures: http://beeman.se/biodling/hive-nf.htm

There are many producers in Europe, it might pay to check around and 
compare before buying. But look out for producers cutting cost by 
making them too soft! Those will not last long....

Those made from 4 parts will be fine if glued properly together. 
Normal wood glue is enough.

And I don't use any inner cover. Plastic lid has good insulation. 
Bottoms can be a problem as they soak up some water during winter and 
slowly can brake up by frost as water is freezing and expanding. 
Bottoms need to be hardest possible, and painted to keep water out.

--
P-O

             ***********************************************
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

Access BEE-L directly at:
http://community.lsoft.com/scripts/wa-LSOFTDONATIONS.exe?A0=BEE-L