Insulating a hive with cork for winter. Is this a good idea?

[admanga]

I saw in the free ads someone selling thick insulating panels that each replace two frames in a Dadant hive (professionals typically use 10 or 12 frame Dadants). The panels on sale are probably polyurethane wrapped in moisture control sheets, which would typically be placed between a plasterboard ceiling and insulation above it. The panels had been used for two winters and appear to have survived the bees very well.

This got me thinking, could I use sheets of cork instead of the polyurethane? As cork is a natural material, is there still a need to wrap it? I have some expanded cork left over from an insulating project, perhaps I could use that. But the cork is porous, almost black, and has a burnt smell. It was messy to cut, producing black dust that sticks to everything.

I cut two sheets with dimensions 45 x 33 cm which are taller than a standard frame, as they reach right to the floor of the hive. These are 40 mm thick which will result in loss of the bee space on the outside of the adjacent frame. The hive is therefore converted from 10 frame to effectively 7. The optimal width of the panels should have been about 36 mm to preserve the bee space, and allowing for some irregularities in the cork.

The photos show a hive I’m preparing as a bait hive to catch a swarm, which was convenient to use to test the insulation. What are the bees going to do with these cork sheets? Will they feel at home because its like the inside of a burnt tree trunk or flee from the burn smell? Should I wrap the cork in sheets like the free-ads insulating panels so there would be no cork dust around the hive and a smoother surface to unglue if the bees coat it in propolis?

Second photo shows a panel cut to fit inside the cover. I guess that will work fine, so long as I allow some airflow below it.

 

[Murox]

That's a fun project. I think its a real "just see what happens" thing. Do let us know what the bees make of your cork. The only comment I would make is to question the insulative 'R' value of thickness of cork versus polyurethane?

 

[admanga]

Searched and found: Cork has a thermal resistance of R-3.6 to R-4.2 per inch; Polyurothane has an R-value of about 3.6 per inch. Both are therefore excellent insulators.

Actually, my house is insulated with blown cork and it worked out quite well. There is 20 cm of blown cork in granules between the joists and this and the joists are covered with 8 cm of wood wool sheets. It provides good protection in summer because the dense material delays transmission of the sun's heat, while in winter this solutions is equivalent to standard insulation of the era that my house was built (20 years ago). What I do remember, was carrying 2 tonnes of cork up and into the loft was hard and dusty, and the house smells of burnt wood on the hottest days in summer.

So long as the bees don't mind the smell or the dust! Actually I would expect them to give it a coat of propolis, which might make it a better material.

 

[oxnatbees]

I know people who have done this.

Leaving aside the insulation benefits, which is a whole other discussion, the odd / interesting thing is that all went well for the first year and, as Admanga suspected, the bees propolised their cork walls heavily.

However in year 2 (or year 1 for someone else) Admanga's other suspicion kicked in and they chewed the cork, creating lits of black dust which got all through the hive, turned honey dark etc. They particularly chewed round the entrances.

We found it particularly strange that the same colony should change behaviour in the same hive.

They still use cork, but on the outside of hives now, or other bee-inaccessible surfaces, or with unchewable surfaces.

 

[admanga]

I have wrapped the cork partitions in vapour control sheets left over from a roof insulating project. They present a plasticy/waxy surface, and all joins with tap would be away from the bees against the walls of the hive.
These are in Dadant 10 hives now set out as traps for swarms, but I would keep them afterwards because I think the Dadant 10 is too big. (but I'm reorienting towards Warre hives after some reading).