On 10/02/2017 12:30, Dave Liquorice wrote:
On Fri, 10 Feb 2017 10:37:02 +0000, Bob Minchin wrote:

An outside container is very difficult to fully seal as thermal
expansion/contraction will act as a pump admitting small amounts of
water vapour laden air. The water condenses and stays inside and on the
next heating cycle the air gets pushed out but the water does not and so
it slowly builds up as the cycle repeats.

Double plus 1. Not many people realise/know that.

So although holes seem bodgy it's the only way. Only need one at the
lowest point and it doesn't need to be very big 1mm is ample. Much
bigger and a decent blow will manage to force water in, not to
mention letting in spiders or other wild life.


Thanks! So, tiny hole to allow air in and out, and use grease to seal
it. Brilliant! Why didn't I think of grease?

Curiously, we have two of these, one under the eaves in a very sheltered
position. That was bone dry inside, so I'm not 100% sure that
condensation is really an issue. It's possibly simpler than that.