The Diffusion lid works, but not as well as calculated:
The calculations for the diffusion lid are basing on approved calculation methods of building physics. The difference in the temperature, the thickness and the type of the material are decisive for the amount of water vapor diffusion.I had to assume the average temperature under the lid, because there were no measurement data about it. I was wrong about that temperature. Measured data show a higher relative humidity than expected. The temperatures measured under the lid are lower than expected.
In a hive (Dadant Blatt) with diffusion lid (hemp wool, closed floor) I have measured a relative humidity of 69% in the middle and 73% at the edge of the cover. This means that the diffusion lid works, the moisture is removed by water vapor diffusion. However, the scale is not satisfactory and cannot rule out condensation and mould.
I have reduced the insulation layer in the lid. I will put an outer insulation layer on the hive in addition. This increases the temperature under the lid (especially in the outer areas) and the amount of water vapor diffusion. Apart from that, the bees have to heat less and there will be less water.
During the winter we will collect measurement data. I will publish the results here. At the beginning of 2018 I will also publish a construction manual for an optimized Diffusion Lid here.