Difference in the balances due to Short-Wave radiation absorptivity

[kamil Hamela]

Hi,

I have a flat roof construction:

- Bituminous felt,
- OSB,
- Mineral wool insulation,
- VCL (Sd = 200 m),
- OSB,
- Mineral wool insulation,
- Air cavity (without additional moisture capacity),
- Plasterboard.

When I run the assessment, Balance 1 and 2 are very different. I have tried to move the VCL and change the insulation, but the balances are still different. The only part of the construction that seems to make a difference to the balances is when the construction in Short-Wave Radiation Absorptivity is specified. When I change from bituminous felt to a white surface the balances become closer, when I turn it off there are exactly the same.

From my understanding, a 'healthy' model should have equal balances and this is not achieved for above construction. I have also followed 'handling of typical construction' pdf.

Any help would be appreciated.

[Christian Bludau]

Hi Kamil,
yes, the balances should be about equal. Try to set an sd-value for the bituminous felt in the exterior transfer coefficients instead of building it into the construction. At the same time switch off rain. In the numeric switch on the adaptive time step control. You also could try to replace the mineral wool insulation by another brand.
Turning of the absorption is not an option.
Best regards
Christian

[kamil Hamela]

HI,

I have changed the mineral wool in the construction to 'Mineral Wool (heat cond.: 0,04 W/mK)' for both layers. this seems to have fixed the issue with the balance. Originally it was 'Rockwool speedrock lamelle'. Why would this have made a difference?

thanks

[Christian Bludau]

(Hydrophobic) materials which have a very low moisture storage up to high relative humidity and a very step increasing up to 100%, like mineral fiber insulations can cause convergence problems when the relative humidity reaches the range of this knick at the end, especially if dew water is involved. So changing the material can solve this issue.
Christian