WUFI

Dear Sir/Madam,

I would like to model an air cavity in the assembly near the outside environment, so with rather cold conditions in winter. Infiltration of this cavity might occur by air exchange with the inside environment. So, condensation might take place.

The help mentions about the moisture source air infiltration model IBP: "...WUFI determines the lowest temperature occurring in the region covered by the source. If this temperature is higher than the dew point temperature of the indoor air, nothing happens".

The 'nothing happens': does that mean that indeed nothing happens with the T/RV in the assembly? So, there is only an effect when condensation occurs? That would seem rather strange to me, as infiltration air flows will always affect the conditions (both temperature and humidity) in the 'infiltrated layer'. Could you please clarify?

When indeed 'nothing happens' in the Moisture source air infiltration model IBP, I was also looking at the air change source model as alternative. I was wondering what happens in this model when condensation occurs. When I test this with an air cavity with no extra moisture capacity, the RH goes quickly up to 100%. But, it seems that 'additional' condensed water is not taken into account in this model. Does this model maybe use the 'Cut-off at Max. Water Saturation' approach? (can't specify this for an air change source).

The IBP infiltration model would be great to use, if it would also take non-condensing conditions into consideration. Now, it seems there is no complete model available for an air cavity in the construction that exchanges air with the inside environment.

Looking forward to your support/comments.

Kind regards,

Willem

Dear Willem,

the infiltration model is a simplified model which does not calculate the airflow or energy exchanges of the air with the construction. Thus if the dewpoint temperature is undercut by the room air conditions, condensation occurs, otherwise not.

The air exchange source is working in another way. Here the air layer conditions are ideally mixed up with the T and RH of the selected boundary conditions depending on the ACH. If there are cold interior surfaces connected to the air gap, condensation might occur.

We are trying to get funding for setting up a more complete model since some time, but did not yet succeed. But beside the funding this is not that simple, as for example the way of the air fluxes can not be defined clearly without CFD approches, which are not included in WUFI. Further they depend on many (mostly unknown) parameters.

Christian