Increased Moisture in the Wall Assembly 4 years into simulation

[bcroylesy]

I am conducting a simulation in WUFI Pro 6.5 on a light wood frame wall assembly with a 20 mm ventilated drainage cavity (with a ventilation rate of 10 ACH). The siding of the wall is 10 mm LP SmartSide Siding. There is a moisture source of 2% of the wind-driven rain leakage into the assembly. I have modelled the moisture source to be applied on the right-hand side of the cladding (on the inner 7 mm of the cladding).

The moisture imbalance in the simulation is 14.93 (balance 1 = 5.94; balance 2 = 2 -8.99). I ran the simulation for 5 years. During the first three and a half years of the simulation, the results seem to be "normal" compared to similar simulations. Over the last 1.5 years of simulation, the relative humidity in the sheathing layer increases to 95-100%, whereas in the previous 3.5 years, it ranged between 65-85% in the sheathing layer. As a result, the water content of the OSB sheathing, the air barrier and the air layer sharply spikes, to (seemingly to me) unrealistically high numbers (air content water content gets up to 200 kg/m^3, OSB jumps from 10-15% moisture content in the first 3.5 years up to ~50% moisture content in the last year of the simulation.

Note, the climate file is from St. John's, Newfoundland, in Canada. I modelled the same wall assembly with a moisture source of only 1% wind-driven rain leakage, and the moisture balances were even, and the moisture content of the sheathing stayed between 9 and 14% moisture content.

I have a couple of questions about my simulation:

1. Is there something wrong with these results? Based on what I know about WUFI simulations, I think the answer is yes, because the moisture balances are not even, and the humidity at the end of the simulation spikes really high. But, if there is something wrong with these results, how can I change the simulation setup (excluding the climate, wind-driven rain leakage and cavity ventilation rates), to improve the moisture balances?

2. Why are these results happening? I am a bit confused as to why these results are happening in the last year and a half? Does it have something to do with the moisture storage function of the air layer or sheathing layer?

Please let me know if there is any additional information I can provide to help in answering these questions. I have attached a screenshot of the OSB sheathing water content and a screenshot of the relative humidity and temperature at a layer within the OSB sheathing.

Thanks,
Ben

water content.png (21.21 KiB) Viewed 1928 times

relative humidity.png (33.43 KiB) Viewed 1928 times