What could be the sources of humidity that build up in a house in an arid environment?

by jdl   Last Updated October 09, 2019 20:21 PM - source

We open our window at night at the temps get down to 70F @20% RH. The temp in the house drops to 75F and the RH is 33%. We close our windows when we go to work.

When mid day comes, outside, the temp is 95F and the RH is 10%. Inside the temp is 76F and RH is 40%. How is this possible?

  • we have no water leaking into the house... with water off... usage is 0.
  • Cement slab.
  • Nobody is home. We have a couple dogs but they can't contribute that much water.
  • There is a temperature difference in the house one side is 74F and another side of the house is 78F.
  • We are waiting on a manual J to be completed, the guy thinks our AC is oversized. But that explains why moisture isn't leaving. But it doesn't explain where the moisture came from to begin with.
  • We had a pressure test done on the house... -0.1 pascals. So we are in good shape there.
  • It seems to me the only way water could get in, if no leaks, is condensation some where. The air handler is in the attic, attic get hot during the day...

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