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My point is that the outside RH is reasonably high anyway. The inside will be higher at the same temp because of cooking, washing, breath etc. The heating will reduce the RH but I would be surprised if you could get it down to 65 (unless heating is running pretty high)
eskay-
If you theoretically replaced all the air in your house with outside air at 5 degrees at 80%RH and heated it to 20 degrees the RH would be 35% - I think.
I did open the windows for an hour before whacking the heating on for this reason.
Another calculator suggests that at 65%RH @ 20C makes the dew point 12 degrees. So if the external wall is 12 degrees there will be condensation (?)
Whilst the sensor may be inaccurate (gonna buy a good one), the condensation mark has reappeared on the wall today.
apc
Depends on the temperature though