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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-
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 haven't checked the numbers but yes, that theory is correct. We have several large sealed environmental chambers at work and if you turn the humidity control off and raise the temperature you will see a drop in RH but that is a 1m3, sealed stainless steel chamber. As soon as you closed your windows, there would have been things increasing the amount of airborne water vapour.
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 (?)
Yes
Edit: Take all of your measurements with a very large pinch of salt.
eskay
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)