what is acceptable humidity reading?

[elmsoft]

Helping a friend renovate a flat I have found some damp- probably rising damp- so put a hygrometer in the room. It typically reads about 70%. But living in the west of Scotland it is usually more than that outside. Information on the internet suggests a room should be about 50% but I have always understood that a well ventilated room shouldn't have damp issues and if the humidity outside is 70% then 70% is as ventilated as you can get!
Obviously I am missing something here but am very confused.
Could someone point me to some good info on the subject?

 

[stuart45]

The comfort zone is generally regarded as being 40% - 70% R/H.
50% is seen as probably about the best, 70% is up the high end.
If it's 70% outside it depends on the outside temp. If it's colder than inside as air comes in and warms the R/H will drop. If the outside temp is the same as inside then it will stay at 70%, or rise from moisture produced inside the house. If you are getting condensation on the walls they must have a temp below the dew point.

 

[elmsoft]

If it's 70% outside it depends on the outside temp. If it's colder than inside as air comes in and warms the R/H will drop. If the outside temp is the same as inside then it will stay at 70%, or rise from moisture produced inside the house. .

Ah, I think I get it! It's probably a bad idea to be measuring this in Summer because the inside temp and outside temp are likely to be similar- the CH is, after all, off!
I will get a better idea when the heating is running in Winter.

Thanks for the reply

 

[jeds]

It's a while since I did one but internal RH on it's own tells you very little. The key measurement you need is the differential vapour pressure. To get that you need to measure internal temperature and RH and mark that on a psychrometric chart. The chart gives you the internal vapour pressure in KPa. Then repeat with external temp and RH and then extract external pressure from the internal. In a normal internal environement the pressure differential should be quite low - in the range of 0 to 0.3KPa. Bear in mind you may not get meaningful readings in hot weather. Ideally you'd do it when there is a clear temperature difference between internal and external.

 

[^woody^]

Could someone point me to some good info on the subject?

Try here

Otherwise its pointless