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Loft Humidity Levels

Discussion in 'Home and Garden' started by relicalpha, 13 Jan 2021.

  1. relicalpha

    Gangster

    Joined: 2 Feb 2020

    Posts: 104

    Location: UK

    Hi,

    I've been improving a few things in my loft recently, but I've noticed relative humidity sits at 90-92% which is obviously pretty bad and needs fixing. I did have a shower extractor leaking up there for a few weeks which I hoped was the cause, but levels are not receding having since fixed that. There's no longer any visible condensation. I've run a decent dehumidifier for a week but it barely makes a dent, then levels rise overnight with low temperatures.

    The big thing is, there seems to be no ventilation at all, so I am getting some felt vents to fit as mentioned in another thread but I'm more concerned about what the underlying cause might be?

    The rest of the house is entirely below 60% humidity and there's no obvious leaks, extractors are all good etc. There is a cold water header tank up there, but that has a lid and appears ok.

    I don't know how humidity works... but could that 90% just be an accumulation of lower low-humidity rising across the rest of the house? If so ventilation should fix that, but if levels that high imply another problem I would rather identify it asap. Or am I worrying over nothing and should just get the ventilation fixed? :D

    Thanks a lot!
     
  2. bremen1874

    Capodecina

    Joined: 20 Oct 2008

    Posts: 11,910

    According to my weather station, the humidity outside is currently 95%.

    I'd assume that my well-ventilated loft is going to contain that same humid air.

    Do you have condensation/damp issues up there or just humid air?
     
  3. relicalpha

    Gangster

    Joined: 2 Feb 2020

    Posts: 104

    Location: UK

    TGha
    Thanks, I have recently had condensation problems from the extractor leak which is why I'm now checking to make sure that's all fixed, I don't have a record of what humidity was before that sadly. I guess if it is that high outside and you would expect a cold loft space to match outside levels, then that doesn't suggest a problem? I had just assumed that humidity that high would be really bad for timbers and could trigger mould etc? It's been sitting at those levels consistently for the week I've been checking.
     
  4. Mel_P

    Wise Guy

    Joined: 27 Feb 2004

    Posts: 2,315

    Location: Kent

    Last edited: 13 Jan 2021
  5. bremen1874

    Capodecina

    Joined: 20 Oct 2008

    Posts: 11,910

    The colder it is the higher the relative humidity will be for the same amount of water vapour.

    If you had a faulty bathroom extractor you'd get condensation because you're blowing warm wet air into a cold area. As the hot air cools it can't hold the water vapour and it comes out as condensation.
     
  6. relicalpha

    Gangster

    Joined: 2 Feb 2020

    Posts: 104

    Location: UK

    Ahh I see thanks, so the key is just ensuring it's a well ventilated space so it can get close to matching outside RH levels? As I currently have no obvious ventilation that does explain it, and will no doubt cause a problem so I'll get the vents fixed. I just wanted to make sure it wasn't indicating a hidden leaking pipe or similar :)
     
  7. bremen1874

    Capodecina

    Joined: 20 Oct 2008

    Posts: 11,910

    My loft doesn't have any 'obvious' ventilation either but it's there. The vents are all hidden behind the guttering (over facia vents).

    The best ventilation in the world wouldn't handle you dumping bathroom extractor air into a cold loft space.
     
  8. relicalpha

    Gangster

    Joined: 2 Feb 2020

    Posts: 104

    Location: UK

    And if everything is as it should be, there's no issue with high humidity if it's equally as high outside?
     
  9. GeX

    Soldato

    Joined: 17 Dec 2002

    Posts: 6,486

    Location: Manchester

    Nope, because it's relative humidity. If you heated the loft to the same temperature as the rest of the house then the RH would be roughly the same.
     
  10. Slade2

    Wise Guy

    Joined: 14 Apr 2011

    Posts: 1,138

    Location: Stafford

    There was a similar thread to this recently but I cant find it so maybe have a look at that one.

    The UK has had some really cold weather and I think a lot of people are finding condensation in their loft mainly like people have posted already because of the colder temperatures and the low pressure has meant hardly any wind as well so air has not been circulating as much as it does normally.

    If you are having issues with ventilation make sure any insulation is not pushed right into the end of the roof space you need air to come in and go out through the softies, as mentioned you could aid it with some tile vents but I think the issue might go away if/when the weather improves.
     
  11. relicalpha

    Gangster

    Joined: 2 Feb 2020

    Posts: 104

    Location: UK

    Thanks, still not quite sure I get it though :D Ive no issues with condensation anymore, it’s just the consistent 90% humidity in there still that confuses me... I get that’s relative to the low temps, and pretty much the same as outside humidity - but how is that high air moisture content not dangerous for the rafters and mold growth?

    even with ventilation, i get your loft will start to match external humidity levels, how does that not by itself cause damp under the current weather patterns? Sure I’m missing a basic understanding of physics here but I’d like to understand it better :)
     
  12. bremen1874

    Capodecina

    Joined: 20 Oct 2008

    Posts: 11,910

    You want to convert relative humidity to absolute humidity. There are online calculators that'll do it for you.

    If the temperature was 20 degrees and the relative humidity 40% would you be concerned? That's roughly the same amount of water in the air as 95% @ 6 degrees.