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Condensation in loft.

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I have posted on a couple of threads about how i have insulated my loft. 
I was pleased with the result, particularly this morning when the roof was completely white.

I went up there this afternoon, and there was condensation on the roof lining, which was dripping. It was all over the loft.

I have been particularly careful in making sure that i left gaps at the eaves and between the insulation and the loft boards.

Was I always going to get this, and haven't realised. Is there anything I can do to prevent it

thanks in advance for any pointers.

Not sure if this picture helps.


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  • stuart45
    stuart45 Posts: 4,121 Forumite
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    How is the roof ventilated?
  • MisterNick
    MisterNick Posts: 1,246 Forumite
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    Not sure I have the terminology right, but in the soffits that run along the eaves for the guttering have almost like vents in them.

  • stuart45
    stuart45 Posts: 4,121 Forumite
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    You probably need more ventilation. You can't have that amount of moisture dropping down all the time. You might need eaves to ridge ventilation.
  • jacko220
    jacko220 Posts: 125 Forumite
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    The roof space is now a "cold" roof space, which means that it cannot hold as much moisture as your previous "warm" roof space. I have pasted a number of times the psychrometric chart. Have a look at this or google it this shows you what I have said. The problems will be if the timber in your trussed rafter roof exceeded its equilibrium moisture content (emc) the roof is held together with pressed metal plates that are galvanized which is good but in the punching out process bare steel is exposed, check to see if there is any white rust or ferric rust, you sometimes get a reaction between the salts in the timber protector and the steel. 

    Make sure the roof space is ventilated or even better cross ventilated with gaps, just make sure that the space is ventilated probably more than it needs, the thermal insulation of your roof stops at the top of the joist insulation. You have got what is known as interstitial condensation. I lectured in this and other Construction Science problems for over 30 years. If you have any questions please feel free to ask.
  • housebuyer143
    housebuyer143 Posts: 3,502 Forumite
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    edited 11 December 2022 at 5:40PM
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    I have posted on a couple of threads about how i have insulated my loft. 
    I was pleased with the result, particularly this morning when the roof was completely white.

    I went up there this afternoon, and there was condensation on the roof lining, which was dripping. It was all over the loft.

    I have been particularly careful in making sure that i left gaps at the eaves and between the insulation and the loft boards.

    Was I always going to get this, and haven't realised. Is there anything I can do to prevent it

    thanks in advance for any pointers.

    Not sure if this picture helps.


    I went into mine yesterday and the felt is wet from condensation. Not sure how it can have anymore ventilation as it actually got loads as it needs it for my whole house fan. 

    It's only the North facing side that's wet so could be to do with the crazy cold weather recently. That or I spilled some water up there a few days earlier.. could have helped cause it.
  • grumbler
    grumbler Posts: 58,629 Forumite
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    I have been particularly careful in making sure that i left gaps at the eaves and between the insulation and the loft boards.

    Was I always going to get this, and haven't realised. Is there anything I can do to prevent it

    Another important step is to prevent moist air getting into a loft from the house. Without supply of moisture there will be some condensation, but no more than on your car outside.

  • stuart45
    stuart45 Posts: 4,121 Forumite
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    grumbler said:

    I have been particularly careful in making sure that i left gaps at the eaves and between the insulation and the loft boards.

    Was I always going to get this, and haven't realised. Is there anything I can do to prevent it

    Another important step is to prevent moist air getting into a loft from the house. Without supply of moisture there will be some condensation, but no more than on your car outside.

    The trap hatch is usually the worst offender.
  • grumbler
    grumbler Posts: 58,629 Forumite
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    edited 11 December 2022 at 6:16PM
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    Recessed lights are often bad offenders too. You can see the gaps and they don't have to be big given the noticeable pressure difference in winter.
  • ashe
    ashe Posts: 1,568 Forumite
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    I've got a similar problem, and was about to update my other thread. I've seen on my local town FB noticeboard a lot of people saying similar things about their loft and condensation, and it was suggested to  insert lap vents. These are pieces of plastic that go between those layers of felt and allow ventilation. I suspect you have a similar problem as me in that once moisture is in there it doesn't have anywhere to go because of the bitumen felt, so it condenses on that then is dripping onto the insulation. 


  • MisterNick
    MisterNick Posts: 1,246 Forumite
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    stuart45 said:
    You probably need more ventilation. You can't have that amount of moisture dropping down all the time. You might need eaves to ridge ventilation.
    Thanks
    Just googled eaves to ridge ventilation, but it all seems to be fitted when installed. Am I missing something?
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