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General: Loft conversion - no roof felt on existing roof

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  1.  
    https://www.proclima.com/products/external-sealing/solitex-mento-3000/application-guide/photo-gallery-retrofitting-underlay-from-the-inside-4597
  2.  
    if the preference to use continuous sheet membrane is for speed of installation, then follow up with a stanley knife, and slice the membrane off the lower part of the rafters.
  3.  
    Thanks. I was considering that option so glad to hear it’s not a crazy idea.
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: modernvictorianThe builder wants to drape the membrane between rafters rather than cut individual strips. The Proclima website shows only the individual strips method. Is there any issue with wrapping the rafters in membrane? Perhaps water pooling in the troughs i.e. under the rafters?

    https://www.proclima.com/products/external-sealing/solitex-mento-3000/application-guide/photo-gallery-retrofitting-underlay-from-the-inside-4597

    Thanks for the link. I don't understand what the builder is proposing? Draping rather than cutting implied to me that you/he has access from the top to fit the membrane, but the link is discussing installing from underneath so I don't understand what your situation is or what the builder was proposing?

    I note also that the application guide clearly says "The membranes are to be installed as an underlay or sarking membrane horizontally (parallel to the eave) in a taut manner with no sagging." So any idea of draping is presumably a non-starter.
  5.  
    'Draping' may not be an accurate description. He is proposing to run the membrane as one continuous sheet by wrapping around the rafters and pushing up the spaces between joists. Using staples to secure it to the side of the rafters. All from underneath.

    Its not ideal but I think it might work as long as they cut the sections under the rafters with a Stanley knife as GreenPaddy suggested.
    •  
      CommentAuthordjh
    • CommentTime6 days ago
     
    Posted By: modernvictorianUsing staples to secure it to the side of the rafters.

    The instructions say to use battens, not staples. i.e. full support that might be vaguely watertight or draught-tight.

    Note that some of what the instructions say about using specific methods and products might not matter to you. They did matter to me since I now have a 25-year warranty on the roof structure courtesy of Pro Clima.
    • CommentAuthorMike1
    • CommentTime6 days ago
     
    I've done similar in the past, but using single strips + battens. I can't see any advantage of not cutting it in strips - not to do so would make it more unwieldy and harder to fit.
  8.  
    Mike it was your advice higher up this thread that led me to this product so thanks for that. Builder has now agreed to install in strips and battens which is likely easier as you say. Thanks all. Months of preparation and I still feel like I'm making decisions on the fly.
  9.  
    The installation instructions for that membrane are quite specific that you use an extra "batten" parallel to the rafters, and halfway between them, to positively create the "valley" that would conventionally be provided by letting the membrane sag between rafters.

    So it seems it would be possible to follow the guidance that it's installed "taut".

    I've not seen this approach before; I wonder why they recommend it. Is it because you can't do a visual check from outside that the valleys are still there, after pushing insulation in? Or maybe it improves the airtightness/draught-proofness at laps between runs.
  10.  
    Yes that is the ideal method. I can' t install the batten to make the valleys unfortunately as I would lose the specified depth of insulation and the U-value would then be out. Its not ideal but hopefully better than nothing in terms of at least keeping the wind away from the insulation. If water does come in it will run down into the woodfibre batts which hopefully can then dry out again at the next opportunity.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Posted By: lineweightI wonder why they recommend it.

    Possibly the membrane won't stand up to the extra movement it will suffer if it is draped - billowing in the wind and so forth. They don't just recommend it, they say 'are to be installed .. in a taut manner with no sagging'. That's a pretty black and white instruction.
  12.  
    Posted By: modernvictorianYes that is the ideal method. I can' t install the batten to make the valleys unfortunately as I would lose the specified depth of insulation and the U-value would then be out. Its not ideal but hopefully better than nothing in terms of at least keeping the wind away from the insulation. If water does come in it will run down into the woodfibre batts which hopefully can then dry out again at the next opportunity.


    Isn't the other purpose of the batten to ensure there is a sufficient ventilation gap between the membrane and the underside of the tiles?

    I would be wary of sacrificing that in order to get the "specified depth of insulation". It seems like the tail wagging the dog.

    I don't see what you mean about water coming in - if it gets in, it should be running on top of the membrane and not getting into the batts at all. The batts are under the membrane aren't they?

    The danger is water pooling against the tiling battens, instead of running underneath them.
  13.  
    There will be a 50mm ventilation gap from under the slate (25mm in batten void and 25mm from top of rafter). There are nails protruding from the battens which also need to fit into this space.

    Without creating the valleys (requiring an additional 25mm which I don't have...) I guess there is more risk of water running down the sides of the rafters and escaping through the edge of the membrane. Not sure a watetight seal against wood is possible.
  14.  
    If it was me, I'd prefer to sacrifice the 20mm of insulation, that would be necessary to achieve the valley they recommend.
  15.  
    I agree but Building Control are only worried about the U-value.
  16.  
    Is the insulation squashy enough that it would fill up to the membrane at the sides, and then be compressed slightly in the middle? Then I would argue that on average only 10mm thickness is lost which will have a negligible effect on things.
    • CommentAuthorkebl2825
    • CommentTime5 days ago edited
     
    Posted By: modernvictorian... I guess there is more risk of water running down the sides of the rafters and escaping through the edge of the membrane. Not sure a watetight seal against wood is possible.


    A detail I've seen mentioned on the forums before (I think on a suspended timber floor insulation thread?) is a glue/sealant line between the rafter and the membrane before mechanically fixing the batten from the other side of the membrane as a belt-and-braces seal. Just needs to be something that plays nicely with both the proclima solitex and the timber and should help confidence in it being watertight once cured.
    •  
      CommentAuthordjh
    • CommentTime5 days ago
     
    Why buy a premium product that you're not prepared to follow the manufacturer's installation instructions on? Find an alternative product with different installation constraints, contact the manufacturer and get a waiver or buy a cheaper product and misinstall that.

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