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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorhepm
    • CommentTimeMay 16th 2021 edited
     
    Build up will be as follows:

    150mm rafters with 50mm air cavity and 100mm STEICOflex between
    70mm PIR under rafter taped
    Airtight INTELLO PLUS membrane
    25mm batten for service void
    15mm plasterboard

    1. What are the correct fixings for PIR to the rafters to avoid thermal bridging?
    2. What are the correct fixings for battens to the rafters to avoid thermal bridging?

    I think these are suitable, are there any recommended alternatives?
    https://www.insulationsuperstore.co.uk/product/surelino-insulation-securing-fixing-85mm-pack-of-200-1.html

    Links to any items are greatly appreciated.
    • CommentAuthortony
    • CommentTimeMay 16th 2021
     
    They are ok for walls but not for deadweight pulling them downwards and out

    I would use wafer screws, few as possible

    Be very careful to fully ensure that outdoor air can’t gain access to your service void
    • CommentAuthorkristeva
    • CommentTimeMay 16th 2021
     
    Posted By: hepmBuild up will be as follows:

    150mm rafters with 100mm STEICOflex between
    70mm PIR under rafter taped
    Airtight INTELLO PLUS membrane
    25mm batten for service void
    15mm plasterboard

    1. What are the correct fixings for PIR to the rafters to avoid thermal bridging?
    2. What are the correct fixings for battens to the rafters to avoid thermal bridging?

    I think these are suitable, are there any recommended alternatives?
    https://www.insulationsuperstore.co.uk/product/surelino-insulation-securing-fixing-85mm-pack-of-200-1.html

    Links to any items are greatly appreciated.


    I also need to purchase some fixings for insulation. Just thinking out loud, would it not be possible to use normal screws and then cover with airtight tape?
    • CommentAuthorhepm
    • CommentTimeMay 16th 2021 edited
     
    A little more searching and I see these frequently:

    https://www.ecomerchant.co.uk/roofing/insulation/insulation-fixings/ejot-str-h.html

    Posted By: tonyI would use wafer screws, few as possible

    Is that for both PIR and battens?

    Posted By: tonyBe very careful to fully ensure that outdoor air can’t gain access to your service void

    Can you elaborate?
    • CommentAuthorjms452
    • CommentTimeMay 16th 2021
     
    Stainless screws have about a third the conductivity of normal steel.

    I'd use as few as possible to hold the insulation in place then screw through the batons and insulation and into the rafters (but would bow to Tony's experience on this).

    Intelleo's a smart membrane so isn't doing much up against a taped foil layer (apart from being a tough air tightness layer).
    • CommentAuthorhepm
    • CommentTimeMay 16th 2021 edited
     
    Posted By: jms452 Intelleo's a smart membrane so isn't doing much up against a taped foil layer (apart from being a tough air tightness layer).


    I was thinking the airtightness membrane would still be needed to deal with the joints where the insulation boards meet purlins, gable/party walls and also where I have roof windows.

    I should add that this is a ventilated roof space that will have a 50mm cavity between the rafters.

    If the opinion is that it isn't worth the effort for the benefit then it would make the installation easier.
    • CommentAuthorjms452
    • CommentTimeMay 16th 2021
     
    Posted By: hepmairtightness membrane would still be needed to deal with the joints where the insulation boards meet purlins, gable/party walls and also where I have roof windows.


    air tightness membrane yes.

    My comment related to intelleo plus being quite expensive and having smart vapour pemeability which you don't need - it would be fine but there are cheaper membranes out there - some people even use polythene although it rips a bit easily for me.

    a few of the popular ones are here:
    https://www.greenbuildingstore.co.uk/product-category/airtightness-products/membranes-interior/
    • CommentAuthortony
    • CommentTimeMay 16th 2021
     
    In Canada they use polythene, not sure about Scandinavia but suspect they use poly too
    • CommentAuthorhepm
    • CommentTimeMay 16th 2021
     
    Posted By: jms452
    Posted By: hepmairtightness membrane would still be needed to deal with the joints where the insulation boards meet purlins, gable/party walls and also where I have roof windows.


    air tightness membrane yes.

    My comment related to intelleo plus being quite expensive and having smart vapour pemeability which you don't need - it would be fine but there are cheaper membranes out there - some people even use polythene although it rips a bit easily for me.

    a few of the popular ones are here:
    https://www.greenbuildingstore.co.uk/product-category/airtightness-products/membranes-interior/


    Thanks for clarifying and the info.
    •  
      CommentAuthordjh
    • CommentTimeMay 16th 2021
     
    Posted By: jms452some people even use polythene although it rips a bit easily for me.

    We used PE membrane in our conservatory and didn't have any problems, but then I suppose we and our builders aren't typical. Perhaps it depends on the gauge (ie. thickness) of the membrane? As a vapour/airtight membrane behind our first floor timber ring beam we cut down a sheet of PE DPM rather than rely on an 'airtightness membrane'. That plus butyl patches where the fixing bolts for our ledger beams went seems to have worked.
  1.  
    Hi Hepm, we insulated several pitched sections on our last house and also had a joiner do some. We had a debate with him about methods, and merits of long screws. His preference, if applied to your roof, would have been:

    -150mm Rafters filled with 100mm PIR insulation leaving 50mm air gap, this layer is most of the insulation value
    -Membrane or taping as required for airtightness
    -Horizontal timbers running across rafters with PIR sheets between, thickness to meet remaining U value requirements eg 40 or 50mm, wiring run in this layer
    - Plasterboard

    Our proposal was
    -Rafters/insulation/airgap
    -Continuous sheet PIR all over the rafters, taped
    -Plasterboard, fixed through PIR to rafters with long fixings

    Our reasoning was to avoid timbers bridging through the inner layer of PIR, and avoid cutting PIR to exactly fit between timbers

    His reasoning was that his productivity depended on using his higher speed tools, collated screws and nailgun, he wasn't keen on long fixings which were slow to drive.

    There were merits for both approaches but in the end we went with his proposal, turned out that the criss cross of timbers and rafters pretty much mitigated the cold bridging of the timbers through the insulation, and avoided having lots of long fixings which themselves would all have been cold bridges. Being much more skilful than us, the joiner was good at cutting PIR to fit without gaps.

    We avoided having service voids, as that house already had lots of draughts and mice, in any little cavity they could find!
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