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General: EWI fixing: mechanical vs adhesive vs mechanical + adhesive

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    • CommentAuthorShevek
    • CommentTimeJul 17th 2013 edited
     
    What method do you recommend and why?

    As I understand it these are the options for fixing external wall insulation:

    1. Mechanical fixings only
    2. Adhesive fixing only
    3. Adhesive + mechanical fixings

    WIth the two adhesive options being:

    A. Mineral adhesive, with 50-100% coverage to the back of the board
    B. PU foam adhesive, with perimeter and zig-zag coverage to back of board

    And in all cases good practice being to fill the gaps between boards with PU foam to optimise thermal performance.

    Discussion here on 100% PU foam adhesive:
    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=9140

    And attached is a document from Parex about why the recommend [mineral] adhesive over mechanical fixing.
    • CommentAuthorShevek
    • CommentTimeJul 17th 2013
     
    In our case we're fitting 150 mm to existing solid brickwork and 250 mm to new solid blockwork. I don't know which method to choose. I'm leaning towards adhesive-only, because I can't be assed with mechanical fixings or the added expense but also because I like the idea of not having any form of thermal bridge and also because our EWI will be supported at the base on concrete foundations (apart from over windows of course).

    Just can't decide between mineral or PU foam. Are the similarly priced?
    • CommentAuthorjamesingram
    • CommentTimeJul 17th 2013 edited
     
    As mentioned , I 've used mineral adhesive with 2 mech. fixing per boards.
    Not used PU foam , but have used mech. fixing only , but this was on 'tin tops' post war prefabs.

    Cant speak from experience but PU only makes me slightly nervous, if you don't mix/shack up the can correctly or get a dodgy batch, PU foam can go a bit dusty over time (happen when exposed also (UV?)

    I like mineral +fixings
    When combed up it give a good full bed with room to level up
    I'm used to spreading stuff with hawk and trowel
    You use the same gear for mess coat
    could be consider an external parge/airtightness layer behind insulation
    mech. fixing hold it in place whilst working on adjancent boards.
    they also pull it in a bit , make it feel more secure.
    Can live with small thermal bridge of 2 fixings if it reduces potential for failure rate.

    PU foams
    nasty , horrible toxic stuff that gets all over your hand and clothes and gives you a headache.

    But as said not used it for EWI, so it might be great !
    • CommentAuthorSwarm
    • CommentTimeJul 18th 2013
     
    Not speaking from experience but I will be in a similar position to you Shevek and will fully favour James's approach and logic.

    I also really don't like the thought of a slab of insulation, plus attached brick slips falling from a great height, or having to get scaffolding back to fix it etc. Even if it's a very remote possibility.

    Thank you for the PDF too, interesting bed time reading. :)
    • CommentAuthorTimSmall
    • CommentTimeJul 18th 2013
     
    I reckoned that there is about a 10x safety margin with the PU foam method (didn't calc for the mineral adhesive but I'd think it's similar), so you have to go some to do it wrong I think.

    With either adhesive, I don't think you'll get either insulation off without destroying the blocks.

    Don't know about GWP of either system, but I know that the PU is quicker (and I believe a bit easier, BICBW I've never used the mineral stuff).

    I believe that you're supposed to use mechanical fixings along with the adhesive above a certain height with EPS above a certain height (two storeys?) - due to risk of the render layer heading downwards in a fire I think.

    Also a requirement to put in a rockwool firebreak between the 2nd and 3rd storeys in my case I think when I looked into it (according to the Weber BBA cert I think). Not done the EWI except on the loft conversion yet, so I'm not 100% sure.

    Tim.
    • CommentAuthorShevek
    • CommentTimeJul 18th 2013 edited
     
    Posted By: TimSmallAlso a requirement to put in a rockwool firebreak between the 2nd and 3rd storeys in my case I think when I looked into it (according to the Weber BBA cert I think).

    Oh, that's a point. We're mid-terrace. Do we need those vertically at the party walls?
  7.  
    Posted By: Shevek
    Posted By: TimSmallAlso a requirement to put in a rockwool firebreak between the 2nd and 3rd storeys in my case I think when I looked into it (according to the Weber BBA cert I think).

    Oh, that's a point. We're mid-terrace. Do we need those vertically at the party walls?

    I'd have thought only if your neighbour EWI or have flammable cladding etc.
    One thought RE PU foam , it's a lot less to ship about so EE (embodied energy) might be much less ??

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