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  1.  
    I am about to install woodfibre boards over both a) stripped back brick walls plastered with Diathonite insulating plaster, or b) existing lime plaster. When fixing over existing plaster the builder wants to using mechanical fixings to mitigate against the existing plaster degrading and the whole thing falling off the wall. This seems logical. For fixing the boards over the new insulating plaster the manufacturer says you can just use adhesive so no fixings required. However the builder still wants to use fixings. I would prefer not to have a load of holes in the newly plastered wall unless absolutely necessary. Any experience with this?
    • CommentAuthorSimonD
    • CommentTimeFeb 12th 2021
     
    Who's the supplier and how thick are your boards?

    I'm guessing you're referring to IWI? I know that Pavatex IWI recommends mechanical fixings for both drylined and wet limed plastered.

    For EWI it's usually both adhesive and mechanical fixings.

    If you're worried about cold bridging with the mechanical fixings, I'm sure you can use thermally broken ones.

    However, happy to have my knowledge corrected and maybe your supplier uses a different system of adhesive?
  2.  
    Thanks. Its the Gutex IWI boards from EcologicalBuildingStore. The boards are only 20mm thick.
    • CommentAuthorjfb
    • CommentTimeFeb 12th 2021
     
    The fixings tend to be plastic with plastic pins, some have metal pins. I'm with the builder - i'd rather make sure they are fixed mechanically

    this sort of thing:

    https://www.ecomerchant.co.uk/universal-insulation-fixings-9988.html
  3.  
    This is what the instructions say:

    4. Installing the boards
    Install the GUTEX Thermoroom® so the full area
    of its rear side adheres to the suitably prepared
    substrate. Make sure the joints and boards are
    vertically and horizontally plumb. Use of additional
    mechanical fasteners is unnecessary when using
    GUTEX Thermoroom®.
  4.  
    Yes, plastic mech. fixings too. At 20mm they won't be T & G (??) so make sure you have the manuf's recommended no. of fixings. I don't know what adhesive you would be using. The Pavatex systems use 'adhesive' - a toothed-trowel-coat of the base-coat plaster - but it wouldn't hold the boards on its own - i 'tacks' them and helps with placing, but mech fixings are essential in my experience (which does not include fixing on top of Diathonite, BTW).
  5.  
    I think the 'no fixings required' is on the assumption that you use their own adhesive:

    https://www.ecologicalbuildingsystems.com/product/thermoroom-adhesive
  6.  
    Will you need to hang radiators shelves curtains etc on the walls?
    • CommentAuthorjfb
    • CommentTimeFeb 12th 2021
     
    I suppose if it complies with the manufacturer's guidelines it should be ok. Depending on the substrate that is. Does the diathonite plaster have a roughish finish? It is basically a scratch coat for the thermoroom adhesive. If the existing lime render is smooth I would definitely put some fixings in.

    Looking at the data sheet for the adhesive it says it is a cement based adhesive and says it is still very water vapour permeable (though no figure to compare with a lime render). Data sheet is here:
    https://gutex.co.uk/fileadmin/uploads/Downloads/Technische_Merkblaetter/GUTEX_EN_TD_Klebe_u_Spachtelputz_rot_2020-12.pdf

    Have you already bought the adhesive and wood fibre boards?
    • CommentAuthorSimonD
    • CommentTimeFeb 12th 2021
     
    I've read the board installation instructions now and have to agree with jfb. It does appear that it seems okay with just the adhesive, even if the price per bag is ridiculous and nearly £30 plus VAT and you need full board coverage. With 20mm boards, there isn't much weight either. The only note about mechanical fixings is if you're subsequently finishing the wall off with heavy tiles.

    You've clearly also done the necessary wall prep.

    If you're concerned about it, the plastic hammer in fixing don't cost much money, or much time to install and would provide extra pair of braces.
  7.  
    I have not bought the boards yet or anything else for that matter. Pencilled in for next week. The Diathonite does have a rough finish. The existing plaster is smooth so would definitely use fixings in this case. Basically the whole of the gable wall will be stripped, levelling coat of Diathonite and then WF boards. The remaining walls will keep their original plaster and the WF boards fixed on top.

    I am happy to use either system but just wary of putting extra holes into a wall where I don't need to. Cost saving is also a consideration. If the Gutex adhesive is expensive there might be a good argument for using another type of adhesive/render (any suggestions?) and fixings on all of the walls.

    Am I worrying unnecessarily about the effect of putting a whole load of holes in the walls (and through the expensive Diathonite layer!) for the fixings, even if thermally broken fixings?
    •  
      CommentAuthordjh
    • CommentTimeFeb 12th 2021
     
    Posted By: SimonDthe plastic hammer in fixing

    They're not actually plastic, I think, rather they're plastic-coated steel. So in theory plastic-coated stainless or even plain stainless might be better. But go with the flow. Whatever components the system has the ETA for.

    If you're worried, the option of examining all the specs in detail is available to you.
    • CommentAuthorArtiglio
    • CommentTimeFeb 12th 2021
     
    I’ve been fitting some steico internal boards today, a lime based plaster adhesive and plastic hammered fixings into a predrilled hole. On a purely instinctive level not sure i’d rely on only the adhesive, but thats exactly what happens in huge numbers of “dot n dabbed” dry lined houses. That said , i’ll be using more fixings tomorrow.
  8.  
    djh said: ''They're not actually plastic, I think, rather they're plastic-coated steel.''

    The ones I use are really a plastic pin in a plastic plug. Not a piece of steel in sight. The fixings NBT used to supply had a stainless pin. I have never seen a plastic-coated metal pin. Even for quite thick EWI many system providers supply all-plastic fixings.
    • CommentAuthorjfb
    • CommentTimeFeb 12th 2021
     
    you can get steico boards at not far off half price the gutex. I just fixed them to lime rendered walls, no adhesive.
    • CommentAuthorSimonD
    • CommentTimeFeb 13th 2021
     
    Posted By: djh
    Posted By: SimonDthe plastic hammer in fixing

    They're not actually plastic, I think, rather they're plastic-coated steel.



    Posted By: Nick Parsons

    The ones I use are really a plastic pin in a plastic plug. Not a piece of steel in sight. The fixings NBT used to supply had a stainless pin. I have never seen a plastic-coated metal pin. Even for quite thick EWI many system providers supply all-plastic fixings.


    It depends on the thickness of the boards and the system but I do think there is a move towards using Ejotherm anchors which are all plastic. Ejotherm plastic screw in anchors can take up to 360mm thick EWI boards. I have thermally broken steel expansion pins for my 140mm EWI boards but I also have a load of shorter full plastic ones for either iwi or ewi.

    Personally, I'm slightly annoyed my supplier provided the steel pin version for me rather than the all plastic.
  9.  
    Posted By: jfbyou can get steico boards at not far off half price the gutex. I just fixed them to lime rendered walls, no adhesive.


    Thanks the Steico boards do look cheaper. Not 50% but maybe when you take out the need for adhesive it could work out a lot cheaper. I hadn't considered the option for fixings only with no adhesive. I assume you would need a very flat substrate for this. I may be able to achieve this given that a plasterer will be applying the Diathonite layer.
    • CommentAuthorSimonD
    • CommentTimeFeb 14th 2021
     
    Posted By: modernvictorian
    Posted By: jfbyou can get steico boards at not far off half price the gutex. I just fixed them to lime rendered walls, no adhesive.


    Thanks the Steico boards do look cheaper. Not 50% but maybe when you take out the need for adhesive it could work out a lot cheaper. I hadn't considered the option for fixings only with no adhesive. I assume you would need a very flat substrate for this. I may be able to achieve this given that a plasterer will be applying the Diathonite layer.


    Be careful with this as part of the job of the adhesive is to prevent air circulation behind the boards due to undulating substrate. It may still be a good idea to use some adhesive. For example, Ty Mawr use ISOVIT E-Cork (AdhereVit) which is £16 + VAT per bag for their IWI system, so definitely half the price of of the Gutex.
    • CommentAuthorArtiglio
    • CommentTimeFeb 14th 2021
     
    I’ve been putting up steico therm 20mm bonded with RK70, i’m getting about 12m2 to a bag, but must be said the walls are pretty flat already, then using the ejotherm fixings to add onother 40mm ( 60mm boards were unavailable until april) No bonding required between the two boards, and as pointed out the adhesive is more to prevent air culation behind the boards than actual fixing ( or so i was told).
    Whilst the costs do add up, it would seem foolhardy to want to save a pound or two per metre on something that if it failed in the futre would be massivley disruptive and expensive to put right. My biggest cost saving was finding a supplier who kept stock near me and so i can pick up, saving a considerable amount on pallet deliveries. Plus i’m ordering reasonable quantities and got a discount on list prices for everything, that was just over the discount offered on pallet quantities.
    So long as it performs as hoped and gives a comfortable home along with a bit of efficiency , i’ll be happy.
  10.  
    Thanks. Looking at the EcoMerchant system they recommend the MultiContact Baumit MC55 W. About the same price as RK70. The builder will probably want to use a recommended system. So looks like the way to go is Steico boards, thin layer of Baumit adhesive type render for levelling, and thermally broken fixings.
    • CommentAuthorArtiglio
    • CommentTime7 days ago
     
    Apologies regarding an earlier post regarding the ejotherm fixings, one came apart and as stated elsewhere it does have a stainless pin in it.
    • CommentAuthorSimonD
    • CommentTime7 days ago
     
    Yes, some of them do have metal pins, some don't:

    All plastic Ejotherm hammer in https://www.ejot.co.uk/Building-Fasteners/Applications/ETICS/H3/p/H3
    With coated metal pins https://www.ejot.co.uk/Building-Fasteners/Products/ETICS-anchors-UK/H1-eco/p/H1-eco
    • CommentAuthorwookey
    • CommentTime7 days ago
     
    Woodfibre is heavy so fat boards really do need mechanical fixings but for 20mm boards glue-only will be just fine. Certainly metal fixings would be a terrible idea on such thin boards.

    Udi do a special hex-pattern board optimised for exactly this thin internal insulation application. You might want to compare claimed performance and cost with the Gutex boards: https://www.udidaemmsysteme.com/udi-system/for-interior-insulation/udiin-2cm-system/ (£14/m2) Lap joint so they'll be flat at joints. Those are for reasonably flat walls - they have a different board for heritage wobbly-walls.

    Glue it on with Baumit Starcontact (cheap@£12/bag) or their foundation coat. (If you are anywhere near Cambridge I have about 6 bags of starcontact left over which you can get a very good price on).
  11.  
    I did look at those honeycomb boards early on but the stated thermal conductivity is 0.089 W/mK which compares badly to a normal woodfibre board of the same thickness. Or am I missing something?

    Still to decide on final spec! Basically I have a gable wall which is still covered (mostly) in original lime plaster. The hallway is only 90cm wide hence the insulation in this area must be slim. Builder advises to keep original plaster and fit insulation over the top. If I do this I can probably only squeeze in 20 - 30 mm of insulation.

    I would like to strip original plaster from the gable wall, replaster using 20 mm insulating plaster (e.g. Diathonite) and then apply 20mm woodfibre panels over the top. The front and back walls will just get woodfibre boards over the original plaster (large windows on these walls so difficult to justify an expensive insulation buildup).

    Cost is a big issue if I use the insulating plaster. Unless I do most of it myself (which I'm now leaning towards).

    If anyone has any better suggestions please fire away.
    • CommentAuthorjfb
    • CommentTime2 days ago
     
    Posted By: modernvictorianI would like to strip original plaster from the gable wall, replaster using 20 mm insulating plaster (e.g. Diathonite) and then apply 20mm woodfibre panels over the top.


    How bad is the gable wall plaster? Still mostly sound (no hollow bits behind) and flat enough?
    If so I wouldn't bother with the insulating plaster as it is major extra cost (both the material itself, time to rip off render and remove it and time for re rendering). Just go for 40mm insulation (or 60).

    If it is a question of patching up a few bits I wouldn't rip it all off.
    Pictures might help.
  12.  
    The gable wall looks very flat. It is hard to tell about the condition of the lime plaster. It certainly crumbles away very easily at the edges and when you drill into it. But the main problem is space. As you can see I only have the width of the doorway to work with. If I laid 40mm over the existing plaster I would be covering the door frame. If I used 20mm I would question whether it is worth bothering? Hence the idea to remove the existing plaster and get at least 40mm of good insulation on that wall (likely the coldest wall of the house as it faces into a gloomy passageway).
      passage.JPG
  13.  
    Wall
  14.  
    Loft.
  15.  
    Had to make a decision today so the plaster on the gable is going/already gone in places. Hope I don’t live to regret it.
  16.  
    But you need an air-tightness layer. Normally parge is min 10mm, but can you get 5mm on each side. Better than nothing.

    (Or is the 40mm insulation you refer to here made up of 20mm diathonite and 20 WF? If so, ignore me - you (will) already have an a/t layer!)
   
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