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    • CommentAuthorgoodevans
    • CommentTime4 days ago edited
     
    Now - don't shout at me - but over the last couple of days I've also been looking of moving the air barrier towards the outside - here's my thinking...

    I plan to have a single blockwork skin with eps and flexible render outside going from under the ground level up to the roof. The roof will have an eps outer insulation on top of osb which is supported on the joists. At present I plan a wet plaster internal skin.

    The thinking to date has been my internal plaster is the air barrier connected to the osb in the roof which is to be my roof air barrier (other threads have discussed the method of connecting the two together).

    Would I be successful in getting the external render to be my air barrier instead of internal plaster? I then have two membrane options on the roof either a membrane on on top of the roof eps (option A) or a barrier on top of the osb (option b) (or both). Option A could also be my waterproof underfelt (non draping and flush against the EPS). Both options can be embedded into the render for a continuous barrier. Option A also has benefits because it removed wind wash through the eps. Option B has a benefit in that it splits convection circulation paths in half as there will be insulation between the rafters also.

    At doors and windows the air barrier would be contiguous with the water barrier - i.e. get a good barrier between the render and the frames.

    Any thoughts or am I on a hiding to nothing. The benefit is simplified details - no tony trays at gable ends, no worrys about barrier gaps behind skirting. But it does mean my render needs to be crack free and well maintained.
      Air Barrier Rev A.jpg
  1.  
    Is your EWI to be fixed with mechanical fixings, adhesive or both? If there is adhesive, and if that is full coverage, then that can be the a/t layer, and the render is a belt-and-braces back-stop. A little bit of convolution at the top brings it up to meet a/t layer A, or it can meet layer B dropped down the front of the joist/rafter ends. How will an external a/t layer connect tightly to your floor a/t layer?
    • CommentAuthorgoodevans
    • CommentTime4 days ago edited
     
    EWI fixing not decided on yet - As a minimum I was going to specify horizontal adhesive stripes top and bottom of each panel - but would be happier with full coverage if it does not make fixing difficult with air pockets (presumably applied to the wall). The blockwork will be clean and flat (inc morter).

    I don't mind the adhesive layer being the backup to the render - I think making the render the primary barrier makes it easier to connect with the windows/doors as they will be set into the EWI. (door/window frames always need to directly interface with the render for waterproofing).

    At the ground floor I could bring the DPC/DPM through the eps and leave enough poking out/lapped down for the render to seal against. (I have no radon issues here).
    •  
      CommentAuthordjh
    • CommentTime4 days ago
     
    If the air barrier is external it often needs to be breathable, in order to prevent condensation forming behind it. The external surface is also subject to much more environmental variation than the inside surface (temperature, wind, rain, solid object impact, plants, insects, rodents, to name a few possible factors) meaning it needs more careful design. Be very careful to design details at the bottom of things carefully - window and door sills, the bottom of walls etc. Think about how any water that somehow gets to those places can escape whilst maintaining airtightness. How is the render connected to the floor slab in an airtight but water permeable way viewed from above whilst being watertight viewed from below, for example? Water can come from temporary failures somewhere (open doors, broken windows, missing tiles, leaking gutters) or from leaks or flooding inside the building.

    You also have to make every fixing into the outside of the wall airtight forevermore. Admittedly having the airtightness barrier inside has a similar problem, although there are standard techniques such as service cavities to ameliorate this.

    A roof air barrier in position A needs careful roof design to handle the risk of condensation inside the roof.

    As you may be able to detect, I think it's easier to design the main air barrier inside a building. :bigsmile:
    • CommentAuthorgoodevans
    • CommentTime4 days ago edited
     
    I'm thinking - make it all 'breathable' except for the dpm/dpc. I'm not planning for weep holes above the windows or dpm so any rain penetration would have to be through evaporation - either through the render or internally - but this is a problem whether the external render is the air barrier or not.

    I was planning on the DPM being just under the screed - flooding moisture is then restricted to 70mm - in this case it should evaporate fairly quickly in an heated house (UVH assistance as well).

    Every fixing outside should be made watertight in any case - I have no cavity protection - ensuring airtight at the same time should not be too bad - and would only be an issue where the fixing is over eps joints and deeper fixings.

    So lets assume a silicone render for waterproofness, flexability, and breathability - how does it fair as an air barrier?

    I have no doubt that at some point there will be a breach of the water barrier allowing water to run down the EWI or block wall. As the windows and doors are located in the insulation layer would some kind of embedded flashing be advised above the windows to divert water to the side or is it better to let the water show on the inside to indicate there is a fault that needs a repair.
    • CommentAuthorgravelld
    • CommentTime4 days ago
     
    On the external face of the block work? Basically what nick said about adhesive, but could also use S&C?
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