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    • CommentAuthorgravelld
    • CommentTimeMar 3rd 2017 edited
     
    What is considered best practice at the moment for air tightness on cavity walls that are being externally insulated?

    I know about stopping convection currents in the cavity by filling and sealing (although maybe this plays into the overall solution).

    However I'm talking about the air tightness of the entire wall itself. Is a membrane mechanically fixed externally before insulation? Parge coat the exterior?

    ** Edit ** I'm interested in _retrofit_ EWI on existing cavity walls.
  1.  
    Hmm, well...

    Most of the plastic insulants use an adhesive as well as mechanical fixings. Though some might 'blob and dob' I intend to use full-coverage adhesive and effectively use it as a parge coat.

    At least one of the wood-fibre EWI suppliers does not recommend an adhesive coat. On the one hand I can see why, as the mech fixings will hold it all fine, but if I use wood-fibre externally I will use a (lime) parge/adhesive coat. The only case where it would be hard to achieve a parge coat is where the substrate is very uneven - too uneven for 'dubbing out' - and a composite board - of rigid wood-fibre as the render carrier and wood-wool 'fluff' behind to take up the unevenness - is used.
    • CommentAuthordjh
    • CommentTimeMar 3rd 2017
     
    I would have thought that you'd want the airtightness barrier on the inside of the wall. That's what you need for integrity of the thermal performance of the house. Further outboard of that, you're simply trying to eliminate 'wind washing' and convection currents in the insulation as I understand it, so there's no need for rigourous airtightness. Just a good attempt to fill voids to prevent convection, and a mostly sealed barrier to prevent the wind blowing in.
    • CommentAuthorMarkyP
    • CommentTimeMar 3rd 2017
     
    my extension is having EPS EWI over new medium density blockwork cavity walls with full fill mineral wool insulation. I think the combination of the adhesive on the EPS and then the render on the EWI will make it air tight externally. And then on the inside, wet plastering the walls (float and set) rather than dot and dab.
  2.  
    ''I would have thought that you'd want the airtightness barrier on the inside of the wall. That's what you need for integrity of the thermal performance of the house. Further outboard of that, you're simply trying to eliminate 'wind washing' and convection currents in the insulation as I understand it, so there's no need for rigourous airtightness. Just a good attempt to fill voids to prevent convection, and a mostly sealed barrier to prevent the wind blowing in.''

    You may be right, depending on how bad the 'wind-wash' is. My take in terms of my post above was that cold air might be entering the cavity through gaps in the masonry. Therefore parging could help. I realise that the air-tightness layer needs to be internal, but one could have a fantastically air-tight house with no insulation, so reducing (to as close to nil as possible) the chance of wind-wash has to be high up on the shopping-list too.
    • CommentAuthordjh
    • CommentTimeMar 3rd 2017
     
    Yes, I've no problem with doing a good job on the outer airtightness; I just wanted to make it clear that the primary airtightness layer should still be inside in this type of construction.

    It's generally best to have the airtightness layer fairly well inboard, either an exposed internal plaster layer or a membrane, ideally protected by a service cavity. There are some constructions that are designed with it towards the outside, but generally it's more difficult to achieve a good result.
  3.  
    Posted By: MarkyPmy extension is having EPS EWI over new medium density blockwork cavity walls with full fill mineral wool insulation. I think the combination of the adhesive on the EPS and then the render on the EWI will make it air tight externally.

    If the EPS is applied with dot and dab then you can (will) get convection air flow between the wall and the EPS. Either use full-coverage adhesive or (what I do is) put a solid line of adhesive (as a minimum) on the top and bottom edge or better on all 4 edges and then a couple of dollops in the middle. I have found that a line around the edge is better than full cover if the wall is uneven.
    • CommentAuthorgravelld
    • CommentTimeMar 3rd 2017 edited
     
    Posted By: djhI would have thought that you'd want the airtightness barrier on the inside of the wall. That's what you need for integrity of the thermal performance of the house.
    This is one of the things I wanted to learn about. I see it written a lot but without explanation. Can you explain more about this?

    There are major issues I perceive with internal air tightness layers:

    - Surely it's more complicated? And therefor more liable to error? It means going around every joist etc
    - Disruption - if EWI is sold as low disruption, and we want to perform proper holistic retrofit, how are we going to do air tightness? Plus I don't fancy ripping out every internal wall covering in my own house.

    I would've thought, conceptually, an external (although internal to insulation) air tightness layer is easier and possible in the same way wrapping a sandwich in some cling film is (well not exactly the same way!)
    • CommentAuthordjh
    • CommentTimeMar 3rd 2017
     
    Posted By: gravelldSurely it's more complicated? And therefor more liable to error? It means going around every joist etc

    No it's generally less complicated. Change the design to avoid problems. For example use a ledger board inside the airtightness barrier to support joists instead of running them into the blockwork or studwork. Outside there are lots of penetrations for things that all would need care, and the external environment is a lot harder on components and junctions. For example how do you ensure drainage of any water that does somehow find its way into a wall? How do you do the wall-floor detail and the wall-roof details?

    Disruption - if EWI is sold as low disruption, and we want to perform proper holistic retrofit, how are we going to do air tightness? Plus I don't fancy ripping out every internal wall covering in my own house.

    I would've thought, conceptually, an external (although internal to insulation) air tightness layer is easier

    I suppose it is less clear cut for a retrofit. The main thing is to have a complete and detailed plan before you start and to think through exactly how to achieve the planned result. Have an airtightness test at the earliest possible opportunity and then another one at the end, and maybe a third in between. The cases of failure that I know of were largely due to incomplete and/or impractical plans, IMHO.

    It is also possible to have part of the barrier inside and part outside, though that makes things more complicated to design.
    • CommentAuthorgravelld
    • CommentTimeMar 3rd 2017 edited
     
    Sorry, I should've said this was for the retrofit case... argggh! Sorry everyone. I'll edit the title.

    Developing a complete and detailed plan is exactly what I am trying to achieve, hence posting.

    Ok, I think I can understand how internal air barriers are easier if you have full control over the design. But that's obviously not the case for retrofit while trying to balance levels of disruption.
    • CommentAuthorMikC
    • CommentTimeMar 4th 2017
     
    If the existing cavity is uninsulated would it be possible to fill it with something which provides airtightness? Not necessarily an insulant, it could be concrete since you are also installing EWI. Is this a bad idea? If so what are the pitfalls? My first thought would be rising moisture at the footing level since there would be no DPC.
    • CommentAuthorgravelld
    • CommentTimeMar 4th 2017
     
    Sounds like a good strategy for that case. It's contentious moisture rises through bricks, surely not solid concrete?

    There's also expanding foam products. What about Icynene?
    • CommentAuthortony
    • CommentTimeMar 4th 2017
     
    Expanding foam might damage the structure so don't take a chance, polystyrene beads are easy,
    • CommentAuthordjh
    • CommentTimeMar 6th 2017 edited
     
    Icynene is specifically marketed for cavity fill insulation and airtightness, so I don't think there's a question of taking a chance.

    http://www.uksprayfoam.co.uk/wall-insulation/

    So are other foams, but look out for the costs.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 6th 2017
     
    Posted By: djhIcynene is specifically marketed for cavity fill insulation
    Didn't know that - probably ideal - expanding to very full fill but permanently rubbery/flexible, unlike other expanding foams. And breatheable. And largely made from organic source, not petrochemical. And not relying on exotic gas fill, just air, so only medium insulativeness but no long-term degradation of same on that account.
    • CommentAuthorgravelld
    • CommentTimeMar 6th 2017
     
    Yep, I agree this might be a good idea for the empty cavity case.

    I actually find the partial fill cavity case (esp. mineral wool) more interesting; I think there's a lot of them.

    Seems like a hack but I'd like to know if you could just pump icynene in anyway and have it compress existing insulants.
    • CommentAuthorringi
    • CommentTimeMar 6th 2017
     
    Or just remove the first row of roof tiles and fill the top of the cavity with expanding foam along with all vents etc. Air movement in the cavity should not be an issue provided the air can't mix with the outside air, and all of the wall is covers with EWI.
    • CommentAuthordjh
    • CommentTimeMar 6th 2017
     
    Deliberately make a sealed space for air to convect in you mean? Doesn't sound terribly smart to me.
    • CommentAuthorDarylP
    • CommentTimeMar 7th 2017
     
    no, the air could set up convection currents, effectively wicking heat away from the inner/warmer leaf and to the colder outer leaf. Better to prevent air movement where ever possible.
    • CommentAuthorgravelld
    • CommentTimeMar 7th 2017
     
    I think a lot of people do this though djh. Not that it makes it the right thing to do. ISTR another poster Tim doing this (might have the name wrong).

    But it's not the convection I'm interested in - there are a number of strategies for sealing.

    It's more the through-wall air tightness. Mortar gaps, cracks in bricks/blocks, cracks in render, all these mean the cavity air can mix with the outside air. And obviously weep holes etc. although I assume as part of EWI work those would be blocked (?)
    • CommentAuthorringi
    • CommentTimeMar 7th 2017
     
    Posted By: DarylPno, the air could set up convection currents, effectively wicking heat away from the inner/warmer leaf and to the colder outer leaf. Better to prevent air movement where ever possible.


    What "colder outer leaf" as it will be covered with EWI......
    • CommentAuthorDarylP
    • CommentTimeMar 7th 2017
     
    well, yes there will be a slight difference in temperature at the two surfaces, but not much!:shamed:
    • CommentAuthorgravelld
    • CommentTimeMar 9th 2017
     
    About sealing up already partial filled cavities, I emailed the UK Icynene people about this but they haven't had a reply. I got the Retrofoam people interested for a while but they didn't call back.

    What about membranes, can they have a place, fixed on the existing external leaf before EPS applied?
  4.  
    Posted By: gravelldWhat about membranes, can they have a place, fixed on the existing external leaf before EPS applied?

    IMO there would be no point in fixing a membrane on the external leaf before EWI
    1 I would make fixing the EPS difficult, to what do you glue the EPS
    2 Any mechanical fixings would pierce the membrane
    3 Any air migrating from the cavity would best be stopped by filling / sealing the cavity rather than by stopping it reaching the EWI which any way IMO should be air tight by virtues of its construction.
    • CommentAuthorgravelld
    • CommentTimeMar 9th 2017
     
    Ok, just wanted to put it out there.

    So it looks like the answer is boiling down to either stopping air leakage in the cavity and/or using render externally or plaster internally to act as a second air tightness layer, to which (for example) joinery can be taped, or Tom's OSB tea cosy on the roof structure.

    Anyone think I've misunderstood that?
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