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Housing - Renovation: EWI on external stud wall without VCL

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  1.  
    • CommentAuthorphiledge
    • CommentTimeFeb 20th 2023
     
    My daughters semi has the first floor external walls made made of timber studs front and rear with a brick insulated cavity gable end. Theres a token bit of mineral wool in the stud walls thats been poorly installed. There's no VCL in the stud walls and they're timber clad on the outside.

    Question is how to insulate. The interior has been decorated in the last year or so and with a second baby on the way they really dont want any/much disruption inside. There's enough eaves overhang to get a bit of EWI fitted so thoughts are to work from the outside, remove the external cladding, fit 50/75mm of PIR between the studs and a further 75 over the outside of the studs. Vertical battens fixed through the PIR into the studs with timber cladding fixed to the batten.

    With no VCL are we going to hit problems even though weve got an equivalent or more insulation on the outside than within the studs? Second question is, if we can use foil faced insulation, do we need a membrane over the outside of the insulation?
  2.  
    Hi,

    This should also give you a chance to improve air-tightness, even though the decorated interior denies you access to the key 'interface'. I would suggest a condensation risk assessment. I don't know of a free version but I think others on here have used free or 'trial' versions. Just finding a risk does not mean you will get an interstitial condensation issue - there are so many variables - but it may help you to find what the model says the risk is and where it predicts it will be. Improving the air-tightness will/may reduce the volume of water vapour 'transported' in any case. If we use the '66% outside;33% inside' rule of thumb for 'hybrid' insulation then it perhaps points to 50 between studs and phenolic, not EPS externally.

    You asked ''With no VCL are we going to hit problems even though weve got an equivalent or more insulation on the outside than within the studs?''
    As above, not necessarily, and in any case more outside is 'prescribed', not more inside.


    ''Second question is, if we can use foil faced insulation, do we need a membrane over the outside of the insulation?''

    I would not use foil-faced outside, but if you watch Grand Designs you'll see lots who do. We don't get the 'after story' though. I suspect foil-faced is only used externally because that's what the merchants most readily sell.
    •  
      CommentAuthordjh
    • CommentTimeFeb 21st 2023
     
    FWIW, there was a project on a house with a similar-sounding construction in the last PH+ magazine (article titled Phit the Bill). They reached EnerPHit.

    IMHO it sounds like now is the wrong time to try to insulate it. Doing the job with no (or little) disruption will probably result in a less than ideal result that will annoy everybody for evermore. Far better to wait until they are in a calmer position and can face doing the job right. JMHO, YMMV. :devil:
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 21st 2023
     
    Surely, wise men, it will only work, without VCL, if all of it is reasonably breatheable? which means not PIR or any of the plastic foam insulants, but good old EPS.
    •  
      CommentAuthordjh
    • CommentTimeFeb 21st 2023
     
    Or Warmcel or ...
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 21st 2023
     
    Sheep, hemp, wood fibre, even glass fibre, mineral wool - almost anything other than PIR and the other 'high performance' insulants.
  7.  
    The condensation problem (if there is one) is due to the insulation between the studs, which in any case will be bridged by the timber, and a faff to cut to fit tightly.

    Could you not bother with the between-studs insulation - just cover the outside of the studs with something airtight, then 100mm PIR/phenolic, then battens, cladding?

    Probably work out cheaper, similar insulation value, and no doubts about condensation. Option to use 150mm EPS wool etc

    But you mentioned this is for the upstairs side walls of a semi, so I guess there will be junctions with the downstairs, the roof, the cavity gables, and the neighbours?

    It will take quite some thought how to do those without thermal bridges and without returning the insulation inside the decorated envelope. In our case, those thermal bridges made it meaningless to go for sub U=0.2 EWI because the bridges lost more heat than the EWI.
    • CommentAuthorphiledge
    • CommentTimeFeb 22nd 2023
     
    Thanks for all the comments so far.

    Still a bit uncertain over the way forward though!

    I had a play with the demo calculator on u-wert.net and ended up with diagrams below that include a statement that "this component is free of condensate under given climatic conditions". Is that the answer Im looking for??
      Screenshot_20230222-113613.png
  9.  
    Only if the indoors air is kept at 50% humidity, and the outdoors at 80%. Perhaps try it with a range of higher humidities, eg 60-70% indoors and 100% outdoors? And a range of temperatures?

    I dislike the humidity method in that software which looks fancy and detailed but is actually an (over) simplified method from an EN standard. EG it shows a big humidity gradient in the air space (would actually get evened out by convection) and no humidity effect of the foil facings on the PIR (would actually trap more humidity on one side than the other, that's what they're for).

    If this calculation is going to change your mind about the design one way or the other then Wufi would be more trustworthy IMO. The PIR/phenolic manufacturers might already have an analysis they could give you for free?
    • CommentAuthorphiledge
    • CommentTimeFeb 24th 2023
     
    Just spoken with Celotex tech and insulation between the studs without a VCL is a non starter. So as you suggested Will, we'll probably go for 100mm PIR on the outside of the studs. From inside out, build up will be-
    Plasterboard
    4x2 timber studs without insulation
    VCL over outside of studs
    100mm PIR
    Breather membrane
    Vertical battens fixed through to studs and hopefully strapped to ends of rafters for support
    Horizontal cladding

    Longer term when interior needs decorating we'll have the option to remove outer VCL, add a bit more insulation and VCL from the inside.

    Thanks for everyone's input
    •  
      CommentAuthordjh
    • CommentTimeFeb 24th 2023
     
    Just curious but why PIR rather than phenolic?
    • CommentAuthorphiledge
    • CommentTimeFeb 24th 2023 edited
     
    Posted By: djhJust curious but why PIR rather than phenolic?


    Availability locally at a good price and Ive already got some large offcuts. I've never really looked at phenolic but it seems slightly better performing but at a disproportionately larger price. Am I missing something??
    •  
      CommentAuthordjh
    • CommentTimeFeb 24th 2023
     
    Performance in a fire?
    • CommentAuthorphiledge
    • CommentTimeFeb 25th 2023
     
    Posted By: djhPerformance in a fire?

    For the limited area we're doing, the benefits of phenolic seem to be trumped by the cost saving and availability of PIR
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 28th 2023 edited
     
    Posted By: philedgeJust spoken with Celotex tech and insulation between the studs without a VCL is a non starter
    Such firms' reps put out that kind of blanket ruling that just shows poor understanding and analysis. They may think it's fail-safe but actually VCLs in many cases make matters more risky, when re-drying ability becomes dominant in the everyday behaviour of a particular configuration of the 'sandwich' plus local climate.

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