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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorbogal2
    • CommentTimeDec 1st 2014 edited
     
    Enerphit retrofit3 bed semi cavity walls. Architect has specified a dpm below 300 mm of EPS and one above below slab. Both come up the inside of the exterior wall behind an upstand of insulation to be plastered into the wall for airtightness. However well I fit the dpm without creases there will be some air gaps surely allowing cold air from the cold side of the insulation through to the platered area on the warm side. Could I cut the lower dpm shorter perhaps below the point where the floor insulation meets some foamglas blocks I have inserted below the slab level in the inner leaf of the exterior wall. What does the lower dpm actually do? Any advice? Thanks.
  1.  
    Hello!

    Tape them.

    Is one a Radon barrier?
    • CommentAuthorringi
    • CommentTimeDec 1st 2014
     
    I expect the top one is to stop the concrete getting into and under the EPS.
    • CommentAuthorbogal2
    • CommentTimeDec 1st 2014 edited
     
    Hi Nick, tape them where? I was planning to tape from the slab to the membrane, is this is what you mean. Does that' stop that cold air getting through though?

    I can understand the top one to separate the concrete and insulation and maybe the bottom one to stop the insulation getting damp, the sand blinding is a bit damp, but joining the two membranes together on the warm side seems counterproductive? Here's a picture or two of of the foamglas row of blocks. Cleaning out the cavity with a brick hammer inside the cavity felt like a remake of the Shawshank Redemption! Sorry can't download picture.
  2.  
    Hi Nick, tape them where?


    Tape the 2 membranes together. Leave the 'tail' on one longer than the other, and tape them together. Hopefully, anyway, there'll be a high degree of 'squish' on the tape. If tere is, there should not be any air getting up to cool anything.

    Pic needs to be less than 500kb, I think.
    • CommentAuthortony
    • CommentTimeDec 1st 2014
     
    Belt and braces insanity, how long before someone specifies three?
    • CommentAuthorringi
    • CommentTimeDec 1st 2014
     
    Tony,

    It is good to keep the EPS dry if possible, and I would gladly pay the cost of a 2nd DPM to stop any risk of floating EPS!
    • CommentAuthortony
    • CommentTimeDec 2nd 2014
     
    it wont float any less with poly under it! Archimedes

    I built floors for many years with eps and poly on top, narry a problem.

    BI made us put poly under one of the last ones I did, loads of problems

    Generally and almost exclusively the eps remains dry, even tinder dry due to the migration od moisture away from warmer areas, partial vapour pressure highest near the room, lower further away so moisture is drawn away, this a down hill process.

    Exceptions would be plumbing leaks, water flowing in rivers, unlikely with hardcore under, flooding, etc
  3.  
    I'd agree with Tony, the bottom DPM is likely to cause trouble. Just do the top one well. EPS doesn't mind if it gets wet unlike other insulants. Manufacturers don't require it, what specific circumstance does the architect have for requesting it? Is there a radon problem?
    • CommentAuthorringi
    • CommentTimeDec 2nd 2014
     
    Tony,

    What issues did you get from putting poly under?

    (I can see it has little benefit)
    • CommentAuthorbogal2
    • CommentTimeDec 3rd 2014
     
    Architect says it's there to prevent damp! No radon issues. What problems did you get Tony?
    • CommentAuthortony
    • CommentTimeDec 3rd 2014
     
    Damp!

    Water from rain and from concrete got on there during their build and could only get out by coming up into the building,
    •  
      CommentAuthordjh
    • CommentTimeDec 3rd 2014
     
    Posted By: bogal2Enerphit retrofit3 bed semi cavity walls. Architect has specified a dpm below 300 mm of EPS and one above below slab. Both come up the inside of the exterior wall behind an upstand of insulation to be plastered into the wall for airtightness. However well I fit the dpm without creases there will be some air gaps surely allowing cold air from the cold side of the insulation through to the platered area on the warm side.

    How high is the water table and can you drain away from under the EPS? If so then I suggest putting a land drain under the EPS instead of a second DPM.

    I'm not clear where your airtightness barrier runs. I'd expect your slab itself to be part of the airtightness barrier, rather than a DPM running under it. And normally either a membrane or plaster on the inside of the wall is the airtightness barrier. So a DPM running up into the cavity would be well outside the airtightness barrier. It would be interesting to see your architect's detail drawing of the airtightness barrier at this junction (I hope s/he has one!)
    • CommentAuthorbogal2
    • CommentTimeDec 3rd 2014
     
    Can't upload his detail as done on programme I haven't got. I tried out his plan in part of the house and found there was nearly always a small gap between the brick wall and the insulation with the dpm running between. The upper dpm is meant to be the airtightness layer. Surely this gap means that there is possible movement between the cold and warm sides of the insulation which is surely what airtightness is there to prevent! Both dpms lap up the wall 15 cm to be covered with steel mesh and plastered to link the internal plaster with the dpm.
  4.  
    Could you photograph the screen, or 'print screen', and send a pic of that?

    Presumably if it is Enerphit, every detail has been run through PHPP - or is PHPP not that specific?
    •  
      CommentAuthordjh
    • CommentTimeDec 4th 2014
     
    Posted By: bogal2Surely this gap means that there is possible movement between the cold and warm sides of the insulation which is surely what airtightness is there to prevent!

    No, airtightness is there to prevent air exchange between the inside of the building and the outside air (what some people summarise as asphyxiating the inhabitants). The phenomenon you describe is a form of thermal bridging known as 'wind washing'.

    I still don't understand the detail you are describing and suggest that you follow Nick's idea and photograph the drawing or screen. You could also photograph the actual wall and DPM, though it's sometimes difficult to understand what such photographs are showing as well!

    I'm curious why the DPM was selected as the airtightness barrier rather than the slab itself? How have joints and penetrations in the DPM been sealed?

    PHPP is a level away from this kind of detail. There needs to be a drawing that is accepted by the certifier as corresponding to what is entered in PHPP, and if there's any question, the certifier will want something like a Therm model, and photos to show it was actually built like the drawings show. At least that's my understanding.
    • CommentAuthorbogal2
    • CommentTimeDec 5th 2014
     
    Yes thermal modelling has been done for this junction, and as a novice I don't know, but would imagine there is often quite a gap between what can be drawn and a real life situation! Which is I suppose where the squirty foam comes in! However carefully I pit the dpm and large insulation boards against this rough brickwork there is always a small gap. Maybe the architect considers the slab as the airtight layer in conjunction with the upper dpm. Can't get the size of the photo small enough to go on here.
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