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General: Very tight cavity, 0.19 u-value needed

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  1.  
    • CommentAuthorShevek
    • CommentTimeOct 9th 2012
     
    What BBA-certified insulation do you think I could use and how thick would it need to be to achieve a 0.19 u-value with the following detail?

    See attached detail. The cavity I'm currently working to is 35 mm but I'm hoping to increase this by reducing the retaining wall. The current build-up is:
    12.5 mm plasterboard
    10 mm dabs
    100 mm Plasmor Stranlite 1375kg/m3 blockwork
    35 mm full-fill insulation (fully dry and above DPC)
    tanking
    225 mm concrete retaining wall

    Is 50 mm of aerogel enough? And even if it was is there a suitable BBA-certified product on the market in the UK?
  2.  
    Dabs? Full perimeter beads to prevent heat-loss through convection?
  3.  
    Can you move the retaining wall away from the inner wall? why are they so close?
    • CommentAuthorRobinB
    • CommentTimeOct 9th 2012
     
    ..and if you can't push out the retaining wall it might be worth making your building a bit smaller to allow a thicker wall. If you could make that insulation continuous from under floor to wall would be very good too.

    Posted By: Nick ParsonsFull perimeter beads to prevent heat-loss through convection?


    These are shown on the drawing but not in the OP's text.
    • CommentAuthorCWatters
    • CommentTimeOct 10th 2012
     
    Might have to persuade your BCO and brickies that not having a cavity is ok.
    • CommentAuthorSaint
    • CommentTimeOct 10th 2012
     
    Quick calculation indicates you'd need 65mm Aerogel Spaceloft!
    Spaceloft has an ETA
    • CommentAuthorRobinB
    • CommentTimeOct 10th 2012
     
    If your dimensions are immutable could the Stranlite (k = 0.46W/m°C) blocks be replaced with something more thermally efficient? May only help a bit. Eg Plasmor Aglite k = 0.32W/m°C and appears to have similar strength.
    • CommentAuthorbarney
    • CommentTimeOct 10th 2012
     
    Do you need 0.19 U at that point - the wall at that point is losing heat to the ground not the air - so the delta t is smaller as the ground temp is higher.

    Work it out for what would be the heat loss at the design condition of say -5C and 0.19 U and then transpose to find the same heat loss at say 10C to find then new u value - the building then does not perform any worse when considerd from an energy perspective rather than a power perspective.

    Regards

    Barney
    • CommentAuthorSaint
    • CommentTimeOct 10th 2012
     
    Barney, good point I missed the underground bit
    • CommentAuthorShevek
    • CommentTimeOct 10th 2012
     
    Thanks for the input guys, we can't push retaining wall out because it's the boundary. We don't want to move the inner leaf in because of the structural implications at this late point in the design (they're digging foundations this week). Funding is based on area of flat so decreasing internal area is going to be problematic but may be unavoidable.

    Sounds like we need between 50-65 mm Aerogel or we have to add insulation to the inside face of wall. Spaceloft is an Amercian company no?
  11.  
    Change the thickness of the retaining wall from 200mm to 100mm at the point where the insulation is needed.
    or
    Increase the size of the steel in the retaining wall and make it 150mm all the way up.

    Why do you need to dig down so far?
    • CommentAuthorShevek
    • CommentTimeOct 10th 2012
     
    I think that's going to be the ticket Seamus, thanks.
    • CommentAuthorShevek
    • CommentTimeOct 10th 2012 edited
     
    If we can increase the size of the cavity what insulation do you think we should use? Would it be okay to full-fill with a foil-backed rigid phenolic foam board?
    • CommentAuthorShevek
    • CommentTimeOct 10th 2012
     
    75 or 100 mm CavityTherm perhaps?
    • CommentAuthorDarylP
    • CommentTimeOct 10th 2012
     
    BC will probably insist on a 50mm clear cavity if you want to use rigid boards....?
    Good luck...:smile:
    • CommentAuthorShevek
    • CommentTimeOct 11th 2012
     
    The engineer has managed to provide us with a 75 mm cavity (well 77.5 mm). With CavityTherm this comes to 0.23 u-value as far as my u-value calcs go.

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