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Effect of air movement within cavity wall on u-value

  • 05-10-2014 01:33AM
    #1
    Superdaddy
    Registered Users, Registered Users 2 Posts: 234 ✭✭


    If a 150mm masonry wall cavity is filled with Xtratherm full fill board giving a u value of 0.12. What is the effect of cold air within the cavity moving around the insulation on the overall u value of the wall? Are you really getting the u value calculated, is this air movement taken into account?


Comments

  • #2
    893bet
    Registered Users, Registered Users 2 Posts: 9,618 ✭✭✭893bet


    Superdaddy wrote: »
    If a 150mm masonry wall cavity is filled with Xtratherm full fill board giving a u value of 0.12. What is the effect of cold air within the cavity moving around the insulation on the overall u value of the wall? Are you really getting the u value calculated, is this air movement taken into account?

    .12 assumes perfect installation and performance.

    Reality is this rarely happens so you will have reduced performance due to thermal looping of air.


  • #3
    Rabbo
    Registered Users, Registered Users 2 Posts: 637 ✭✭✭Rabbo


    The risk of air movement or thermal looping in a full filled cavity should be greatly reduced in comparison to the traditional partial cavity fill


  • #4
    Superdaddy
    Registered Users, Registered Users 2 Posts: 234 ✭✭Superdaddy


    Rabbo wrote: »
    The risk of air movement or thermal looping in a full filled cavity should be greatly reduced in comparison to the traditional partial cavity fill
    Thermal looping, so that's what it's called. Its not really a full fill as there is a gap albeit small to prevent water ingress. I think a lot would depend on the brickie installing it. I will look into thermal looping and try figure out how much of the u-value you could potentially lose I am leaning towards a Walltite cavity fill but it is very expensive and I am trying to justify the cost of it. You only get one shot at it.
    I am also considering 100mm of xtratherm and 50mm of walltite but i doubt it will make economic sense.


  • #5
    galwaytt
    Registered Users, Registered Users 2 Posts: 12,754 ✭✭✭✭galwaytt


    Superdaddy wrote: »
    If a 150mm masonry wall cavity is filled with Xtratherm full fill board giving a u value of 0.12. What is the effect of cold air within the cavity moving around the insulation on the overall u value of the wall? Are you really getting the u value calculated, is this air movement taken into account?

    As pointed out, thermal looping occurs where there is a gap. There is a number out there from years ago, but I can't remember where - iirc the gap was quite small - 5mm - 10mm.
    And that's the problem with subjective build methods - it won't match the on-paper values. When you get to the roof it gets even fuzzier.

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  • #6
    heavydawson
    Registered Users, Registered Users 2 Posts: 1,231 ✭✭✭heavydawson


    galwaytt wrote: »
    As pointed out, thermal looping occurs where there is a gap. There is a number out there from years ago, but I can't remember where - iirc the gap was quite small - 5mm - 10mm.
    And that's the problem with subjective build methods - it won't match the on-paper values. When you get to the roof it gets even fuzzier.

    I used that exact board and width in my build and while I can't tell you it's operating at its paper-value of 0.12, I support what's already been said. The board is un-forgiving unless you've got a careful block-layer. Any snots on the inner leaf can cause issues. The alternative as others have pointed our here is to go wider (200->250mm) and pump, which will get you to your 0.12.
    I'm happy enough with the end results on my build. The temp inside the kitchen here now hasn't dropped below 21 since April, when we turned the heat off (its still off).
    Others on the forum have mentioned wall-tite, which is a foam pumped in the same manner as the beads. Don't know a whole lot about it, but is probably worth looking into.


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  • #7
    MicktheMan
    Registered Users, Registered Users 2 Posts: 2,952 ✭✭✭MicktheMan


    galwaytt wrote: »
    iirc the gap was quite small - 5mm - 10mm.
    6mm:)


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