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help!! fitting insulated slab to cavity wall??

  • 31-01-2013 7:31pm
    #1
    delta1980
    Registered Users, Registered Users 2 Posts: 42 ✭


    looking at buying a a 1960's bungalow in need of renovation was wondering can i fill the cavity with beaded bond insulation AND slab the inside walls with an insulated plaster board slab. I will be replastering the inside anyway. can i attach the slab directly to the wall or would i need to leave an air gap?? would i need to put in an airtight membrane?? any experts to advise exactly what to do it to make it fairly air tight and avoid condensation issues??


Comments

  • #2
    sydthebeat
    Subscribers Posts: 42,171 ✭✭✭✭sydthebeat


    what size cavity do you have? is anything in it currently?


  • #3
    delta1980
    Registered Users, Registered Users 2 Posts: 42 ✭delta1980


    from looking in it, it seems to be maybe 4'', nothing in it currently


  • #4
    sydthebeat
    Subscribers Posts: 42,171 ✭✭✭✭sydthebeat


    theres an approximate rule of thumb that you should have no more than 1/3 of the insulation internally and 2/3 in the cavity to be confident of having no interstitial condensation issues. When you apply internal insulation you are making the inner block wall colder thus more prone to having the dew point closer to the inside.

    On your build, if you are sure of using internal insulation, personally i wouldnt apply more than a 32.5 PIR insulated board internally.
    If the walls are in good "nick" id even question if that is worth doing at all. If your budget stretches that far, great, but if its tight it wouldnt be top of my priority list. A 4"cavity pumped would increase a u value from about 1.6 to 0.27... thats a very significant increase in anyones language. the extra 32.5 board would bring that u value down to 0.22ish.

    How sure of you as regards cold bridges in the build? i have seen cavity builds from that era having huge ring beams cast over openings. Try your best to see if the whole cavity is open for pumping. If there are lots of cold bridges it makes more of an argument to use the insulated slabs... if not, then perhaps the slabs could be left off.


  • #5
    delta1980
    Registered Users, Registered Users 2 Posts: 42 ✭delta1980


    thanks for the reply, ya were getting an engineer to have a look at the house first so ill get him to look at the cavity in detail. if we put a third of the insulation on the inside can we apply it directly to the inner wall or would we have to construct a stud for the slabs


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