Cold flat roof insulation

Discussion in 'Builders' Talk' started by A500-Dave, Oct 27, 2010.

  1. A500-Dave

    A500-Dave New Member


    Have recently had two "cold" flat roofs installed: one over a downstairs room and the other over part of the kitchen. The old roof beams were so badly rotted that all were replaced. So no ceilings at present..

    I need to get insulation installed and then, presumably, plaster boards fitted.

    As per the roofers advice' I would like to use 100mm of rockwool insulation between joists; leaving a 50mm air gap below the decking.

    I see one can buy rockwool of this thickness
    in "slabs".

    A couple of question:

    1.) how would such rockwool slabs
    be fixed between the joists?

    2,) The joists have holes at the top for the electric
    cabling: Would these be suffient for ventilation?

    Thanks in advance,
  2. G Brown

    G Brown New Member

    Use Celotex or Kingspan, much better.
  3. Andy the roofer

    Andy the roofer New Member

    Hi mate. Rockwool is very expensive and heavy. Nobody uses it in cold roof specs. Any 'wool' insulation will perform perfectly as long as you leave the gap between the boards and the insulation.

    Chances are if you cut the insulation 50mm bigger than the centres of the joists, it will hold itself in place. If not, run a series of thin string bands under the joists to help hold it up.


    andy - 25 yrs in flat roofing

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