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SuperHomes Revamp Forums General Discussion ?OPTIONS FOR ROOF INSULATION

This topic contains 5 replies, has 5 voices, and was last updated by  David Hatchman 4 years, 8 months ago.

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    Does anybody know whether it is possible to use some rigid insulation like kingspan over the top of joists ( between which is a currently insufficient thickness of rockwool) .

    The reason I would do this would be to allow reboarding, as more rockwool would come over the joists and would not allow the boards to be replaced easily for storage space.

    Are there any condensation issues- or could these be avoided by leaving a cm or so gap between the solid insulation boards and reboarding again with a little gap at 90 degrees to this??

    If this is realistic what thickness of kingspan or the like, needs to go on 7 cms of rockwool to come  up to the equivalent of the 270 mm recommended.


    Thanks- Jane




    Dear Jane

    Don’t do this. See the bit about insulating loft floors in ( ).

    Good luck. Bob Pringle





    Is the only option to insulate plaster and lath ceilings in the loft using rockwool ?


    – Jane




    Jane, Speak with KIngspan they are very helpful, having said that, I doubt they would recommend the method you describe because of condensation forming.
    2 ideas come to mind, either remove the existing rockwool and then add Kingspan panels with thier advise or :- add more Rockwool to the desired thickness and place joists at 90 deg to the existing joists, these will then carry the floor boards for storage. I have done the latter and am happy with the result



    SuperHomer Mark Brown has written a helpful article about boarding out a loft.

    Note the link at the bottom of this article to a new system now available for lifting your loft floor – could be worth a look, particularly I’d think if you won’t need the whole loft for storage but have some fairly heavy stuff you’d want to store there in a compact area.


    David Hatchman

    Pretty much – in general – the better the insulation the more the system is prone to produce condensation. To prevent the condensation from entering the living space a vapour block (plastic membrane) must be on the warm side of the insulation. So in a loft that’s either on the ceiling inside the room or under the insulation in the loft. When insulation was only 50mm – back in the mists of time – we didn’t worry about condensation. . . . So with 270mm of glass mat now the norm we must address condensation and provide sufficient air movement to get rid of it. So to reduce heat losses through a roof my suggestion – at least one of them – is to lay a plastic sheet over the entire roof space such that any condensation that does form will be contained and not allowed to soak into wood, plasterboard or brick. Then apply your insulation. The original post was concerned about boarding over the insulation. There is a product on the market made from dense foam – fibran. 100mm has a heat loss equivalent to 200mm of fibre glass wool so that laid between the joists would show a great energy saving and boarding would be easy as it wouldn’t protrude above the joists. It also has other properties, such that if you could already walk on the floor then you can lay this stuff straight onto the floor and walk on it without damaging it or reducing its thermal properties.

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