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Quick question, is it possible to mix insulation types?

Discussion in 'Building' started by dagoof, 26 Mar 2015.

  1. dagoof

    dagoof

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    I've got an extension with a flat/slightly pitched roof, with a vinyl covering. There's 2 rooms, 1 has rockwool, the other has an old rigid pink insulation.

    On room 1, can I leave the wool in place and add Kingspan below to increase U value? (it should still leave a few cm breathing space above the wool).

    Room 2 has two layers of pink board insulation packed right into the underside of the roof (no air gap, which has been causing condensation to drip down).

    Again, to increase U value, could I remove 1 layer of the old stuff and lower the remaining layer down slightly to allow a 50mm air gap above - then fit the kingspan further below, with another air gap between the pink & kingspan?

    Room 1:

    Room 2:

     
  2. Joe Malone

    Joe Malone

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    Essentially you have a cold roof deck as opposed to a warm roof deck where the insulation is installed on top of the roof. The problem here is that if you maintain a 50mm air gap above the insulation then this needs to be ventilated to prevent interstitial condensation. I suspect this hash' been provided for and this would be the reason that you have suffered with interstitial condensation.
    Normal practice is to have an airflow running through the roof soffits. Technically I see no reason not to mix insulation types because rock wool is fairly inert. However I would look to install whatever product has the best U-value for its given thickness after ensuring that you have dealt with provided a through flow of air to the 50mm headspace. Phenolic boards generally have the best U-Value.
     
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  3. Tipper

    Tipper

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    Can you not make it warm deck by stuffing the space fully with insulation? Why have an airspace at all?
     
  4. jeds

    jeds

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    No. 'Warm deck' means the insulation is above the deck keeping the deck warm so it doesn't require ventilation. In your scenario the deck would be cold - in fact colder than at present!, which would increase condensation.
     
  5. dagoof

    dagoof

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    Thanks Joe - there are some little vents visible externally, but there's been cavity wall insulation done since, so I'm not sure if that's since blocked them.
    Out of interest, can you just drill a few holes in the brickwork (at the airgap level) to create ventilation?
     
  6. Tipper

    Tipper

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    So adding insulation to the topside of a roof deck would change it to warmdeck?
    Do you then need to create a cavity and ventilate somewhere below even if various previous layers of insulation below the deck are continuous from outside to inside?

    I'm thinking:

    Outside
    New single ply waterproof membrane
    new 50mm Iso
    old felt roof
    old 25mm Iso
    even older felt roof
    50mm woodwool slabs/deck
    100mm timber joists interspace filled with fibre glass wool
    plasterboard
    Inside

    This is the roof currently on a late 1970's bungalow with a 10deg pitched roof.
     
  7. noseall

    noseall

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    Think...

    125mm Kingspan insulated plywood deck above the joists.....job done.

    No need for venting.

    No need for any other insulation.
     
  8. DIYnot Local

    DIYnot Local

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