Rockwool vs glass fibre and availablity of 150mm thickness

Discussion in 'Eco Talk' started by benawhile, Nov 13, 2018.

  1. benawhile

    benawhile New Member

    I have previously asked about materials for loft insulation and have come down in favour of insulation roll, loft legs and standard loft board.

    I want to increase my loft insulation from 4” glass fibre to 10” and retain access and storage using wickes 7” loft legs and standard loft boards.
    Problem is according to all sources I can find, rock wool is a superior materila to glass fibre and while I would like to use it, I can't find it in 6” thickness, only 4 or 8, (100mm and 100mm)

    There is something called rockwool rollbatt at Jewsons

    Could I use Rockwool “rollbatt “ as if ti was standard roll to insulate across the joists in my loft, and board over it using loft legs?

    Is rock wool available anywhere in 150mm
    am I misinformed about the superiority of rockwool?

    Finally a gripe:

    It is stated on US websites that rockwool has a better u value, repels moisture where glass fibre soaks it up, and is more fire resistant.

    It seems little has changed since I insulated my last house in 1979. There are the same two basic materials, glass wool and rock wool. Glass wool, confusingly also called glass mineral wool for some reason, confirmed by all sources as inferior to rock wool, is nonetheless almost universally available by all manufacturers, but rock wool is only made by one firm.

    Even products like Knauf Earthwool, which sound as though they would be more natural, and actually do feel more natural, are made of the same old glass “mineral” wool
  2. Jord86

    Jord86 Well-Known Member

    What you do is insulate between your joists with 100mm, then cross layer at 90 degrees over the top with 200mm rockwool. Add more if you feel the need, but that satisfies the regs as far as I'm aware and keeps your place toasty.
    KIAB likes this.
  3. benawhile

    benawhile New Member

    Thank you for replying, but I have to board over the insulation somehow, and the ideal method, using loft legs means I have to use 150mm.
  4. Heat

    Heat Well-Known Member

    Have you tried some of the 8” Rockwool to see if it easily compresses down to 6” thickness you require?
    Fibreglass and Rockwool are not high density products.
    Obviously you do not want to put pressure on the plasterboard ceiling by sandwiching too heavy insulation between it and flooring you are going to put down, but worth buying a roll to see it it fits near flush with joists
    Last edited: Nov 14, 2018
  5. benawhile

    benawhile New Member

    I've read that that would make the insulation less efficient
  6. Heat

    Heat Well-Known Member

    It might, but only if it does need massively compressed.
    The rolls of insulation are already very compressed.
    Would it not be worth buying one roll of the thicker Rockwool to see the actual thickness of it?
  7. chippie244

    chippie244 Well-Known Member

    The increase in heat gain will be mimimal anyway so squashing it a bit will be infintessimal.
    Heat likes this.

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