Can Joist Insulation be used in Rafters?

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skidbum1
skidbum1 Posts: 2 Newbie
in In my home (includes DIY) MoneySaving
Hello, I am putting loft panels down and as a result a lot of the insulation (put down new in 2007) is becoming redundant (a small bit is still under the panels). Has anyone come up with a method of using it the rafters? It is still in good condition. If not what is the most cost effective and easiest method of insulating my roof to compensate for the loss of joist insulation?
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  • FreeBear
    FreeBear Posts: 14,631 Forumite
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    The conventional way of boarding over thick insulation is to use stilts to support the boards. e.g. https://www.screwfix.com/p/diall-loft-storage-stilts-210mm-12-pack/243gf
    Other lengths and sizes to suit larger joists are available.
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  • skidbum1
    skidbum1 Posts: 2 Newbie
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    I have already gone down the non-stilts approach!
  • Doozergirl
    Doozergirl Posts: 33,813 Forumite
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    Then you'll need solid insulation cut between your rafters as the rafters are never going to hold 30cm of rockwool and it will slump over time anyway, I would suspect, if you try and suspend it.

    Solid insulation is thinner than you'd need to Achieve the same uValues with rockwool. Not cheap, but at least it will pay you back relatively quickly in savings. The stilts were probably the easiest way, tbh.
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  • ANDY597
    ANDY597 Posts: 430 Forumite
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    I would follow doozers advice and cut solid insulation between the rafters ensuring a tight fit, but don't forget the air gap between outside and the insulation.

    In answer to your question though if you were adamant you wanted to, then you would leave an air gap then the rockwool, then suspend it in there with a breathable membrane to stop it from sagging out. How efficient it would be in there would be debatable.

    From your question though it struck me. What size are there joists if most of the insulation is going? Joists may not be suitable to support a floor.

    Possible solution is to increase the joist height to accommodate more insulation underneath. Hanging new joists alongside existing is within a diyers capability thanks to modern joists hangers.

    Hope this helps.
  • Davesnave
    Davesnave Posts: 34,741 Forumite
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    We get a good bit of air whistling through our loft and I suspect this has some bearing on our lack of condensation issues. Frankly, I wouldn't think insulating our rafters would do a lot to keep us warm, unless we also restricted air flow.

    I wouldn't be doing that.

    I've used solid insulation down the central aisle in our loft, where there's a boarded walkway. Everywhere else, it's about 300mm of fluffy stuff. Because I still want to store Christmas decorations, baby chair, buggy, and other rarely-used, light paraphernalia, I've designed platforms to fit in the 'W' of the roof trusses that may be reached from the walkway. Works OK.
  • TELLIT01
    TELLIT01 Posts: 16,487 Forumite
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    If you do want to reuse the current insulation, you could try using plywood as a backing, fixed over the rafters. It will definitely be a two person job.
  • Doozergirl
    Doozergirl Posts: 33,813 Forumite
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    TELLIT01 wrote: »
    If you do want to reuse the current insulation, you could try using plywood as a backing, fixed over the rafters. It will definitely be a two person job.

    There won't be any airflow?
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  • ANDY597
    ANDY597 Posts: 430 Forumite
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    Dooze, that might work as long as there's an air gap between the insulation and rafters.

    I think what they meant was

    Airflow gap

    Recyled insulation

    Board to face it off and stop it falling down like plasterboarding a wall.

    Sounds like a lot of effort when solid insulation is available to make life easier.

    OP check out seconds and Co for slightly damaged Kingspan at a fraction of the cost. Buyer beware board won't be perfect. May have a chunk etc missing or an odd shape etc
  • Doozergirl
    Doozergirl Posts: 33,813 Forumite
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    Okay, but I'm not convinced that one can maintain a consistent air gap with fluffy insulation, especially if it's 2nd hand, and then if you screw boards on it, they have to be able to screw to something and then you don't know if you've bridged the air gap with the pressure of the thing holding it on.

    It's just a bad idea all round.
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  • Tom99
    Tom99 Posts: 5,371 Forumite
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    Doozergirl wrote: »
    Okay, but I'm not convinced that one can maintain a consistent air gap with fluffy insulation, especially if it's 2nd hand, and then if you screw boards on it, they have to be able to screw to something and then you don't know if you've bridged the air gap with the pressure of the thing holding it on.

    It's just a bad idea all round.
    And if there is an air gap to start with then the insulation is likely to slump towards the bottom.
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