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  1. #1
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    Insulating a boarded loft

    My loft is currently boarded, but the insulation under the boards isn't very good - just a bit of fibreglass, and most of that is missing. I wanted to improve on it, and I discussed it with a chap at Sheffield Insulations who told me that 100mm depth of Celotex/Kingspan between the joists would be as good as 270mm of fibreglass. (Yep, I know it's pricey stuff - but I don't mind shelling out if it's the easiest way of doing the job. The alternatives involve putting the boarding on props to take a greater depth of fibreglass underneath, or laying insulated boards under the hardboard - both of which seem like too much work.)

    However, I've done a bit of reading, and heard warnings about doing this as moist air from the house would condense on it. The bloke at Sheffield Insulations reckoned this wouldn't be a problem as the bottom surface of the Kingspan/Celotex wouldn't get that cold, as it would be insulated from the cold roofspace.

    Any insulation experts on here have any views? And what's the score on throwing away bags of old fibreglass insulation - will a council tip take it?

    Cheers.

  2. #2
    Nicotine, valium, vicodin... notdeadyet's Avatar
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    Kingspan will do dew point and condensation risk calcs if you phone their technical department. I think Celotex will as well. Both products are very similar, PU foam with a facing layer.

    There are other alternatives to mineral fibre, there is recycled paper insulation which is allegedly more efficient but still cheaper than foam board. I've used the spray on version in the past but I believe it is available in rolls too.

    If you're going to the hassle of taking up the boards then I would've thought propping them and another layer of mineral wool would cost about the same as Kingspan or Celotex.

    Can't help you with disposal, trade waste magics it away. Don't know what councils are like, I know they are arsey with building materials other than wood.
    Caliph Hamish Aw-Michty Ay-Ya-Bastard, Spiritual leader of Scottish State in England

  3. #3
    Formerly sbc23cam Steve Clark's Avatar
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    Quote Originally Posted by PlymouthD View Post
    My loft is currently boarded, but the insulation under the boards isn't very good - just a bit of fibreglass, and most of that is missing. I wanted to improve on it, and I discussed it with a chap at Sheffield Insulations who told me that 100mm depth of Celotex/Kingspan between the joists would be as good as 270mm of fibreglass. (Yep, I know it's pricey stuff - but I don't mind shelling out if it's the easiest way of doing the job. The alternatives involve putting the boarding on props to take a greater depth of fibreglass underneath, or laying insulated boards under the hardboard - both of which seem like too much work.)

    However, I've done a bit of reading, and heard warnings about doing this as moist air from the house would condense on it. The bloke at Sheffield Insulations reckoned this wouldn't be a problem as the bottom surface of the Kingspan/Celotex wouldn't get that cold, as it would be insulated from the cold roofspace.

    Any insulation experts on here have any views? And what's the score on throwing away bags of old fibreglass insulation - will a council tip take it?

    Cheers.
    You don't need to worry about the lower surface because it is a vapour barrier (foil) and will be warm (above the dew point) in just about every condition.

    You could get condensation on the upper surface in unusual conditions (e.g. if you leave your loft hatch open with no outside breeze in winter), but in these conditions you are likely to get it on every other surface in your loft too, including the timber.

    How deep are your ceiling joists? Will there be a bit of a gap above the insulation, below the boards? This will help with cross-flow ventilation, but not essential here. Be careful if the 100mm insulation sticks out above 3" joists. When you stand on the boards it will push the plasterboard off the ceiling!

    For fitting, you should try and get it to fit as tight as possible between the joists and foam the gaps to limit air leakage into the loft. If you can get it, there is a product called Xtratherm Rafterlock which has clever grooves in it so it squashes in width so you don't need to cut it exact. At the eaves, it's quite important to cut the insulation at an angle to match the pitch of the roof. You should leave a 50mm gap below the sarking felt/underside of slates, but cover the entire on the plasterboard/lath so you don't get a cold spot in the room below that will go black and mouldy.

    Just take the fibreglass to the tip in a normal car and you should be fine.

    Steve

  4. #4
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    I had a similar problem with my loft. I added extra 50mm battens on top of the joists which allowed me to put normal rockwool insulation under the boarding. I then found that the loft was very cold and damp so I have lied the underside of the roof with the quilted aluminium foil insulation. This has made the loft much warmer and drier - and made the rest of the house warmer too because it has reduced the temperature gradient through the roof. In order to provide ventilation to the roof struts, I've put vents in the soffit boards to allow air flow in the 4 inch gap between the insulaton and the roof liner.

  5. #5
    TDF Member Sparky's Avatar
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    I also had a similar problem, noticed there was a lot of condensation on the inside of the membrane that was dripping onto boxes, on looking closer noticed that the insulation was pushed all the way to the edges to the extent that I couldn't see the vents in the sofits, replaced the insulation and boards anyway, but left a gap at the edges, all the condensation has gone as now have a through breeze.

    If you're worried about too much condensation you can fit vents into the membrane to help with air flow.

    I used normal 100mm thick insulation didn't go for the added thickness as the loft is not that high and we do use it, it took a while to remove the old stuff and put new in.

    I just bagged the old stuff up and took it to the tip, after asking where they wanted it I put it in the general waste skip.

  6. #6
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    I've just read up on that rafterlock stuff and it seems like it makes fitting them between the rafters much easier, however my roof trusses are spaced very irregularly, e.g. some are 40cm whereas others are as little as 30cm, so I may end up wasting a large percentage of the rafterlock boards as they come ready cut to fit standard spacings. I'll go and measure again and look into whether I can make rafterlock work. If not, it's Celotex or Kingspan - you've put my mind at rest on the condensation issue. I'll first check to make sure I've got at least 100mm height in the gap between the joists.

    Thanks for all the advice - you can always rely on good advice on this forum regardless of how non-diving the question.


 

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