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Products: Insulation to hang off ground floor joists from the cellar

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  1.  
    • CommentAuthorStuartB
    • CommentTimeApr 9th 2010
     
    Can anyone recommend affordable insulation that I can hang off ground floor joists?

    I have easy access to the joists from the cellar, apart from pipes and cables there are just joists with the ground floor floorboards sitting on top. There is currently no insulation whatsoever.

    My initial thoughts are to use something like space blankets and tack them to the bottom side of the joists, this would cover all pipes and cables. The alternative would be to stuff insulation in between joists and around pipes and cables and use straps to keep it up - this may be the cheaper option.

    Can you see any problems with these options or are there any other options? One of my concerns is that you won't be able to spot a leaky pipe which could lead to more extensive damage. The area is about 120sqm.

    Cheers
    • CommentAuthorevan
    • CommentTimeApr 9th 2010
     
    The standard option at the moment seems to be glasswool (because it's cheap, as loft insulation). It's a bit fiddly to hold up, especially if you are not taking the floorboards up.
    Water will drip through it if there's a leak.

    The next question is whether you should put a layer of plastic between the floorboards and the joists :)
    • CommentAuthorStuartB
    • CommentTimeApr 9th 2010
     
    Yes that also crossed my mind, but if I pinned plastic to the joists and then insulation on the outside of that wouldn't moisture be retained?
    • CommentAuthorevan
    • CommentTimeApr 9th 2010 edited
     
    Moisture in the floorboards is OK, it can evaporate back into the room, because it is warm.
    But if it gets through into the insulation and onto any cold-ish woodwork it will condense and stay there.

    So you can add a plastic barrier layer on the warm side of the insulation to stop the vapour going down.

    I think that's the theory anyway?
    • CommentAuthorcromar
    • CommentTimeApr 11th 2010
     
    I prefer solid polyurethane insulation cut to achieve a push fit between the joists. better performance than glass wool and no more fiddly. Can't think of a downside but I bet someone will!
  6.  
    The great thing about mineral wool is it squashes down enough to push fit in between the joist.

    Downsides to using insulating boards is that you have to pretty darn precise with your cutting of polyurethane to prevent gaps being left between joist and inusulation - air leakage gaps render polyurethane board virtually useless. Leaving a gap and then in-filling this with expanding foam will give good results. In old houses, the joist spacing varies a lot so there's additional measuring and cutting involved for polyurethane boards, potential lot of material wastage too.
    • CommentAuthorchuckey
    • CommentTimeApr 11th 2010
     
    What I did was to drive screws in every 12" on the side, 1/2" up from the bottom of the joist. Use 2" screws and leave 1" sticking out. On the other side of the space(between the joists) repeat but with the screws offset by 6". Now get some soft garden wire twist around the screw nearest a wall, string across to the first screw on the "other" side. Do the whole length of joist. Now thread your glass fibre through from one end (or do it one diagonal wire at a time). This is a bit time consuming but is easy to modify for adding pipes etc.
    Frank
    • CommentAuthorsquowse
    • CommentTimeApr 12th 2010
     
    can you get mineral wool bats (or preferably recycled but "woven" type insulation) in 400mm widths for this purpose?
    • CommentAuthorRobinB
    • CommentTimeApr 14th 2010
     
    The mineral wool that's wrapped could work well, e.g. Knauf space blanket, easy to handle and relatively inexpensive. I've fastened it between our joists with a bead of pink-grip foaming adhesive each side at the upper edge of the joist. So far so good. The adhesive seems to set quick enough but not too quick. Wear a hat or a hood!
    RobinB

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