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Housing - Renovation: Where to put wiring with under-floorboard insulation

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    • CommentAuthorWeeBeastie
    • CommentTimeNov 24th 2019
     
    My small house needs a rewire. I'd like to add insulation under the floorboards in the future and would appreciate advice on where best to put the new wiring with this in mind.

    The insulation may be a novice DIY job so I'd prefer not to be working around lots of wiring. I'd also like to have the option of fixing rigid insulation boards under the joists, in addition to the quilt + netting between.

    My electrician says he would normally put the wires along the bottom of the joists, but this would conflict with the fitting of the rigid boards.

    In an ideal world I'd somehow have all the wiring above the floor, but this seems difficult and expensive. I know of the rebated skirting boards but these are mdf which I'm hoping to avoid. One (discarded?) thought was to reinstate the picture rails and tuck wires in above these and then drop down, but some walls are solid so there would be a lot of chasing into the wall.

    So do the wires just go further up the joists? Any thoughts?

    Thanks again.
    • CommentAuthortony
    • CommentTimeNov 24th 2019
     
    Loose laid across the oversize or fixed under the sheet insulation..

    In badly trays hung just below the sheets

    If it was mine I would derate the cables, 4mm instead of 2.5 in the insulation.

    Or run in first floor void with drops (ceiling if single storey)
    • CommentAuthorMatBlack
    • CommentTimeNov 24th 2019 edited
     
    Under floorboardswise, are you talking about just upstairs? If so stick to the guidelines on drilling joists, holes 1/4 max of the joist depth, etc and de-rate the cable.

    Edit - just read it again. If you didnt mind the extra wall chasing/patching. First floor wiring with drops would probably be best. If you're insulating the upsatirs too, then de-rate still.

    I guess it depends on what you do first, wiring or insulation. If wiring first, is there enough crawl space to clip your cable to the inside face of the joists?
    • CommentAuthorphiledge
    • CommentTimeNov 24th 2019
     
    In a smallish house and modern low energy appliances youll probably not need 32 amp circuits so can derate the MCB protection to 20 amp and leave the cabling at standard 2.5mm. Bar the kitchen ring, all our sockets are on 20amp circuits without any tripping issues.
    • CommentAuthorMatBlack
    • CommentTimeNov 24th 2019 edited
     
    Good idea philedge, as long as the electrician is happy with diversity and max demand of the install. (Only in the indirect sense that the circuit loads have been thought out)
    • CommentAuthorWeeBeastie
    • CommentTimeNov 24th 2019
     
    It is a bungalow with generous space under the floor. The electrician will be in before I do the insulation. I like to know as much as possible before talking to tradesmen so thanks for the advice.

    I will ask him about dropping the cables down, perhaps from the picture rail hiding spot (if I put the cables in the loft I have the same issue with wiring-buried-in-insulation, no?) Some walls have lathe and plaster so not all will need to be chased, though it will be annoying to have to patch as the plaster is in very good condition and I don't want to wallpaper.

    I like to have everything readily accessible and as worry-free as possible. In my last property, a second floor flat, I put in high level shelves to hide the plumbing as I didn't want it mysteriously under the floor....
    •  
      CommentAuthordjh
    • CommentTimeNov 24th 2019
     
    Posted By: WeeBeastieI like to have everything readily accessible

    Whichever way you choose, put the cables in conduit then.
    • CommentAuthorMatBlack
    • CommentTimeNov 24th 2019 edited
     
    Oh right, if you want the least patching possible, and you have generous floor space, maybe have a chat with the spark about the future insulation?

    Some sparks would literally fish under a floor void and drape the cables under there! Perhaps ask if he could maybe screw some 4 x4 inch plastic trunking under the floor to a wall under the floor, going the lengthwise run of the house (if that makes sense) and he can branch off as needed. I guess you could just have neat chases up to the sockets then.

    In the loft you should be ok, he could run a plank low down across the rafters, and make the long lighting + shower cable runs on that, avoiding lengthy runs in the insulation? Just a couple of options.
    • CommentAuthorMatBlack
    • CommentTimeNov 24th 2019
     
    Posted By: djh
    Posted By: WeeBeastieI like to have everything readily accessible

    Whichever way you choose, put the cables in conduit then.


    Good idea, could be sound for under the floor ( plastic though), not so much inside the rooms, its a bit more destructive installing it on show.
    •  
      CommentAuthordjh
    • CommentTimeNov 24th 2019
     
    Posted By: MatBlackPerhaps ask if he could maybe screw some 4 x4 inch plastic trunking under the floor to a wall under the floor,

    I think that's kind of what Tony was suggesting with laying the cables in a tray, which keeps them accessible as an alternative to conduit.
    • CommentAuthorMatBlack
    • CommentTimeNov 25th 2019
     
    In badly trays hung just below the sheets.

    Oh yesðŸ‘Â
    • CommentAuthorwookey
    • CommentTimeDec 2nd 2019
     
    If you hang the wiring under the floor insulation then you are introducing an airtightness problem where it goes past the insulation (assuming the floor void is still insulated?). Not sure where your airtightness line is going but I'd keep the cabling inside it rather than going out and back times N cables. Get/make some grommets if you really must do that.

    I have a concrete floor so all the cabling is in the 1st floor void, except front room sockets which are cabled behind the skirting (but in front of airtightness line). You don't have to use rebated MDF skirting. You can use any skirting you like and rebate it yourself, or just add a spacer at top and bottom to create a void. There is also PVC skirting with channels in and the front handily clips off for later fettling, but it's not the prettiest option.
    • CommentAuthorHoveTom
    • CommentTimeDec 4th 2019
     
    I’m presently renovating and converting a bungalow and mid rewire. I’m also pulling up all the floors to insulate between them before I install ufh.

    The cabling was put under the floors a few weeks ago when first fix was done. It’s absolutely no problem working around them as you pull the floors up and insulate. Having said that I know all my cables aren’t live as the power is still all switched off to the house.

    I’d certainly be hesitant using skirting board as until you’ve really thought your re wire and lighting plan out you won’t know how many cables you need. Lighting can take up more than you think, especially if you want two way switching between different entry and exit points in rooms. I wouldn’t want to limit myself on options for that just because of cable space.

    I think a lot of this depends on what your particular electrician is happy to do so I’d speak to them. Some have their preferred way of doing things, some will charge a lot more for doing things 'none standard'

    Good luck with it.
    •  
      CommentAuthordjh
    • CommentTimeDec 4th 2019
     
    Posted By: HoveTomLighting can take up more than you think, especially if you want two way switching between different entry and exit points in rooms.

    That used to be the way of things, but we recently had a discussion about 'kinetic' switches that need no wiring. I for one am happy with them.
    • CommentAuthorrevor
    • CommentTimeDec 29th 2019
     
    I am concerned about the comments re wiring behind skirtings, This is not a permitted zone. Permitted zones are within 150 mm of the top of a wall 150 mm within an angle formed by adjacent walls and zones vertically or horizontally to a accessory or switchgear. You can go outside these zones but the cable run must be in an earthed conduit. The rebated skirtings were for hiding speaker cables and ELV wiring. Your electrician should know these regs but I mention it here in case anyone reading the comments may think it is a permitted way of wiring.

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