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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Hi all,

    I am a novice renovator, and have been doing as much reading as possible on the considerations to some major renovation work I am doing to my early edwardian mid-terrace house. To fill in the gaps and satisfy my anxiety, it would be great to hear some advice on my questions below. This forum seems the best place to ask such questions as i've learned significantly more from this forum than any other.

    To describe the house a bit:
    I have a ground floor suspended timber floor void that is only accessible from above There's a subfloor supporting wall in the centre, and 4-5 vents placed around the base of the external walls.

    My ultimate aim is to install electric UFH as the solitary means of heating this room (a new kitchen). All in an effort to save space by not having a rad(s). I'll be lifting the old floorboards, to make required changes and inspect, but wanted to check that my plans won't cause me any issues with condensation/rot

    There's two aspects to this:

    1. I wanted to install UFH. There is already a nice full set of original 20mm thick pine floorboards which i want to leave intact under the new floor. I planned to gently pull them up, install UFI, a vapour barrier over top of joists, put the boards back, put 9mm ply on top, and then UFH wiring and tiles. Are there any glaring issues with this? I don't want to cause rot of my floor above the vapour barrier, but then surely the UFH will keep the boards and ply dry? Am I missing something, and worrying over nothing?

    2. Joists are 100mm deep, and I wanted to use PIR as thick as possible between these (400mm centres). Should i be considering a specific thickness of PIR to reduce risk of interstitial condensation in the joists? Or is it that there will be no vapour from inside the room to condense because of the aforementioned vapour barrier under the floorboards?

    Any advice on hanging 100mm PIR between 100mm deep joists?

    I've got a few more questions, but they're interrelated to this to a lesser extent, so i'll open another thread.

    Thanks for any help/responses provided!
    • CommentAuthorbarney
    • CommentTimeOct 17th 2018
    Stand a glass of water on the floor - get someone to walk around the room - if you can detect vibrations on the surface of the water, forget tiling the floor

    Assuming from the test above, you deduce that putting tiles on a 4" x 2" joist at 400mm centres isn't a sound plan, then potentially you can:

    1 - Rip the floor out and insulate in the void and then add a structural slab above

    2 - Insulate under new, bigger joists and then between them and add a new floor that can accept tiles

    In either case, you'll need to work out what those vents actually are for - they may be ventilating under the suspended floor - so a solid floor solution renders them pointless - and equally, if you go for a suspended floor, then they may be pointless provided you can encapsulate the underside of any new joists in the insulation layer - ie the whole joist is always warm.

    You may get comments that says the joist ends are still at risk with no ventilation - or a few more that say, just fill the whole void with EPS beads, block up the vents, tile over as planned and get on with life


    Thanks Barney,

    Ah, another worry! I'll have to check re the rigidity of the floor. Replacing the joists is not something I want to have to do. Nor is pouring a slab if i'm honest.

    Venting in the subfloor walls is definitely for venting the joists, and nothing else. This room has 3 exterior walls, and a solid floor on the 4th side.

    Assuming tiling is fine, my mind has been racing for the past few days, and some advice on the build-up i mentioned would help:
    - Joists with PIR
    - Vapour barrier fixed to top of joists
    - Old pine floorboards
    - 9mm ply
    - UFH wiring in tile adhesive
    - floor tiles

    • CommentAuthorgyrogear
    • CommentTimeOct 17th 2018 edited
    Posted By: austingeorgeThis room has 3 exterior walls

    Therefore it might be worthwhile insulating the walls, rather than the underfloor; leave one vent open, then condition the underfloor void with internal (house) air at a rate of 1 cfm for every 50 square feet.

    One way to do it is to fit a low-power exhaust fan blowing through a hole in the exterior wall; the makeup air entering the ''crawlspace'' is conditioned air from the house upstairs: since this conditioned air is drier than outdoor air, it should not lead to condensation problems.

    The underfloor void is now heated.
    It will likely get furhter heated by heat leaking through from your UFH, but that is another issue (-:

    Hi Gyro,

    I was planning to insulate those externally facing walls aswell (and not the other, equally solid internal wall), 50mm celotex with warm battens, ply & plasterboard surface.
    What about access to the fan? It'll be under the floor in a void that isn't accessible from the outside of the house, mind.

    Wouldn't be worried about condensation on the subfloor walls?

    • CommentAuthorgyrogear
    • CommentTimeOct 17th 2018
    Hiya, Austin,

    if no access to the fan, it can be installed inside the heated volume (= home), blowing down...

    To insulate the walls, just XPS, glued on, top to bottom. It will resist condensation.

    Any moisture will run down & be absorbed in the floor - especially if latter treated with a sack or two of lime, raked in...

    • CommentAuthorCWatters
    • CommentTimeOct 19th 2018 edited
    Just for info - There are fan assisted wet rads available intended to fit under kitchen units. Not used them myself. Perhaps someone else can comment on any down sides...

    ... or ditto over doors - Myson used to make them IIRC.
    • CommentAuthorowlman
    • CommentTimeOct 19th 2018
    I've done a couple of jobs tiling over a suspended floor but any floor "bounce" is obviously not good. I've also done several electric UFH cable jobs albeit onto concrete. In the past for suspended floors I've inserted "pit prop" type supports under the joists, onto a concrete sub floor. and extra stiffening noggins between joists,
    Replacing the old floorboards is not the best idea IMO, better to use them elsewhere if they are decent and use T&G ply flooring (e.g. 22mm ), get a really nice level finish. Then insulate onto the ply with this type of thing, to give you a good tile key.


    Alternatively if you're happy with your between joist insulation, then use a decoupling mat onto the new ply flooring, before tiling.
    Thanks. I'll probably stick with vapour barrier over the joists and loft roll between with noggings to stiffen. Also slightly thicker ply overboard, and 300mm and 400mm centred fixings to joists.

    UFH and decoupling membrane set into flex tile adhesive with flex grout.

    Largest span is about 2m, and floor feels quite stiff already, but it's worth the added expense as I do not want to re-do this.

    Thanks for all your help,
    If you have changed away from rigid plastic, can I get you to use breathable membrane 'hammocks' to hang the insulation in? Tape all joints and perimeters and you have a secondary (or primary, depending on which way you are facing!) air-tightness layer to prevent 'windwash' (thermal by-pass).
    Posted By: austingeorgeAlso slightly thicker ply overboard,

    Is there an advantage of using ply rather than OSB which would be much cheaper?
    • CommentAuthorowlman
    • CommentTimeOct 23rd 2018 edited
    I admit Peter I have an aversion to particle board flooring, maybe in upper floor bedrooms is OK, but I don't like it anywhere near wet rooms, incl. kitchens. and as far as bath and shower rooms IMO it should be banned.
    I've done a couple of re-furb jobs in bathrooms albeit with chipboard, not OSB, floors that were like the proverbial wet "Weetabix", and I just refuse to use it now.
    I haven't used OSB for flooring, only for roofing, and I do prefer it to chipboard.
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