insulating draughty cavity under ground floor boards?

Discussion in 'UK DIY' started by Jim, Dec 20, 2010.

  1. Jim

    Jim Guest

    I plan to redirect our ground floor front room next year.

    Currently, the walls are unplastered, and we've rewired so floorboards
    are loose, so any option is possible at this point.

    My question is what, if anything to do about draughts from under the
    floor.

    We live at a T junction, with a road running off our road straight in
    front of our house. This road is a bit of a wind tunnel, and the room
    we plan to decorate gets the brunt of it.

    The house is Victorian, and slightly raised with large cavities under
    the floor-boards. There is also a metal decorated air-grill at the
    front of our house which ventilates the space under this room.

    Net result is that it is seriously cold and draughty at the moment,
    and I can feel quite strong cold draughts if I put my hands over the
    floorboard gaps in this room.

    When we've decorated the room, I plan to get an underlayed berber
    style carpet, which should prevent the draughts, but I'd like it to be
    as cosy as possible for my children.

    Is there anything else I can do to improve insulation/carpet warmth?

    I presume blocking up the air vent completely would be a bad thing? Is
    it worthwhile attempting any other sort of under-floor insulation?

    Thanks for your input.
     
    Jim, Dec 20, 2010
    #1
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  2. Jim

    stuart noble Guest

    On 20/12/2010 12:31, Jim wrote:
    > I plan to redirect our ground floor front room next year.
    >
    > Currently, the walls are unplastered, and we've rewired so floorboards
    > are loose, so any option is possible at this point.
    >
    > My question is what, if anything to do about draughts from under the
    > floor.
    >
    > We live at a T junction, with a road running off our road straight in
    > front of our house. This road is a bit of a wind tunnel, and the room
    > we plan to decorate gets the brunt of it.
    >
    > The house is Victorian, and slightly raised with large cavities under
    > the floor-boards. There is also a metal decorated air-grill at the
    > front of our house which ventilates the space under this room.
    >
    > Net result is that it is seriously cold and draughty at the moment,
    > and I can feel quite strong cold draughts if I put my hands over the
    > floorboard gaps in this room.
    >
    > When we've decorated the room, I plan to get an underlayed berber
    > style carpet, which should prevent the draughts, but I'd like it to be
    > as cosy as possible for my children.
    >
    > Is there anything else I can do to improve insulation/carpet warmth?
    >
    > I presume blocking up the air vent completely would be a bad thing? Is
    > it worthwhile attempting any other sort of under-floor insulation?
    >
    > Thanks for your input.


    The idea on the ground floor is to stop draughts coming up through the
    boards, but allow a free flow of air under them via the air bricks, so
    your carpet should be enough. You don't lose heat downwards, so I'd say
    there's no point in insulating.
     
    stuart noble, Dec 20, 2010
    #2
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  3. stuart noble wrote:

    > The idea on the ground floor is to stop draughts coming up through the
    > boards, but allow a free flow of air under them via the air bricks, so
    > your carpet should be enough. You don't lose heat downwards,


    You fucking well do.
     
    The Natural Philosopher, Dec 20, 2010
    #3
  4. In article <
    ps.com>, Jim <> writes

    >I presume blocking up the air vent completely would be a bad thing?


    Yes. The airflow prevents the joists from rotting. Don't do it.

    > Is
    >it worthwhile attempting any other sort of under-floor insulation?


    No.

    Perhaps put down a layer of thick polythene tucked under the skirting
    boards if there is enough of a gap, then the underlay, then the carpet?

    FWIW, I live in s similar house to that you have described. The floors
    are stripped and varnished, and I love the look, but it's too draughty
    and difficult to heat, especially in this weather, so I'll also be
    putting carpet down when I get round to decorating.

    --
    (\__/)
    (='.'=)
    (")_(")
     
    Mike Tomlinson, Dec 20, 2010
    #4
  5. Jim

    Tabby Guest

    On Dec 20, 2:23 pm, The Natural Philosopher <>
    wrote:
    > stuart noble wrote:
    > > The idea on the ground floor is to stop draughts coming up through the
    > > boards, but allow a free flow of air under them via the air bricks, so
    > > your carpet should be enough. You don't lose heat downwards,

    >
    > You fucking well do.


    Yeah, that was a bit daft.

    Don't block the vents, unless you want the whole floor structure to
    slowly rot away. The ideal thing t do is lift floorboards, fit
    insulation between joists, add a VB over the top and re-lay the
    boards. The not so ideal thing is to jsut rely on underlay to stop
    drafts and not have insulation - but it sounds like now is as good a
    time as any to insulate it.


    NT
     
    Tabby, Dec 20, 2010
    #5
  6. Jim

    stuart noble Guest

    On 20/12/2010 14:23, The Natural Philosopher wrote:
    > stuart noble wrote:
    >
    >> The idea on the ground floor is to stop draughts coming up through the
    >> boards, but allow a free flow of air under them via the air bricks, so
    >> your carpet should be enough. You don't lose heat downwards,

    >
    > You fucking well do.


    With a decent carpet/underlay you're going to lose what? 5% of the total
    heat loss? In most cases that would be a small saving for a lot of work
     
    stuart noble, Dec 20, 2010
    #6
  7. Mike Tomlinson wrote:
    > In article <
    > ps.com>, Jim <> writes
    >
    >> I presume blocking up the air vent completely would be a bad thing?

    >
    > Yes. The airflow prevents the joists from rotting. Don't do it.
    >
    >> Is
    >> it worthwhile attempting any other sort of under-floor insulation?

    >
    > No.
    >

    yes.

    lift boardsd, fit celotex/kingspan between, foil tape over the whole
    lot, refit boards.


    result no draughts and about 5 times better insulation than carpet.
     
    The Natural Philosopher, Dec 20, 2010
    #7
  8. Tabby wrote:

    > Don't block the vents, unless you want the whole floor structure to
    > slowly rot away. The ideal thing t do is lift floorboards, fit
    > insulation between joists, add a VB over the top and re-lay the
    > boards.


    I don't think that's a good idea. The whole point of under floor
    ventilation is to eliminate stagnant air which promotes rot, which
    affects not only the joists but the floorboards as well. If you're
    going to wrap (even just the under-side of) the floorboards in a vapour
    proof skin, you are *creating* pockets of stagnant air, defeating the
    purpose of the ventilation, thereby putting the floor boards at risk.

    You need to remember that a certain amount of draught is actually a
    good thing, otherwise the room becomes too stuffy.

    > The not so ideal thing is to jsut rely on underlay to stop
    > drafts and not have insulation - but it sounds like now is as good a
    > time as any to insulate it.


    I think it's best not to insulate with anything non-permeable. Use
    underlay if you must, but with beautifully varnished floorboards, the
    in-character thing is to have rugs, not wall-to-wall carpet. With rugs
    you don't want an underlay that's too thick, or you'll keep tripping over
    the rug edges. Worth a try is thin cardboard.

    If you can identify any spots where the draught is excessive, caulking
    the gaps with ordinary string may be effective.
     
    Ronald Raygun, Dec 20, 2010
    #8
  9. Jim

    harry Guest

    On Dec 20, 12:31 pm, Jim <> wrote:
    > I plan to redirect our ground floor front room next year.
    >
    > Currently, the walls are unplastered, and we've rewired so floorboards
    > are loose, so any option is possible at this point.
    >
    > My question is what, if anything to do about draughts from under the
    > floor.
    >
    > We live at a T junction, with a road running off our road straight in
    > front of our house. This road is a bit of a wind tunnel, and the room
    > we plan to decorate gets the brunt of it.
    >
    > The house is Victorian, and slightly raised with large cavities under
    > the floor-boards. There is also a metal decorated air-grill at the
    > front of our house which ventilates the space under this room.
    >
    > Net result is that it is seriously cold and draughty at the moment,
    > and I can feel quite strong cold draughts if I put my hands over the
    > floorboard gaps in this room.
    >
    > When we've decorated the room, I plan to get an underlayed berber
    > style carpet, which should prevent the draughts, but I'd like it to be
    > as cosy as possible for my children.
    >
    > Is there anything else I can do to improve insulation/carpet warmth?
    >
    > I presume blocking up the air vent completely would be a bad thing? Is
    > it worthwhile attempting any other sort of under-floor insulation?
    >
    > Thanks for your input.


    If you can get under your floor I would insulate between the joists
    with rigid foam boards, stuck in with foam. (Cut them undersize,
    support with temporary nails & fill the gap with the foam) You must
    fill every little gap.
    If you have to lift the boards, they never go back the same, may will
    get broken and the edges damaged so you may as well replace them. New
    T&G i floorboards are available but expensive. There is special T&G
    chipboard for flooring, thats what I have used. When complete go round
    the floorboard/skirting gap and fill up with canned foam.
    Don't block the air vents it encourages rot.
     
    harry, Dec 20, 2010
    #9
  10. Jim

    Jim Guest

    On Dec 20, 2:07 pm, "Andrew Mawson"
    <andrew@no_spam_please_mawson.org.uk> wrote:
    > "Jim" <> wrote in message
    >
    > news:....
    >
    > > I plan to redirect our ground floor front room next year.

    >
    > ...snip...
    > tion?
    >
    >
    >
    > > Thanks for your input.

    >
    > To WHERE are you going to redirect it? <G>
    >
    > AWEM


    ...oops :) times like this when you think maybe you're too geeky...
     
    Jim, Dec 20, 2010
    #10
  11. Jim

    Jim Guest

    On Dec 20, 4:49 pm, The Natural Philosopher <>
    wrote:
    > Mike Tomlinson wrote:
    > > In article <
    > > ps.com>, Jim <> writes

    >
    > >> I presume blocking up the air vent completely would be a bad thing?

    >
    > > Yes.  The airflow prevents the joists from rotting. Don't do it.

    >
    > >> Is
    > >> it worthwhile attempting any other sort of under-floor insulation?

    >
    > > No.

    >
    > yes.
    >
    >   lift boardsd, fit celotex/kingspan between, foil tape over the whole
    > lot, refit boards.
    >
    > result no draughts and about 5 times better insulation than carpet.


    How thick is a layer of celotex/kingspan? Also, what do you mean by
    foil tape?

    Also, only some of the boards are already up, so I'd rather not lift
    them all unless I had to. Would a combination of your's and Mike's
    suggestions work?

    That is to say, fix all the floorboards, then lay celotex/kingspan
    (I'll have to look those up), then foil tape on top of that, and then
    let the fitters lay underlay and carpet ontop of that? That would be a
    lot less effort intensive.
     
    Jim, Dec 20, 2010
    #11
  12. Jim

    Owain Guest

    On Dec 20, 12:31 pm, Jim wrote:
    > When we've decorated the room, I plan to get an underlayed berber
    > style carpet, which should prevent the draughts, but I'd like it to be
    > as cosy as possible for my children.


    Hardboarding the floor would stop draughts between the boards and
    would be fairly cheap and easy.

    Owain
     
    Owain, Dec 21, 2010
    #12
  13. Jim

    Tabby Guest

    On Dec 20, 5:37 pm, Ronald Raygun <>
    wrote:
    > Tabby wrote:


    > > Don't block the vents, unless you want the whole floor structure to
    > > slowly rot away. The ideal thing t do is lift floorboards, fit
    > > insulation between joists, add a VB over the top and re-lay the
    > > boards.

    >
    > I don't think that's a good idea.  The whole point of under floor
    > ventilation is to eliminate stagnant air which promotes rot, which
    > affects not only the joists but the floorboards as well.  If you're
    > going to wrap (even just the under-side of) the floorboards in a vapour
    > proof skin, you are *creating* pockets of stagnant air, defeating the
    > purpose of the ventilation, thereby putting the floor boards at risk.
    >
    > You need to remember that a certain amount of draught is actually a
    > good thing, otherwise the room becomes too stuffy.
    >
    > > The not so ideal thing is to jsut rely on underlay to stop
    > > drafts and not have insulation - but it sounds like now is as good a
    > > time as any to insulate it.

    >
    > I think it's best not to insulate with anything non-permeable.  Use
    > underlay if you must, but with beautifully varnished floorboards, the
    > in-character thing is to have rugs, not wall-to-wall carpet.  With rugs
    > you don't want an underlay that's too thick, or you'll keep tripping over
    > the rug edges.  Worth a try is thin cardboard.
    >
    > If you can identify any spots where the draught is excessive, caulking
    > the gaps with ordinary string may be effective.



    The point of underfloor ventilation is to conduct away the damp that
    evaporates from the earth below, so RH doesnt rise high enough to
    cause rot. If you insulate between the josits with no VB, you'd be
    right, condensation and rot could occur. But with insulation and VB
    above all is well.


    NT
     
    Tabby, Dec 21, 2010
    #13
  14. In article <
    s.com>, Jim <> writes

    >That is to say, fix all the floorboards, then lay celotex/kingspan
    >(I'll have to look those up), then foil tape on top of that, and then
    >let the fitters lay underlay and carpet ontop of that?


    Celotex is a foil-backed insulation mat, about an inch thick. It
    wouldn't stand up to being walked on, even with underlay and carpet on
    top. It's used for things like insulation between the roof joists
    during a loft conversion, for example.

    I don't agree with TNP here. There would be some heat loss through the
    floor, but not much, certainly not worth the effort of following his
    suggestion of lifting the boards and fitting celotex between the joists,
    then re-laying. As others have said, you'd also be creating pockets of
    stagnant air between the celotex and the floorboards.

    My opinion.

    --
    (\__/)
    (='.'=)
    (")_(")
     
    Mike Tomlinson, Dec 21, 2010
    #14
  15. Jim

    Tim Watts Guest

    On 21/12/10 06:07, Mike Tomlinson wrote:
    > In article<
    > s.com>, Jim<> writes
    >
    >> That is to say, fix all the floorboards, then lay celotex/kingspan
    >> (I'll have to look those up), then foil tape on top of that, and then
    >> let the fitters lay underlay and carpet ontop of that?

    >
    > Celotex is a foil-backed insulation mat, about an inch thick. It
    > wouldn't stand up to being walked on, even with underlay and carpet on
    > top. It's used for things like insulation between the roof joists
    > during a loft conversion, for example.
    >
    > I don't agree with TNP here. There would be some heat loss through the
    > floor, but not much, certainly not worth the effort of following his
    > suggestion of lifting the boards and fitting celotex between the joists,
    > then re-laying. As others have said, you'd also be creating pockets of
    > stagnant air between the celotex and the floorboards.
    >
    > My opinion.
    >


    Can the small amount of vapour in the floorboards breath up through the
    underlay and carpet?

    Unless there is a vapour barrier, I would have thought so.

    I see no problems with this approach as long as there is a breathing
    path from everything above the celotext into the room, and what's under
    the celotex being open to the underfloor vents.

    --
    Tim Watts
     
    Tim Watts, Dec 21, 2010
    #15
  16. On 20 Dec, 12:31, Jim <> wrote:
    > I plan to redirect our ground floor front room next year.
    >
    > Currently, the walls are unplastered, and we've rewired so floorboards
    > are loose, so any option is possible at this point.
    >
    > My question is what, if anything to do about draughts from under the
    > floor.
    >
    > We live at a T junction, with a road running off our road straight in
    > front of our house. This road is a bit of a wind tunnel, and the room
    > we plan to decorate gets the brunt of it.
    >
    > The house is Victorian, and slightly raised with large cavities under
    > the floor-boards. There is also a metal decorated air-grill at the
    > front of our house which ventilates the space under this room.
    >
    > Net result is that it is seriously cold and draughty at the moment,
    > and I can feel quite strong cold draughts if I put my hands over the
    > floorboard gaps in this room.
    >
    > When we've decorated the room, I plan to get an underlayed berber
    > style carpet, which should prevent the draughts, but I'd like it to be
    > as cosy as possible for my children.
    >
    > Is there anything else I can do to improve insulation/carpet warmth?
    >
    > I presume blocking up the air vent completely would be a bad thing? Is
    > it worthwhile attempting any other sort of under-floor insulation?
    >
    > Thanks for your input.


    I'm in a similar position and this is what I plan to do.

    I sanded and varnished several of my downstairs floors last year, and
    am now kicking myself for not taking the opportunity to take all or
    most of the boards up and insulate underneath first. Not only are the
    boards cold under foot (even through rugs), but despite the greatest
    care I have gaps and draughts where boards have had to come up for
    plumbing and electrics (someone said in a similar thread in an other
    place that they'd totted up all the gaps and holes in their floor, and
    if they were all in once place they'd amount to an eight inch square
    hole).

    I have one more room to do, and I'm going to start by lifting all the
    boards. It's a job, but a job you only have to do once. I might cut
    the tongues out first, to get them up easier, as once I've insulated,
    tongues won't matter.

    What you need to do is insulate between the joists, putting the
    floorboards into the warm zone of the room.

    The easiest way to insulate is probably to use space blanket, which is
    a roll insulation encapsulated in plastic to avoid mess. It is rolled
    out between the joists and supported on netting or similar. My problem
    with this is that the standard size is 370mm wide, and assumes that
    your joists are at 400 centres, whereas mine, annoyingly, are at 430
    centres, so I'd have a gap. This leaves me the option of using
    messier, unencapsulated roll insulation, or what I'll probably do
    which is buy seconds of 50mm Kingspan, cut to size and fix in place
    with PU foam.

    Where my previously-sanded floors are concerned, I had a plan to lift
    a couple of floorboards in four or five places across the floor, and
    pull space blanket through, under the floor, between the joists,
    supporting it where I could. Unfortunately I have this width issue so
    I'm not sure what approach I'll take to that now - possibly pull up a
    lot more boards than I had intended, and use Kingspan again.

    Important issues are electric cabling and central heating pipes -
    cable should not be buried in insulation as it can theoretically
    overheat and become a fire hazard, so it needs to run above it. I also
    need access to my central heating pipework (in case I want to
    reconfigure the radiators or access the Speedfit connectors, so I need
    to make sure that's abpve the insulation too - which has an obvious
    other benefit).

    As people have said, on no account block up your air bricks, as air
    circulation to the bottom of the joists is essential for the avoidance
    of rot. I think worrying about the floorboards is unnecessary, as Tim
    Watts says.

    I've had three winters here now and dealt with a lot of the other
    insulation issues that the house has - I'm convinced now that a major
    part of the cold feel that is left is down to the floors. Especially
    when outside is minus fifteen. I've yet to hear anyone say they
    insulated their floor, but considered it a waste of time. You can't
    build anything new without insulating the floor...

    Cheers
    Richard
     
    geraldthehamster, Dec 21, 2010
    #16
  17. On 21 Dec, 06:07, Mike Tomlinson <> wrote:

    > Celotex is a foil-backed insulation mat, about an inch thick.  


    That's a bit misleading. There is more than one Celotex product, but
    the most common is a rigid foam board, usually foil backed, that comes
    in sizes from one inch up to - actually I don't know, but certainly
    more than seven inches. It's not a mat that you can lay on top of
    floorboards and fix carpet to.

    Cheers
    Richard
     
    geraldthehamster, Dec 21, 2010
    #17
  18. stuart noble wrote:
    > On 20/12/2010 14:23, The Natural Philosopher wrote:
    >> stuart noble wrote:
    >>
    >>> The idea on the ground floor is to stop draughts coming up through the
    >>> boards, but allow a free flow of air under them via the air bricks, so
    >>> your carpet should be enough. You don't lose heat downwards,

    >>
    >> You fucking well do.

    >
    > With a decent carpet/underlay you're going to lose what? 5% of the total
    > heat loss? In most cases that would be a small saving for a lot of work


    make that 85% if the room is wall insulated with DG.
     
    The Natural Philosopher, Dec 21, 2010
    #18
  19. Jim wrote:
    > On Dec 20, 4:49 pm, The Natural Philosopher <>
    > wrote:
    >> Mike Tomlinson wrote:
    >>> In article <
    >>> ps.com>, Jim <> writes
    >>>> I presume blocking up the air vent completely would be a bad thing?
    >>> Yes. The airflow prevents the joists from rotting. Don't do it.
    >>>> Is
    >>>> it worthwhile attempting any other sort of under-floor insulation?
    >>> No.

    >> yes.
    >>
    >> lift boardsd, fit celotex/kingspan between, foil tape over the whole
    >> lot, refit boards.
    >>
    >> result no draughts and about 5 times better insulation than carpet.

    >
    > How thick is a layer of celotex/kingspan? Also, what do you mean by
    > foil tape?


    variable. Suggest 50mm minimum.

    Celotex is foill faced, you buy Al. tape to go over the board face and
    the joists its wedged between. Makes a contonuous draught and vapour
    barrier.
     
    The Natural Philosopher, Dec 21, 2010
    #19
  20. Mike Tomlinson wrote:
    > In article <
    > s.com>, Jim <> writes
    >
    >> That is to say, fix all the floorboards, then lay celotex/kingspan
    >> (I'll have to look those up), then foil tape on top of that, and then
    >> let the fitters lay underlay and carpet ontop of that?

    >
    > Celotex is a foil-backed insulation mat, about an inch thick. It
    > wouldn't stand up to being walked on, even with underlay and carpet on
    > top. It's used for things like insulation between the roof joists
    > during a loft conversion, for example.
    >
    > I don't agree with TNP here. There would be some heat loss through the
    > floor, but not much, certainly not worth the effort of following his
    > suggestion of lifting the boards and fitting celotex between the joists,
    > then re-laying.


    Do the calculations before you make such opinions.

    Would you want a wall the size of your floor, cosnsisting of leaky
    floorboards just covered with a carpet and underlay with a -5C 15mph
    wind the other side?

    yet a well vented underfloor space is just that.

    Compared with a solid floor, where the ground itself acts as an
    insulator, suspended wood is a complete heat loss disaster area
    especially when its well ventilated.
    ..



    As others have said, you'd also be creating pockets of
    > stagnant air between the celotex and the floorboards.
    >


    The celotex is flush with the top of the boards.

    BUT creating pockets if stagnant air is precisely what an insulation
    material does anyway, so its a silly thing for you to say, twice over.

    > My opinion.
    >
     
    The Natural Philosopher, Dec 21, 2010
    #20
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