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Housing - Renovation: EPS fill under suspended timber floor?

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    There seem to have strong opinions for and against doing this on several threads over the years.

    I can see the attraction of a quick and easy DIY way of insulating the floor void, but even though some people seem to have been doing this for the last 10 years I am assuming that the issue of removing ventilation and the risk (even if small) of damp issues arising still make this impossible to get building control sign off for?

    I am looking to dig down an existing 1.5m basement to give useable head height, which I'm assuming will be specified with a Delta membrane or similar system. Doing this will cut off the front ventilation from the rear of the house, so I assume I for BC I would need to install new cross vents to compensate for this?

    Thanks,

    Nick
    • CommentAuthorrevor
    • CommentTimeJan 9th 2022
     
    I had a house with similar height basement and thought of doing the same thing. The issue I had was being a late Victorian house the foundations would have needed underpinning as removing the basement floor would have gone deeper than the footings. It was also a semi so it had complications from that also. Presumably you have had the effect on your footings checked out?
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    Yes, I will need to underpin. The house next door has done the same and c50% of the street.
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    Nick
    I'm not sure why you are considering EPS (bead fill ?) under the suspended floor.
    The EPS fill usually discussed here is a full fill with EPS bead of the void under the floor - and yes there is a discussion about the potential issues of no ventilation that starts with if there is no void (because you just filled it with EPS) then no ventilation is needed.
    In your case it sounds like you are creating a useable cellar under the floor which is quite different to the normal under floor insulation.
    If you are insulating between the joists then it is usually thought to be easier to get a good fit with soft insulation (wool of some type) than board insulation which would need to be foamed in at the edges to be effective.
    What are you planning to have as the ceiling of the cellar - that is under the joists?
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    Hi Peter,

    The basement only covers 50% of the ground floor, I was looking at how best to insulate the remaining suspended ground floor at the rear, EPS full sounds of interest, however I need to do something that building control will sign off on.

    For the cellar I was just assuming plasterboard ceiling and just to insulate the perimeter against external wall, probably with wood fibre.

    Thanks,

    Nick
    • CommentAuthortony
    • CommentTimeJan 9th 2022
     
    It is a bit semantic, if there is a void then it has to be ventilated but with no void it becomes akin to a double negative and nothing to say about it

    Lots worry about damp and moisture but that moves out of a heated house into the ground, see previous threads. I would fill it right up.
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    Posted By: nickbond11For the cellar I was just assuming plasterboard ceiling and just to insulate the perimeter against external wall, probably with wood fibre.

    I would insulate the whole floor between the joists because it will make for a warmer floor and in the scheme of things the cost would be marginal.


    Posted By: nickbond11EPS full sounds of interest, however I need to do something that building control will sign off on.


    Posted By: tonyIt is a bit semantic, if there is a void then it has to be ventilated but with no void it becomes akin to a double negative and nothing to say about it

    Lots worry about damp and moisture but that moves out of a heated house into the ground, see previous threads. I would fill it right up.

    As I said above and as Tony reiterated with full fill if there is no void ('cos you filled it) so what's to ventilate. This is the question you ask BC if they want a ventilated after you filled the void.
    Full fill or insulate between and a bit more below the joists usually comes down to how much depth you have to fill and how much depth you have to work in to do between the joists. Its a cost/effort balance.
    If you full fill with EPS then any PVC cables would need protection from the EPS
    • CommentAuthornickbond11
    • CommentTimeJan 10th 2022
     
    Thanks both.

    I will see how I get on with BC on the full fill, certainly much quicker than trying to use nets and filling between the joists.

    Peter - if the basement becomes a heated room once dug out, though I appreciate this may help slow the rise of heat throughout the house generally, is there much of a benefit with trying to use a thin sheet of insulated plasterboard for the ceiling or in insulating ceilings generally, rather than just tackling the perimeter?
  9.  
    If the basement is going to be a heated room then the walls and floor of the basement will need insulating (or is this what you meant by perimeter) and there is not much point in insulating the ceiling unless the heating is occasional use only. If it is unheated then insulated plasterboard will help but wool type (glass rock etc.) is cheap enough so it is a cost/benefit decision. Oh and if it is unheated or occasional use only then something better than a standard internal door would help.
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      CommentAuthordjh
    • CommentTimeJan 10th 2022
     
    In a new building the ceiling needs insulating anyway, for noise reduction purposes. I don't know what the rules are for extensions/conversions but I think it's a sensible idea in any case. Acoustic mineral wool is usual.

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