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25mm celotex floor insulation

Discussion in 'Building' started by Custom45, 1 Jan 2020.

  1. Custom45

    Custom45

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    Hi
    I have a utility room with a concrete floor & no insulation in it. The floor level needs raising by about 30-35mm to bring it level with the adjoining kitchen floor.

    I was going to lay a dpm then a 30-35mm concrete slab to bring it to level, but was thinking instead of including some insulation into it.

    Can I lay 25mm celotex boards on existing floor, then screed/floor level compound over it? Will I have issues with compression and have too thin a layer (5-10mm) of screed/self level compound?

    25mm celotex

    Fibre reinforced self leveller


    They also do a 12mm board, but wanted 25mm to add as much insulation as I can.

    Cheers & happy new year.
     
    Last edited: 1 Jan 2020
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  3. 23vc

    23vc

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    Don’t think that’s a goer. Also 25mm insulation may be hardly worth it. Don’t believe you can SLC over celotex and any screed this thin (even 35mm) needs to be the right type, most likely supplied off a truck not a traditional sand and cement jobbie
     
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  4. ^woody^

    ^woody^

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    That will be fine, as long as no one ever walks on it.

    Your better of using chip or ply board.
     
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  5. Custom45

    Custom45

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    Cheers for the replies.

    So, 25mm celotex, then a layer or chip/ply board over the top then floor covering (some vinyl roll)?

    25mm won't make a huge difference I know, but thought a little was better than none.
     
  6. Alexnh

    Alexnh

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    Dunno about the practicalities of getting it in but would agree that some is better than none. Especially if it is providing a break from a big slab of cold concrete.

    Might you need floor rated insulation board?
     
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  7. rssteve

    rssteve

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    I guess 12mm celotex will leave just enough for 18mm of wood flooring. What you planning on covering it with though if its a utility room? Wood would feel warmer to walk on unless you tile over it.
     
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  8. ^woody^

    ^woody^

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    In context, a utility is not that big, and the visible floor area is often much reduced if base units are in there too, and it's a utility not a lounge (it should have extract ventilation), and heat goes up, and millions of utilities don't have insulated floors and no one notices one jot.

    So IMO, and in practical terms, it's not even worth thinking about, there is no advantage, benefit or need to insulate. Do whatever is easiest and cheapest to get the floor exactly to the right height (that is what you will be noticing every single day).
     
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  9. Custom45

    Custom45

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    Planning on covering with a bit of vinyl flooring, nothing fancy.
     
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  11. John D v2.0

    John D v2.0

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    I'll go against the grain and say 25mm with ply on top against nothing will be the difference between night and day if you ever walk on it with only socks on (even for 10 seconds to grab something from the dryer)
    I've lived in a house where the kitchen was like that and the bathroom off the kitchen and you needed shoes to dash across.
    The first thickness of insulation is always the most important, double it again and it won't make as much difference.
     
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  12. Custom45

    Custom45

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    I think it's worth doing. Cost wise is minimal, £11 for a sheet of 25mm celotex plus some timber board.

    I have some 11mm osb3 left from another project if that's suitable? Or am I better off buying some 18mm??
     
  13. Alexnh

    Alexnh

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    I agree. What is the area? Is the slab dead flat? What's wrong with a nice bit of ply to allow you to minimize the wood and maximize the insulation thickness?
     
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  14. John D v2.0

    John D v2.0

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    Normally structural floor should be 18mm at standard centres but since it is fully supported you would probably get away with 11mm. Just don't put any grand pianos on it!
     
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  15. Custom45

    Custom45

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    Existing slab is nice and flat, yes. I guess I can go with ply, if that means a thinner layer of timber for more insulation. What thickness of ply should it be?
     
  16. Alexnh

    Alexnh

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