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Housing - Renovation: Insulating a suspending timber floor - critque

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  1.  
    • CommentAuthormattrgee
    • CommentTimeApr 22nd 2019
     
    Hi all,

    First post *Hello*

    I need to insulate the suspended timber floor in my living room, could you critique the below approach?

    Once the existing chipboard flooring has been lifted (limited crawl space), I intend to remove any builders waste from under the floor. For two reasons: it will reduce material that could hold moisture and it will ensure airflow is unrestricted.

    Joists are 120mm deep and I intend to fill them with 100mm celotex supported on 20mm treated battens fixed to the inside of the joists with galvanised nails.

    The celotex boards will be carefully cut to width with any slight gaps filled with expanding foam. Joints between boards and joists will be sealed with aluminium tape.

    Once the celotex is installed I will lay P5 moisture resistant chipboard, gluing the t&g joints and screwing the boards to the joists.

    Is the above acceptable? Is there anything I have missed or overlooked?

    Thanks
    • CommentAuthortony
    • CommentTimeApr 22nd 2019
     
    I would much rather see some insulation under the joists.
    How will you deal with moisture migrating downwards
    How will you deal with air tightness long term and especially at the edges and doorway
    • CommentAuthormattrgee
    • CommentTimeApr 22nd 2019
     
    Hi Tony,

    I've not seen any examples of insulation under the joists. I was under the impression that the joists should be left 'exposed' to the air flow within the cavity of the floor.

    In terms of airtightness, the room also needs plastering, I've pondered with the idea of installing a single piece of DPM over the joists (and insulation) and bring it up the wall slightly to be fixed in place and ultimately skimmed over. Obviously, this would be punctured by screws when the sub floor is installed but I think this would 'self seal' as the subfloor is screwed home.
    •  
      CommentAuthordjh
    • CommentTimeApr 22nd 2019
     
    Aluminium foil does not have a good reputation as an airtightness seal. It has a tendency to come unstuck after a while, I believe. I would prefer to rely on one of the tapes with a good reputation (and claims of 100 year life!)

    Insulation under the joists would usually be of a vapour-open type such as mineral wool or sheeps wool. Given the greater depth available, it often then makes sense to use the same material for the insulation between the joists, which makes fitting a bit easier.
    • CommentAuthortony
    • CommentTimeApr 23rd 2019
     
    Or EPS if you want to use a non biodegradable sheet material
    • CommentAuthormattrgee
    • CommentTimeApr 23rd 2019
     
    Thanks DJH, I will have a look at the airtight tapes. Would you recommend laying a DPM over the joists and insulation and using tape to seal it behind the skirting boards?

    I’m having to use rigid insulation to achieve a reasonable u value as my joists are too shall to accommodate rock wool etc that would give a good enough u value. I dont really want to mix the insulation types with one type between the joists and another below.

    Thanks
    •  
      CommentAuthordjh
    • CommentTimeApr 23rd 2019
     
    Posted By: mattrgeeWould you recommend laying a DPM over the joists and insulation and using tape to seal it behind the skirting boards?

    I don't really know; I suspect a lot depends on your overall goal. If you are just going for a general improvement in a safe way then providing the space under the insulation remains ventilated and providing the insulation is vapour permeable then I don't think I would add a DPM. I'd be tempted to rely on the glued and screwed chipboard to provide adequate airtightness whilst still hopefully allowing spills to eventually dry out. But I'm sure others have more qualified opinions.
  8.  
    We found that draught elimination from the floor/wall junction was surprisingly one of the biggest gains from insulating our suspended floors. We did ours with glued t&g chipboard, no airtightness membrane or tapes , but obsessively shaping the skirting and sealing around/ behind / under the skirting with flexible sealant. Maybe obvious, but skirting is supplied with straight edges, whereas floor joists naturally sag and are misaligned, so the floor is not perfectly flat and there are inevitable mm gaps between skirting and floor which flex in use.

    Your size of joist is likely to be supported midway on several dwarf walls with airflow between the joists over the wall, don't block this airflow. We used thinner rigid boards in this area and thick wool elsewhere slung between joists .

    A recent thread described nicely how you tweak the depth/compression of the wool and tightness of the hanging so the wool bulges below the joists to envelop them from both sides and below.
    Initially we used netting but later used breathable roof membrane to sling the wool while avoiding wind blowing through it.

    Having warm feet gives especially great improvement in comfort so well worth doing, good luck!
    •  
      CommentAuthordjh
    • CommentTimeApr 23rd 2019
     
    Sorry, I forgot to say anything about the floor-wall junction. I would use tape for this. It needs to be fairly wide and flexible and don't forget to leave some slack widthways to allow for subsequent movement. I wouldn't want the skirting itself to be part of the airtightness, myself.
  10.  
    In our case the existing airtightness layer in the wall was the existing plasterboard, which starts ca 20mm above the floorboard level, where it was foot-lifted into place when originally fitted. I am sceptical whether any of the magic wonder tapes will last long-term while bridging this gap (or in fact how long they will last at all, but that's just me!). Our policy was to keep our airtightness layer visible everywhere, so it can be maintained as the building settles into place. We did find that both squirty foam and acrylic caulk cracked after a few years and needed re-sealing.
    • CommentAuthormattrgee
    • CommentTimeApr 24th 2019
     
    Whilst the living room floor is the first to be insulated, other ground floors will also be eventually insulated. Could the efforts being made to insulate and make airtight the living room be undone by adjacent rooms that aren't insulated? Obviously, heat will be lost through the other un-insulated floors and draughts will be felt. However, is there any possibility of warm air from an adjacent room, passing under the floor and condensing in the floor space of the insulated living room floor?

    Hope that makes sense.
    •  
      CommentAuthordjh
    • CommentTimeApr 24th 2019
     
    Posted By: mattrgeeis there any possibility of warm air from an adjacent room, passing under the floor and condensing in the floor space of the insulated living room floor?

    I wouldn't have thought so, although a barrier around the edge might give some peace of mind. The temperature inside the insulation will be warmer than at its bottom surface, so condensation is likely to form at that surface or elsewhere in the void rather than within the insulation.
    • CommentAuthormattrgee
    • CommentTimeApr 28th 2019
     
    Hi Tony,

    Just looking back at your original reply...

    >I would much rather see some insulation under the joists.
    Could you elaborate on this as it was my belief the joists should still be allowed the 'breath' within the airflow of the floor.

    >How will you deal with moisture migrating downwards
    I believe the foil backing of the Celotex is a vapour barrier, therefore no additional vapour barrier is required. However, I intend to seal the joints using an airtight tape, not the foil tape following the advice above.

    >How will you deal with air tightness long term and especially at the edges and doorway
    The Celotex joints will be sealed using airtight tapes and also foamed when installed. The edge of the Celotex where they meet the wall will also be foamed. Once the chipboard has been laid (glued and screwed) the room will be plastered down to the chipboard rather than a gap being left. The join between the chipboard and the new plaster could then be sealed using an airtight tape. Perhaps this is unnecessary?

    Thanks in advance.
    • CommentAuthortony
    • CommentTimeApr 29th 2019
     
    1 leaving the joists exposed under the insulation is not great as moisture could migrate down through them, this is ok but in very cold conditions moisture will condemned in or on the bottom of the joists and soak in, ok if it can dry out again, if a continuous vapour check is present then there should not be a problem.

    2 do the tapes extend fully over the top of the joists? ie is there a vapour barrier?

    3 the floor joists if new will shrink and a gap will open between the floor and the wall. I like the foaming and this will help. I do not like relying on tapes for air tightness preferring to use a membrane plastered in to the wall with EML retaining it.

    I am a bit OCD about this and most likely you will be OK , I don’t think there will be sufficient insulation in the floor without going under the joists with it.
  15.  
    In our experience, foam was not much good in this application, the joists flex 1-2 mm with seasons and when the floor was loaded, and then the foam developed lots of cracks, which cannot be repaired unless it is left somewhere visible. I am doubtful whether a tape would survive this either, and certainly not plaster. Better off using a flexible joinery sealant in a wide bead that can accommodate movement.

    Joists dry out ('breathe') into regions of low Relative humidity, which is found inside the room rather than in the void. I am comfortable insulating under the joists to keep them warm, but would rather not vapour-seal them from the room. Moisture in Timber works differently from condensation in other materials. However, air tightness is more important than vapour tightness.
  16.  
    My understanding, which might be a misunderstanding, was that the building regs don't allow you to insulate under the joists, because of 4.13 in Approved document C, which says

    Any suspended timber floor next to the ground will meet the requirement if:
    a. the ground is covered so as to resist moisture and prevent plant growth; and
    b. there is a ventilated air space between the ground covering and the timber; and
    c. there are damp-proof courses between the timber and any material which can carry moisture from the ground.


    and that requirement (b) becomes all the more important in a renovation situation because (a) is likely not to be satisfied.

    And that this is why, when you look at all of the insulation manufacturers' standard details, the insulation is always between the joists and never underneath.
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2019
     
    Posted By: lineweightMy understanding, which might be a misunderstanding, was that the building regs don't allow you to insulate under the joists, because of 4.13 in Approved document C, which says

    Any suspended timber floor next to the ground will meet the requirement if:
    a. the ground is covered so as to resist moisture and prevent plant growth; and
    b. there is a ventilated air space between the ground covering and the timber; and
    c. there are damp-proof courses between the timber and any material which can carry moisture from the ground.

    I think it is a misunderstanding, although it might well be that manufacturers share (or choose to share) the same misunderstanding.

    Firstly it's important to realize that this text is only guidance, not a requirement. The legal requirements are on a green background near the beginning of the document. The guidance provides examples of ways that the requirements can be met, so that designers, builders and building inspectors can quickly agree that a particular method is acceptable. But nothing requires you to follow the guidance. You won't find any examples of walls built using straw bales in the building regs approved documents, for example, and especially not load-bearing ones. What you do have to do is agree the design and build meets the requirements with your chosen building inspector. Typically that might involve providing additional evidence that the construction meets the requirements.

    Secondly, this text doesn't say that you can't have insulation below the joists, just as long as there is still a ventilated air-space below that.

    Thirdly, and as an example of the first point, Tom Foster has built examples, approved by inspectors, of fully-filled spaces under timber floors. He did/does that by providing additional evidence in the form of drawings and condensation analysis.
  18.  
    Yes, I agree, it's guidance and it's the green box text that ultimately you have to comply with - however it all depends a bit on the building control inspector; if you are stuck with a cautious one you might find it very difficult to deviate from what's stated in the guidance text.

    I'd say the guidance text is a bit ambiguous about where the ventilation should be, relative to the timber, though.

    One reading would be that 'between the ground covering and the timber" means that *all* that space must be ventilated. Another is that there must be a ventilated air space *somewhere* between the timber and the ground.

    The latter reading would allow you to put anything in between the joist-bottoms and the ground, including something that prevented ventilation to the joists, and that's why I've been inclined to assume that the former reading is the one intended.
    •  
      CommentAuthordjh
    • CommentTimeMay 3rd 2019
     
    Posted By: lineweightYes, I agree, it's guidance and it's the green box text that ultimately you have to comply with - however it all depends a bit on the building control inspector; if you are stuck with a cautious one

    But that's the point of the current regime. You aren't stuck with a cautious one - you chose to have a cautious one if that's what you have. (Cautious is the wrong word actually, one unfamiliar with whatever technology or unwilling to come to terms with some new information).

    One reading would be that 'between the ground covering and the timber" means that *all* that space must be ventilated.

    I don't see how since it doesn't say that.

    The latter reading would allow you to put anything in between the joist-bottoms and the ground, including something that prevented ventilation to the joists, and that's why I've been inclined to assume that the former reading is the one intended.

    I'd simply read it as yet another piece of poor drafting in the building regs. Nothing to see here, please move along :)
    • CommentAuthormattrgee
    • CommentTimeMay 3rd 2019
     
    Based on the suggestions above, I'm planning on laying a vapour barrier over the top of the insulation boards and joists. Consequently, can anyone recommend a vapour barrier?
    • CommentAuthormattrgee
    • CommentTimeMay 3rd 2019
     
    Hi Tony,

    Sorry just another query. You mentioned above: "I don’t think there will be sufficient insulation in the floor without going under the joists with it."

    Are you saying that the 100mm PIR won't be enough to insulate the floor from heat loss to the void below? If so, presumably that would present a condensation risk? The 100mm PIR boards are giving a U value of 0.22.

    Thanks again.
  22.  
    Posted By: djh
    Posted By: lineweightYes, I agree, it's guidance and it's the green box text that ultimately you have to comply with - however it all depends a bit on the building control inspector; if you are stuck with a cautious one

    But that's the point of the current regime. You aren't stuck with a cautious one - you chose to have a cautious one if that's what you have.


    It's not always the case that the designer is the person who pays for or chooses the BCI. Nor is it always the case that when you choose the BC company, you know everything about the attitude of the person who is then assigned to the job.
    •  
      CommentAuthordjh
    • CommentTimeMay 6th 2019
     
    Posted By: lineweightIt's not always the case that the designer is the person who pays for or chooses the BCI.

    Indeed - my architect designed my house. I chose and paid for the BCI. The key point is that the person who does choose the BCI should know what they're looking for - perhaps by being briefed by the designer, perhaps in other ways.

    Nor is it always the case that when you choose the BC company, you know everything about the attitude of the person who is then assigned to the job.

    Also true and happened to me. But when after I chose the firm on the basis of the chief of the office, he assigned a muppet, I let the chief know very clearly what I thought and he assumed personal control. If he hadn't, I would have found a different building control firm.
  24.  
    The point is, while it's true that yes, it's the 'green box' text that you ultimately have to comply with in the Approved docs, and yes, in a well run and well funded job, the AI will be chosen carefully and yes, if they don't perform, then with a not inconsiderable amount of hassle you can change to another one, there are all sorts of reasons why jobs end up in a situation where the pragmatic route to getting things done is to do them in a way that can be easily shown to follow the guidance in the ADs, which is why that part of the text is significant.

    If you're a specifier or designer who feels they want to do the best that is reasonably possible in whatever you're working on - it's sometimes the case that the person paying for the work is not greatly interested in the technical stuff, or inclined to understand its implications. So, if you are trying to make a decision about the insulation strategy for a ground floor, and the building inspector you have wants you to do something that follows the text I quoted above, then it just is not an option to ask the person paying their fees to replace them, in order that you can change the specification of the insulation such that a somewhat better U value results, when that person barely understands what a U value is or why it should matter to them.

    Hence my original comment that it is sometimes really not easy to deviate from the guidance text.
    •  
      CommentAuthordjh
    • CommentTimeMay 6th 2019
     
    Posted By: lineweightthere are all sorts of reasons why jobs end up in a situation where the pragmatic route to getting things done is to do them in a way that can be easily shown to follow the guidance in the ADs

    Yes, I accept what you say and it does drag us out of the rabbit hole we were descending into but ...

    If you're a specifier or designer who feels they want to do the best that is reasonably possible in whatever you're working on

    ... then you have to fight your corner sometimes. So I'd argue that you should specify or design what is sensible and be prepared to explain it to the paymaster if necessary, and discuss the point with building control if necessary, and if that fails there's always:

    https://www.gov.uk/guidance/building-regulations-determinations

    (which also illustrates perfectly the incompetence of government ...


    www.planningportal.gov.uk uses an invalid security certificate.

    The certificate is only valid for the following names:
    www.gov.uk, *.businesslink.gov.uk, *.direct.gov.uk, *.preview.alphagov.co.uk, *.production.alphagov.co.uk, *.publishing.service.gov.uk, *.cabinet-office.gov.uk, assets.digital.cabinet-office.gov.uk, service.gov.uk, data.gov.uk, dfid.gov.uk, cabinet-office.gov.uk, www.data.gov.uk, gov.uk

    Error code: SSL_ERROR_BAD_CERT_DOMAIN


    !! :devil: )

    Setting yourself up to fail (to implement the best that is reasonably possible) by proposing the 'pessimistic' approach in the first place is not a good path, IMHO.
    • CommentAuthorlineweight
    • CommentTimeMay 6th 2019 edited
     
    Posted By: djh
    ... then you have to fight your corner sometimes. So I'd argue that you should specify or design what is sensible and be prepared to explain it to the paymaster if necessary, and discuss the point with building control if necessary, and if that fails there's always:


    Yes in principle I agree. But I'm afraid it has to be proportionate to the size of the job and what you are getting paid for it. There's always a point where you have to make a pragmatic decision not to push further, if client, BC inspector, contractor, whoever, are being resistant to something. Because otherwise the result is that you can end up sinking many hours of time that you effectively don't get paid for, trying to achieve something that you might still not manage to achieve, antagonising people along the way and making them even more resistant to other points of detail you want to be fussy about. I'm always conscious that other people out there will not only design stuff on the basis of what's easy to get approved, but what's easy for them to design, which means things like cold bridges all over the shop. Unfortunately that's the reality of much of the UK construction industry, as I'm sure you know. I wish it wasn't but that's the context one has to work in unless you are operating only in a very slim segment of the market where people are able and willing to invest extra resource into actually doing things completely properly.
  27.  
    (Sorry OP, this has gone a bit off topic)
    • CommentAuthorcjard
    • CommentTimeMay 7th 2019
     
    Not sure I'd bother obsessing too much over the insulation detail of a floor that wasn't being used as a heat emitter (underfloor heating) - the air tightness detail will have a much bigger impact on the the warmth of the house, and if the flooring is chipboard I presume it's going to be covered with carpet?
  29.  

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