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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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  1.  
    Partway through renovation process we have now taken up floorboards over the suspended floor and inspected the void underneath. It is not as deep as expected and according to the builder the perfect depth to pour in concrete and convert to a solid floor with a celotex layer, screed layer with UFH pipes set in.

    I have to make a decision quickly on whether to go with this (after spending weeks meticulously planning the detailing on insulating and installing UFH between the floor joists!!).

    Any downsides to the concrete floor?
      3EFCD4B3-8CC8-4D2D-9A3D-68D4C5FB5D44.jpeg
    • CommentAuthorandyman99
    • CommentTimeJan 26th 2021
     
    Not an expert but, having recently insulated a suspended floor, I think if I had the opportunity to convert to a well insulated solid floor I would go that way. I'm pleased with the result, but its a lot of work to do well and in the long term there are obviously events that could degrade my work. What are the comparative costs?
  2.  
    The costs are similar apparently. The solid floor maybe slightly more. Builder will send over breakdown tonight.

    The DPC course which is a layer of slate (above the black line on photo) would be below the top floor layers.
    • CommentAuthorandyman99
    • CommentTimeJan 26th 2021
     
    I'll leave others to comment on that, but I just remembered that although most of the house has what appears to be a similar depth void to yours, there is an area that is much deeper (we are on sloping ground), you might want to check all floor voids are similar before making your choice?
    • CommentAuthorphiledge
    • CommentTimeJan 26th 2021
     
    Personally, I think anything at ground level needs to be moisture resistant, so a generously insulated solid floor would be my choice. If theres meticulous attention to detail, Im sure an insulated timber floor can last a very long time but its greener credentials over the solid option are probably negated if it needs reworking in a decade or two.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 26th 2021
     
    Would you consider insulation onto the solum (with DPM's etc), then concrete slab on top, with UFH tied to steel mesh sitting on mesh stools?

    Why pay for concrete AND screed? You've more depth for insulation (or thicker cheaper insulation) without the screed. Been doing it that way 20 years. Save yourself a few grand on screed. If your builder can't pour a decently flat floor in conc, try another builder.
  3.  
    Posted By: philedge If theres meticulous attention to detail, I'm sure an insulated timber floor can last a very long time but its greener credentials over the solid option are probably negated if it needs reworking in a decade or two.


    The 'meticulous' detailing worries me as neither myself or the builder have done this before.

    What would you class as 'generous insulation' - is 100mm celotex enough? I am having 200mm in the new extension but will struggle for space I think in the void.
  4.  
    Posted By: GreenPaddyWould you consider insulation onto the solum (with DPM's etc), then concrete slab on top, with UFH tied to steel mesh sitting on mesh stools?

    Why pay for concrete AND screed? You've more depth for insulation (or thicker cheaper insulation) without the screed. Been doing it that way 20 years. Save yourself a few grand on screed. If your builder can't pour a decently flat floor in conc, try another builder.


    Interesting. So that would result in potentially more insulation? The ground floor joins up with the new extension which will have a screed layer so suspect the builder will want to match up with that and do it all at the same time.
  5.  
    A further question is whether I should I parge plaster the wall down to the DPC layer (before either type of floor). The joists are not set into the wall (although they are on the first floor).
    •  
      CommentAuthordjh
    • CommentTimeJan 26th 2021 edited
     
    Posted By: modernvictorianThe ground floor joins up with the new extension which will have a screed layer so suspect the builder will want to match up with that and do it all at the same time.

    You're likely to get cracking in the doorway anyway, AIUI, so perhaps best to deliberately engineer a crack (i.e. a joint) at that location.

    A further question is whether I should I parge plaster the wall down to the DPC layer

    Well yes or no. If the plaster on that section of wall is part of your airtightness envelope, then yes you need to parge it. If the airtightness envelope is at the top of floor then no, you probably don't need to parge below the floor, unless there's some other factor. Maybe easiest just to do it in any case rather than think about it.
  6.  
    We did both methods in different parts of our last house, both gave great improvement and toasty feet. The concrete was easier to draught proof. Enjoy!

    You need upstands of insulation around the bases of the walls, so that there is no path for 'cold' to flow in through the base of the walls and into the edges of the new insulated slab. If that happened, then you'd get a cold bridge area in the wall/floor corner, which attracts condensation. Don't forget internal walls if they go down to foundations in the ground.

    What are your thoughts about keeping your floor moisture permeable (limecrete, no PIR so poorer insulation value) versus sealing it all up moisture proof (DPM to join onto DPCs in all the walls including internal walls)
  7.  
    My thoughts are limited somewhat given it was suggested today and I probably have to decide by tomorrow.

    I am worried about a non-permeable floor and how that might affect the walls but a) there is a slate DPM which seems to be doing its job, and b) the ground currently looks very dry (good?) and I am at the top of a hill so not exactly sitting on top of a water table.

    So basically the options are stick to the original plan and insulate the suspended floor or go for the last minute change of plan to solid concrete/PIR.

    The edge insulation is a good point - what material would you use for that? I assume that would be inside the DPM?
    • CommentAuthorbhommels
    • CommentTimeJan 26th 2021 edited
     
    Fully agree with WiA!

    In our case, a rotting joist tilted the balance to get rid of the suspended timber floor and go for a solid infill.
    Underneath the suspended floor there turned out to be a sort of rough, uneven floor. After some serious poking with a steel bar, our builder reckoned that the floor was sound and dry and since it had been there for 70+ years he saw no need to pour a layer of concrete first. Instead he used a layer of hardcore and sand to level the floor before the DPM and insulation went in. The polyethylene DPM was folded up the walls to meet the slate DPM embedded in the wall.

    Insulation wise we could fit 200mm (repurposed) EPS and a 80mm PIR layer on top, with 80mm screed with UFH pipes embedded on top of the insulation. I used 50mm PIR strips all around the perimeter, which is a bit much as 10mm still stuck out from under the skirting boards: not good when putting legs of bookcases right up to the skirting, although the flooring has been able to cope with it so far. Airtightness is secured by taping the screed to the walls and plastering over.

    Although we have had an issue with water ingress at one of the walls where a gully had clogged up, causing the joists to rot in the first place, I have not had any issues at all after the full filled floor went in.
    • CommentAuthorSimonD
    • CommentTimeJan 27th 2021 edited
     
    In my most humble opinion, I would think very carefully about dumping a load of concrete and screed into that floor space and I'd be really suprised if it was comparable in cost to refitting joists and insulation once all the detailing has been done properly.

    I recently took out a concrete floor and replaced it with a suspended timber floor. The old concrete floor had been poured into what had been a suspended timber floor with dpm up to the dpc. When I stripped out all the kitchen units, I found a terrible amount of damp along the edges of the wall at the join between the dmp and dpc. Overall it had been done pretty badly by the previous owners.

    My understanding is that to do this properly, the dpm needs to tuck into the dpc of the existing wall, not just be brought up the side of the existing wall to dpc. This is likely to be a pain to get done right.

    The other thing is that timber floors are much more comfortable under foot and when insulated will always feel warmer than even an insulated concrete floor (unless you've got some underfloor heating).

    From an environmental perspective,I'd suggest that the suspended timber floor also wins hands down.

    From a historical perspective,it find it interesting that the UK transferred to solid floors only after we'd destroyed all of our local timber during the second world war and haven't gone back to it since. I don't think it's necessarily because a solid floor is any better.

    I attach a photo of the wonderful time I had digging out that concrete floor!
      floor.jpg
    • CommentAuthordereke
    • CommentTimeJan 29th 2021
     
    This system might also be of interest - https://www.youtube.com/watch?v=NbC-EfQ_u3E
    I'll summarise:

    - concrete foundation around the outside
    - compacted aggregate floor
    - EPS on top of that
    - two layers (glued and screwed) of what I think is some fancy OSB

    I'm also considering converting my floors to some sort of solid floor so I'd be interested in thoughts on this system.
    I'd like UFH in mine though which maybe wouldn't work great with this build up.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 30th 2021 edited
     
    Dereke - like this a lot. I always do compacted solum, insulation, and then concrete, so part way there. In a basement renovation I've also done 100mm kingspan pink foamed to the existing slab, and pink foamed T&G 22mm chipboard on top, with vinyl finish - works a treat 4 years on.

    I think I'll try the hybrid of these two, which is what is shown in the video, next project. UFH could be done using the pre-routered chipboard which takes the UFH pipes, and a second layer bonded on top. Maybe paint a liquid DPM over the whole lot, for the spills scenario.

    So, need to get a struct eng'r on side to check it out, and get it through building control, and satisfy myself, that enough heat will be discharged from the UFH pipes into the timber floor build up. I normally run from a thermal store, so could run the UFH circ pump longer, with out high return temps causing a problem (direct from a heat pump could cause short cycling).

    Thanks dereke for pointing this video out, and giving me a shove. All that lovely cement can stay in Portland :bigsmile:
    •  
      CommentAuthordjh
    • CommentTimeJan 30th 2021
     
    Posted By: derekeI'm also considering converting my floors to some sort of solid floor so I'd be interested in thoughts on this system.

    Seems to be one major problem though. The structure sits straight on/in the ground, so there's a major thermal bridge at the external wall-floor junction. Carrying and distributing that load is what the concrete and steel do in a passive slab. At least, that's a big part of what it does.
    • CommentAuthorPetlyn
    • CommentTimeJan 30th 2021
     
    We have 12m3 of expanded glass bead 4-8mm in diameter remaining from our low carbon build which have excellent insulation properties, are non-flammable, inert, easy to pour or blow if they were of any interest. You could lay your underfloor heating directly on top of the beads, creating a warm envelope. We are in Norfolk but do have a contractor with a curtainsider and tail lift who has moved some in the past for us.
  8.  
    Posted By: djha major thermal bridge at the external wall-floor junction


    We are thinking about EWI, but a major advantage of IWI for retrofit is that you can easily join it to the under-floor and loft insulation.
    •  
      CommentAuthordjh
    • CommentTimeJan 31st 2021
     
    Posted By: WillInAberdeen
    Posted By: djha major thermal bridge at the external wall-floor junction


    We are thinking about EWI, but a major advantage of IWI for retrofit is that you can easily join it to the under-floor and loft insulation.

    Indeed. The youtube was about new build I think, where it is possible to avoid such problems.
  9.  
    Just to update that I decided to go with the solid floor/UFH in screed layer in the end. After costing it all up it was 250 quid more than laying UFH between the joists. I'm h happy with the decision.

    Another factor was the fact that we are currently at the end of a very wet winter yet the ground under the floor is bone dry (I was worried about pushing moisture up the walls).
    • CommentAuthordereke
    • CommentTimeFeb 4th 2021
     
    Posted By: GreenPaddyThanks dereke for pointing this video out, and giving me a shove. All that lovely cement can stay in Portland


    Yes Portland has been deprived of its cement for far too long! :wink:

    Keep us posted if you do end up doing a floor this way. I might need to cite you in a building application!



    Posted By: djhSeems to be one major problem though. The structure sits straight on/in the ground, so there's a major thermal bridge at the external wall-floor junction. Carrying and distributing that load is what the concrete and steel do in a passive slab. At least, that's a big part of what it does.


    We could insulate the walls on the inside from the current ground level up to the floor level - I think that would eliminate the bridge you are referring to?
    • CommentAuthorbhommels
    • CommentTimeFeb 4th 2021
     
    Posted By: modernvictorianJust to update that I decided to go with the solid floor/UFH in screed layer in the end. After costing it all up it was 250 quid more than laying UFH between the joists. I'm h happy with the decision.

    What final build up did you decide on in the end? Concrete sub floor or just hardcore underneath the insulation, and how much screed for the UFH on top? BTW I never regretted it either. The high thermal mass of UFH in screed really makes it all very comfortable.
    •  
      CommentAuthordjh
    • CommentTimeFeb 4th 2021
     
    Posted By: derekeWe could insulate the walls on the inside from the current ground level up to the floor level - I think that would eliminate the bridge you are referring to?

    Probably. My comment related to the video's proposal, which was for a new build situation, where I think the suggested technique is less than optimal.
  10.  
    I went with a fairly standard buildup of sand base, 100mm concrete, 100 celotex and 75mm screed. There's not room for anything extra without digging down. I have 200mm celotex in the new extension but couldn't do it in the old part.
    • CommentAuthorwookey
    • CommentTimeFeb 23rd 2021
     
    I'd have dug an extra 100mm to get some more insulation in but you've done it now. I'd always vote for a solid floor over a suspended timber ventilated one because all that cold air and infiltration potential is bad. The only thing in favour of a suspended floor is lower embodied emissions, but you have to work really hard to get as good in-use emissions as solid.

    Having dug up the floor I'd very seriously think about putting in a row of perinsul or quinn-lite in the external wall to minimise the foundation thermal bridge. It's then or never.
    • CommentAuthorSimonD
    • CommentTimeFeb 23rd 2021
     
    Posted By: wookeyI'd always vote for a solid floor over a suspended timber ventilated one because all that cold air and infiltration potential is bad. The only thing in favour of a suspended floor is lower embodied emissions, but you have to work really hard to get as good in-use emissions as solid.


    Why would that be? Isn't that a bit like saying an insulated masonry cavity wall or ICf is always going to perform better than a timber frame? If it's insulated and detailed correctly, a suspended timber frame floor will perform just as well, if not better, for the lifetime of the building. There are lots of other advantages to suspended timber floor that people often overlook. One of these is user comfort - a suspended timber floor is more comfortable under foot, partly because of its 'give' and it feels warmer. Another is when used with underfloor heating, the reduced thermal mass can improve responsiveness of the heating system, avoiding some of the disadvantages of lag experienced with underfloor heating on slabs.

    From a DIY perspective it can be much easier and cheaper to retrofit a suspended timber floor. This includes situations when paying for professional installation too. For example, where I live a solid floor would have to be pumped in, and after having mine priced up, for an area of approximatley 24sqm, the suspended timber floor worked out at about 1/3 the cost of solid floor.

    Detailing of a solid floor retrofit is more involved than many people think and it does come with risks from ground moisture as well as heat transfer/cold bridging and may be unsuitable over some ground conditions.
    • CommentAuthorCWatters
    • CommentTime6 days ago
     
    Posted By: SimonDIf it's insulated and detailed correctly, a suspended timber frame floor will perform just as well, if not better, for the lifetime of the building.


    +1 but I just wouldn't trust most builders to do it right. Most will just hack bits of insulation to death with a hand saw and jam it between the joists leaving gaps all over the place. I reckon its more likely a builder could install a solid floor correctly. If you DIY that's another issue.

    Its probably too late for the OP but the question I would ask is this.. Is the building and your lifestyle suitable for UFH?

    The power you can get out of UFH is limited to about 150W per sqm. That should be OK but if there are a lot of small rooms the actual area that isn't partly insulated by furniture can be small. Are you fitting carpets? Tiles? Wood floor? Carpet and wood can also reduce heat output. The TOG of carpet and underlay should be as low as possible. I think they recommend no more than 2.0-2.5. Special low TOG underlay is available.

    Because UFH takes longer to heat up I think its best suited to families with at least one person home in the daytime. If you are both working then I think rads make more sense. This is only based on my personal experience with rads and UFH.
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