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  1.  
    Hi all,

    I am building a new house and have had a few restrictions placed upon me, including a really short time-frame.

    The foundations must be piled and also have a suspended RC slab

    With this as a starting point, I am building a timber frame house with block-work exterior (rendered) but the plot also comes with a height restriction, so trying to make sure I keep the finished floor level as low as possible.

    I have had a good search through the forums and most of my question have been answered, but I need to double check I am heading in the right direction with the foundation detail before I career off in the wrong direction.

    http://www.ukrobotgroup.com/images/build/Foundation%20section%20plot%208.png

    Some specific issues I am struggling with:

    Most of the slabs I have seen on line seem to have their top edge pretty much level with the outside ground level. If the slab is sunk a little lower than ground level, do you start to move into tanking territory? Also, a builder told me that if I raised the level of the screed above the DPC I would "be asking for trouble", not quite sure why.

    At the doorways I am struggling with the detail. Front door needs to be a level threshold, but not quite sure if the detail I have drawn makes sense. Most of the doors I have been looking at seem to limit the sill to around 150mm which would put the door roughly in the front part of the 50mm cavity.

    Also, most people seem to be saying that the door frame should be mounted back to the timber frame and a movement gap be left underneath it. Not quite sure how to create a gap under the sill and frame, doesn't sound right to me.

    http://www.ukrobotgroup.com/images/build/Foundation%20section%20plot%208%20flush%20front%20door%20050217.png

    The rear has a bi-fold door which I also want to get as flush as possible. The trouble with these is that the channels the doors run in need to be sunk down into the block-work. As my DPC is at 150mm and the finished floor level is not much higher, not sure if I'm allowed to install the channel lower than the DPC so it's top edge is flush enough. See example from Kloeber (but their design allows them to sit above the DPC)

    http://www.kloeber.co.uk/shared/media/documents/87/Standard%20Track%20Flush%20Installation%20Aligned%20-%20Externally%20Opening.pdf


    Thanks for any assistance.

    Pete
    • CommentAuthortony
    • CommentTimeFeb 5th 2017
     
    It might be easier to reduce ceiling height than to mess with stuff below ground

    Anecdotal, someone near me built a total monster house by digging it into a hollow in the site, it flooded in its own pond

    I would say that you can step the dpc 150 across the cavity without tanking.

    No problem with chanel below dpc but use slot boxes or drains Chanel under the doors outside.

    I would aim for ffl slightly above outside ground level.



    I would insulate dround flow with 300mm of eps as a minimum and mitigate thermal bridging at the sole plate or it will rot prematurely due to condensation on top of its dpc.
    •  
      CommentAuthordjh
    • CommentTimeFeb 5th 2017
     
    Posted By: OracsRevengeI am building a new house and have had a few restrictions placed upon me, including a really short time-frame.

    Hi Pete.

    I would do my utmost to get that restriction lifted. Things don't happen to a planned timeline when you are building, unless you are a miracle worker. Plus things that need to happen to time rapidly get a lot more expensive.

    The foundations must be piled and also have a suspended RC slab

    Is a passive slab feasible? If so have you costed it as an alternative?

    I have had a good search through the forums and most of my question have been answered, but I need to double check I am heading in the right direction with the foundation detail before I career off in the wrong direction.

    You seem to have an awful lot of timber underneath the frame. What is the reason for that? It's a big thermal bridge, especially with what looks like a concrete block underneath it. Why does the internal plasterboard stop so high above the floor? Why a full block wall rather than using render boards to carry the render? Have you considered using EWI on the outside of the frame?

    Slabs tend to be level with the ground to facilitate level thresholds. They tend to be higher (150 mm above ground) when there is timber construction that needs to be kept dry, although dwarf masonry walls can accomplish the same thing. If you go lower you either need to be very sure of your drainage or tank it.

    At the doorways I am struggling with the detail. Front door needs to be a level threshold, but not quite sure if the detail I have drawn makes sense. Most of the doors I have been looking at seem to limit the sill to around 150mm which would put the door roughly in the front part of the 50mm cavity.

    Also, most people seem to be saying that the door frame should be mounted back to the timber frame and a movement gap be left underneath it. Not quite sure how to create a gap under the sill and frame, doesn't sound right to me.

    Can you stack the Marmox blocks? What is in the cavity under the threshold? You can buy extension sills or get custom ones made. We just made some in oak for some of our doors. Door frames are secured by screws through the sides into the timber frame. The sill doesn't carry any weight, except that of people standing on it as they pass through. Normally the blockwork at the threshold will be lower than the internal subfloor, I think. In our case the internal floor is 65 mm above the slab, to match a lift-and-slid stood on the slab, so there is a 40 mm or so high gap underneath the threshold of the other doors, filled with EPS.

    Bi-fold doors are bad news. It's very difficult to make them airtight and to keep them airtight. I don't know, but I expect they need to be above the DPC, which may need to be stepped down at that location (or lift the internal floor level as we did).

    HTH, Dave
  2.  
    Hi, thanks for the replies

    Unfortunately, due to restrictions on the site which are immovable, it has to be a piled foundation and an RC slab. However, I can play with the slab location and size. Once the piles and slab are built, the site then passes over to me to continue the build.

    The details of the double sole plate and the block under the frame come from the Timber Frame companies standard foundation design which they have passed on to me. I suspect I can get the sole plate reduced to one timber if required. I suspect the service cavity and plasterboard stopping short is something they did for another client and can be ignored.

    I did originally want to use render-boards, but eventually back-tracked to block-work for several reasons. Firstly, it's close to a railway line and I need to improve the sound insulation for the building. Secondly, building without a block-work outer skin was classed as "non-standard" by the insurers and mortgage company. Companies like the Ecology Building society are happy to help, but at a premium. Thirdly, I just got tired of everyone battering me about having a more solid outer skin, plus the TF company said it improves the racking of the timber frame.

    The Thermoblocks are only stacked at the door thresholds, so hopefully shouldn't be an issue, but I am open to other designs and options. The rest of the upstands for the timber frame around the slab are normal block-work.

    I have looked at dozens of different different foundation drawings around the web and nothing seems to be quite right, if anyone has a link to a design that the think might work in my situation, that would be great.

    Thanks

    Pete
  3.  
    Sorry, forgot about the cavity. There is nothing currently in the cavity.

    We need to reach a bit a wall and floor U value of 0.15, so the wall is 140 timber frame and an extra 25mm insulation on the inside.

    For the cavity below dpc its empty at the moment, but planning on having drain holes at the slab. As the distance to the slab is short, I believe the regs say I need a cavity tray just above dpc all the way round the building. Were you considering insulation in the cavity below dpc at. The threshold?

    Thanks
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2017
     
    Posted By: OracsRevengeSorry, forgot about the cavity. There is nothing currently in the cavity.

    The threshold drawing shows a lump of something in the cavity.
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2017
     
    I think I'd be looking for a timber frame company that shows greater experience of building highly insulated houses. Somebody who showed you a thermal bridge free design out of the starting gate would be helpful.
    • CommentAuthorcjard
    • CommentTimeFeb 6th 2017
     
    100mm of insulation and UFH?

    That's... Unexpected
    • CommentAuthorcjard
    • CommentTimeFeb 6th 2017
     
    You can install your DPC as per that kloeber drawing; step it down outside, just make sure the external ground level is below it (it should be, as the kloeber drawing features a drain) including some way either side of the window

    The co that provided my bifolds (smarts visofold 1000 profile) supplied a 225 mm cill. Bows and arrows against the lightning of my 450mm walls, so the sill finishes before the wall does, but I'm going to slope the wall away to the outside, add a membrane, have a gravel soakaway, and cantilever paving over the sill anyway, as per your drawing. Effectively the membrane extends the sill, and it will then be covered, protecting it

    +1 to the comment about mounting the frames in the timber frame - not much point having a thermally broken window if it's solid stone either side of the break!
  4.  
    Hi

    The item showing in the cavity is just something I put there to signify a cavity closer. I'm going to redraw this again as I now realise that the whole threshold area doesn't need to support any significant weight.

    Still not quite sure of the best way forward for minimising the thermal bridge and condensation issue as mentioned .

    Tony - can you expand on this comment

    "I would insulate dround flow with 300mm of eps as a minimum and mitigate thermal bridging at the sole plate or it will rot prematurely due to condensation on top of its dpc."

    Not sure where you meant the eps could go.

    Thanks

    Pete
    • CommentAuthortony
    • CommentTimeFeb 6th 2017
     
    Tony - can you expand on this comment

    "I would insulate ground floor with 300mm of eps as a minimum in or under the floor and mitigate thermal bridging at the sole plate (additional insulation in the cavity in that area) or it will rot prematurely due to condensation on top of its dpc."
  5.  
    Hi,

    I have been checking lots of Timber Frame companies and they all seem to have a similar design as the one I have been given

    Looking through the TRADA construction book, they give this detail, which seems to be almost identical everywhere I look.

    Is the condensation/cold-bridge/rot issue going to be the same for everyone who uses the official design?

    This is why I made the first block of the upstand a Thermoblock, to try to mitigate the issue a little.

    I want to keep the design similar to the TRADA diagram as I it's also what the BC (Premier) rule book seems to be wanting to see (will double check with them in the morning).

    Is there is anything simple that can be done with the TF upstand?

    There is a picture in this thread which shows some insulation on the outside face of the upstand and frame, not sure if this is better, or if it would trap more moisture in the bottom of the frame? (also seems to be bridging the DPC)

    http://www.ebuild.co.uk/topic/15950-kingspan-tek-sips-panels/page__st__80


    Thanks

    Pete
    •  
      CommentAuthordjh
    • CommentTimeFeb 7th 2017
     
    Posted By: OracsRevengeLooking through the TRADA construction book, they give this detail

    Your copy must be different to mine then, because my 5th ed copy only shows two pieces of wood stacked. The TRADA book is not state of the art (perhaps Timber can confirm this). The Passivhaus Details book has better details.

    Anything based on government approved details cannot be trusted when it comes to thermal bridging, IMHO.

    Additional insulation on the outside is probably the most common way around the issue.

    Perhaps see what MBC or Touchwood can do (there are doubtless others as well).

    This thread may be worth reading:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10226
  6.  
    Apologies if I query anything that's already been answered - I have only had a skim-read of the replies hitherto, but is that really 150mm of timber (2 x sole-plates and bottom rail) above the DPC? That's a hell of a thermal bridge! And the 100 u/floor and 20 upstands compares poorly with 270 EPS and 75 Pu u/floor and 100 Pu upstands in a house I was involved with a couple of years ago.

    One of the general lessons I learned was that any measurement errors (openings made too large, for example) tended to be 'made good' with solid baulks of timber, with no provision for insulation - again, huge thermal bridges compared with the 0.12 U value of the frame overall.

    What is the proposed wall insulation? I am more comfortable with quilt or loose-fill than I am with Pu or similar. With the latter I feel there may be risks of trapping moisture in 'capillary interstices'. Build it fully breathable, without a VCL (but with a tight air-tightness layer) and, in my view, you build in fewer risks. Don't automatically expect general contractors (and particularly sub-contractors) to understand air-tightness, though.

    Sorry, I have digressed from foundations, but if you have a big thermal bridge at the footing to start with, you probably need to think what might arise further up.
    • CommentAuthorDarylP
    • CommentTimeFeb 7th 2017
     
    TRADA vol2 std detail:
      TRADA Vol2 StdDetail_GBSJunction.JPG
    • CommentAuthortony
    • CommentTimeFeb 7th 2017
     
    As the walls become more highly insulated and windows have lower U values the coldest places will pick up condensation. This is likely to be the dpc under the sole plate.

    The world is changing and TRADA has yet to catch up -- they are sleeping, problems will come to light

    The insulation is nothing like joined up between the floor and the walls, monster thermal bridge
    • CommentAuthorDarylP
    • CommentTimeFeb 7th 2017
     
    @tony, Yes I agree with you. I have had several 'conversations' with TRADA about this.
    Nothing....they do not see the need to change/improve, and will not, until forced to. As the majority of the industry use these details as a 'fall back', we are stuck with them, until the revised Part L is trailed, with revised treatments of heat-loss through thermal bridges (Psi-values).
    :neutral:
  7.  
    Thanks, I have taken on the advice above and will ask them to remove the second sole plat (I too have no idea why it's there)

    Had a fiddle with the design, advice on the below welcomed.

    Removed extra sole plate, added an extra line of engineering bricks and dropped the slab level as the site survey shows a slight slope on the site which would mean the rear end of the slab poking out of the ground by around 8cm.

    Brought the OSB and plasterboard down to the correct level, added in the cavity tray and perpend which seem to be required due to the distance from the DPC to the slab. Cavity tray lapped under TF membrane.

    Added in the flooring and increased the floor insulation due to the extra Engineering brick level. Also added the extra 25mm insulation mounted on the inside of the Timber Frame to get to U0.15

    Lightweight thermal block shown to allow shooting nails through the sole plate.

    One thing I have been advised is that if there is a membrane under the slab, then there is no need to ventilate the void where the Clayboard is showing. If this is correct, then does that DPM need to come up the side of the slab and head back into the cavity somewhere?

    Thanks

    Pete
      Foundation section plot 8 0702172.jpg
    • CommentAuthorcjard
    • CommentTimeFeb 7th 2017 edited
     
    Is the aircrete sufficient to withstand the loads imposed by being sandwiched between marmox and studwork?

    I had in mind that marmox were essentially a big EPS version of a clay brick, with solid plastic where a normal brick would have a hole right through.. As such your aircrete blocks sit atop some point loads formed by the plastic cores in the marmox..
    • CommentAuthortony
    • CommentTimeFeb 7th 2017
     
    Yes but aircrete easily moves or shifts about during construction

    Best do 50mm eps below where the sheathing stops, 30 mm above with a chamfered top edge and breather layer going over the top of it. I would like to have seem even more insulation but this is a good start
  8.  
    Hi both,

    The cavity is currently 50mm, if I put 50mm of eps in, it would fill the cavity which I suspect may be an issue. I can't make the cavity bigger as that would mean a re-design and re-submitting of the plans (£££).


    The following text is from the Marmox site:

    "How to Design a Thermally Efficient Wall-Floor Junction

    Marmox Thermoblock is not an insulating building block like an aircrete block. Typically it replaces just the bottom row of blocks at floor level and in conjunction is used with aircrete blocks in the rest of the wall to stop the cold bridge at the critical wall/floor junction.

    The standard block thickness in the UK as a 65mm high block, this is because this is the typical UK brick height and this thickness has been designed to give the necessary insulation required. To achieve an R Value of 2, we can also manufacture a 100mm thick version"


    From their pictures, looks like the standard use is one layer of Thermoblock at the bottom, then aircrete up to the sole plate.

    All this info entering my brain is wonderful, but I do worry what is being pushed out of the side into my mental recycle bin. :bigsmile:

    Thanks

    Pete
    •  
      CommentAuthordjh
    • CommentTimeFeb 7th 2017
     
    You could always use aerogel instead of EPS :bigsmile:

    Just having a quick look at the Touchwood pictures, it seems they sometimes put in a slab or beam-and-block, then 300 mm EPS on top, then a slab on top of that and then the timber frame on top. Maybe that's a possibility for you? Increase the width of your screed and build the timber frame on top. You'd probably have to reinforce and/or increase the depth of the screed I suppose.
  9.  
    Thanks, looking through it now.

    I have engaged a Structural Engineer to examine the levels around the plot and determine the optimum slab level, this will then help us determine the options we have with extra insulation, etc.

    Pete
    • CommentAuthortony
    • CommentTimeFeb 8th 2017
     
    I dont think an engineer is the right person to advise about slab level, they design things to be strong enough, most likely they would like the slab a few hundred millimetres above ground ideally.

    I think you tell them where you want it
  10.  
    He seems confident enough, and comes highly recommended.

    He is looking through it all today, so hopefully will provide some advice on the foundation design and levels.
    • CommentAuthorMarkyP
    • CommentTimeFeb 8th 2017
     
    why cant you fully fill the cavity with EPS?

    why do you need to re-submit plans to make your cavity wider? Will anyone notice or care if you've made it 75mm - 100mm instead of 50mm?

    consider carefully whether the heat loss of going with 50mm EPS outside 3courses of aerated blocks is bad enough to warrant the extra expense of thermoblock, etc. I would think the risk of condensation would be mitigated with the eps/aerated option. So the question is how much heat would be lost and is it enough to warrant thermoblock?
  11.  
    Hi Marky P,

    It's a narrow plot and I am already up against the limit on the external measurement, If I alter the internal measurement to increase the cavity, the plans would need to be-redrawn. This would mean paying for the Engineering design (steels, etc) to be re-done, etc which has cost me several thousand so far.

    I have the tables from the Thermoblock BRE datasheets, so will try to work it out.

    Thanks

    Pete
  12.  
    Not sure if this will help in anyway, but for my SIPs extensions I ended up with the following design (see last image on page 2): http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=10226&page=2

    My exterior is clad though.

    Sole plate was bolted through thermoblock into celcon blocks below.
  13.  
    Hi,

    Did the Thermoblock and Celcon cause you any issues at all during the cutting/laying, and did you suffer any Celcon cracking when the frame was bolted through to them?

    Thanks
    • CommentAuthorcjard
    • CommentTimeFeb 9th 2017 edited
     
    Tony. How can you say "yes the aircrete blocks will withstand the imposed loads" when you know neither what the imposed loads are, nor what the compressive strength of the aircrete bricks is?

    This is a question for Oracs' structural engineer, not you


    Tony's advice in another post has also implied that heavily insulating a wall causes condensation on the sole plate, and that's not necessarily the case- condensation occurs when the air close to a surface holds too much moisture for the temperature that the cooler surface reduces the air to. Thermal bridges, by definition are warmer than the surrounding unbridged area, because more heat is flowing through them because they're a bridge. As such, your soleplate will quite reasonably be warmer than the rest of the wall construction, and you just need sufficient insulation outside of it to establish a temperature gradient whereby the condensation point occurs within the insulation.

    Oracs, bolting a frame to celcon isn't really to stop it lifting up/blowing away, it's more lateral movement until the weight of the frame inhibits the same. As such, predrilling the holes wouldn't cause any harm. In my experience, you can whack a concrete screw into an aerated block without splitting it, but as I'm usually drilling through something else first, there's no harm in sending the bit into the block too

    Dont screw yourself into the ground too much over tiny details; if you try to get everything absolutely perfect you'll never finish, and the stress isn't worth it. 99% perfect is still about 100% better than the average uk house..
   
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