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General: Raft slab insulation

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  1.  
    • CommentAuthorShevek
    • CommentTimeJul 7th 2012 edited
     
    The attached images are my structural engineer's sections for our ground floor extension. It's a raft slab with a couple of 1 metre square pile foundations, because of big poplar tree nearby.

    1. We're aiming for Passivhaus
    2. The new walls will be EWI, single skin aircrete or maybe hemp blocks or something.
    3. There's potential to insulate under the slab and wrap the edge but we're ripping out the timber suspended floor to the front room and adding an insulated slab there too and we obviously won't have the same potential, so perhaps isn't worth it.

    What do think the best insulation strategy is?
      cross-section.png
      boundary-wall-section.png
    • CommentAuthoran02ew
    • CommentTimeJul 7th 2012
     
    I have produced a passive standard insulated slab detail with 9 insulated piles allowing the hole slab to be completely non ground bearing so completly insulated.
    • CommentAuthoran02ew
    • CommentTimeJul 7th 2012
     
    Ok hope this works,
      detail pads pilars and founds 1.20.jpg
    • CommentAuthoran02ew
    • CommentTimeJul 7th 2012
     
    yes, it worked
    • CommentAuthorShevek
    • CommentTimeJul 7th 2012 edited
     
    Thanks.

    Oh and the other thing, it would take quite an effort to insulate around those 1m square 2m deep piles. Another reason to wonder whether it's worth insulating under the slab?
    • CommentAuthorTimSmall
    • CommentTimeJul 7th 2012
     
    Posted By: ShevekThanks.

    Oh and the other thing, it would take quite an effort to insulate around those 1m square 2m deep piles. Another reason to wonder whether it's worth insulating under the slab?


    "claymaster" is low density EPS. Dunno what the insulation value is, but you have a bit of that included in the design already. You wouldn't need to insulate the whole depth of the pillars. Maybe do as an02ew, and use block pillars instead for the top meter (with insulation around the edges of it)? You could then use AAC blocks which have some insulation value.
    • CommentAuthorTimSmall
    • CommentTimeJul 7th 2012
     
    Posted By: an02ewOk hope this works,
      http:///forum114/extensions/InlineImages/image.php?AttachmentID=3159" alt="detail pads pilars and founds 1.20.jpg" >


    Very interesting detail, thanks an02ew - spurred on by one of fostertom's posts in another thread, I was looking at something similar, but using pre-stressed concrete lintels instead of the cast ring beam.

    I came to the conclusion that since the roof and walls brace the structure, and that all the significant lateral forces are inwards - butting the lintels up to the floor slab would be sufficient restrain them. This is the only slightly hand-wavy bit of my design I think. I'm building onto chalk with a shallow soil on top so no heave problems, and my slab is ground-borne, so relatively small lintels are sufficient, but I'm wondering if some lateral restraint in the lintels (beyond bedding into strong mortar) might be necessary - however bit tricky to know how to do that.
    • CommentAuthorShevek
    • CommentTimeJul 8th 2012 edited
     
    So you all think it's worth insulating under the slab, including the piles? I'll have to talk to my engineer about the piles. I'd need to damp proof them too, or do some form of what an02ew has done.

    What about where we connect to the existing house and create a new ground bearing slab? The insulation under the raft slab would need to stop because it would hit the existing external wall below ground. Seems we're going to end up with cold-bridges no matter what we do.

    Wouldn't we be better to accept the cold-bridge around the perimeter of the extension and control it by using an aircrete block to the first course? That way we can use the same insulation approach throughout the new extension and the existing house, and form an upstand of insulation around the entire perimeter. (playing devils advocate here a little, I'd rather insulate under the slab as well)

    Edit: just adding structural engineer's current floor plan.
      ground-plan.png
    • CommentAuthordjh
    • CommentTimeJul 9th 2012
     
    Posted By: ShevekSeems we're going to end up with cold-bridges no matter what we do.

    Since you're aiming for Passivhaus, who's doing your PHPP model and what does s/he say? It seems to me that their opinion is worth at least as much as the structural engineer's.
    • CommentAuthorShevek
    • CommentTimeJul 9th 2012
     
    Yeah you're right, I'm best to get their direction on this one. I'm using Green Building Store. I need to draw something to send them so I'll go with insulation on top for now and see what they say. Thanks.
    • CommentAuthoran02ew
    • CommentTimeJul 9th 2012
     
    If your going for an EWI system then i would think the only way to get a proper continuity of insulation would be under slab and connect to EWI? check with your PHPP model,

    IF under slab insulation then the slab would non ground bearing but held up on piles as many as engineer calculates, so light wieght timber super structure helps this method.

    Connection of slab to existing house need not be so he heavey as miniumum load can be expected hear, cold bridge reduced by insluation exteding under slab and as far out to boundry and beyond if possible:smile:

    If you could convice neighbours to EWI there houses either side of you this would massivly reduce cold bridge from abutment of extension to house
    • CommentAuthorShevek
    • CommentTimeJul 10th 2012 edited
     
    I managed to get some preliminary advice off our PHPP man and he confers. We're going to end up with a cold-bridge somewhere but wrapping the insulation under is probably the best approach if not for anything else than the massive thermal store that the piles represent.

    Spoke to engineer and no probs at his end either. In fact he may have a few tricks to minimise or eliminate the cold bridge where it meets the existing building line below ground level too.
    • CommentAuthorShevek
    • CommentTimeJul 10th 2012 edited
     
    One thought: if settlement is such a high risk for us (because a massive poplar tree 15 m away) then if we insulate under the slab aren't we at risk of having that insulation settle away from the slab? How big of a problem would this be thermally?
    • CommentAuthorTimSmall
    • CommentTimeJul 10th 2012 edited
     
    Posted By: ShevekOne thought: if settlement is such a high risk for us (because a massive poplar tree 15 m away) then if we insulate under the slab aren't we at risk of having that insulation settle away from the slab? How big of a problem would this be thermally?


    If your slab isn't ground borne, then you could perhaps stick down 100mm (more?) of claymaster spongy type EPS under your conventional EPS. The claymaster will compress somewhat under the weight of the poured slab, and if the ground drops by 5mm at a later date, it should then expand to fill this. Maybe? It should also "soak up" any subsequent heave too...

    I've got a load of claymaster sitting at the end of my garden which I bought by mistake (cheap from Seconds and Co' "it's EPS, we don't have any other info on it" they said - so I thought, the worst it was likely to be was EPS70. Wrong. grrr).
    • CommentAuthorShevek
    • CommentTimeJul 10th 2012
     
    Cool idea but someone in my office is telling me it doesn't work like this; that it will collapse but won't expand again?
    • CommentAuthoran02ew
    • CommentTimeJul 10th 2012
     
    Maybe shrinkage from tree roots will be localized to the area of the root? so wont effect the large 8x4 sheet of EPS, also use my idea about laminating the insulation together with good staggered joints and more DM Easy adhesive that way the under slab insulation will act as one piece, even less chance of dropping.

    IF the insulation does drop it wll create a gap as it joins to the EWI with careful planing that joint could be made maintainable or at least visable and then repaired?
    • CommentAuthorShevek
    • CommentTimeJul 10th 2012 edited
     
    Okay, thanks, so I've updated my section with the insulation wrapped under the slab.

    How do you form EPS/XPS insulation around a slab thickening with an angle? Do you mitre the insulation or do you just not form an angle with the concrete and square off the thickening instead?

    And what do you reckon about the DPM where the raft slab meets the ground bearing slab? See attached sketch.

    https://dl.dropbox.com/s/qtzvf04w44qcdwm/section-through-slabs.pdf?dl=1
    • CommentAuthoran02ew
    • CommentTimeJul 11th 2012
     
    ok, check with engineer but i,m not totally convinced that the slope is needed? if not then just square it off, much much easier.

    Remember that all footings, slab or raft what ever you want to call it are non ground bearing once on EPS70 there are much stronger EPS that could tack the loads of biulding but they come at a price, that is why i encorporated small support piers every so often (designed by engineer) and suspended slab between these.

    DPM runs under eps and then up walls
    • CommentAuthorShevek
    • CommentTimeJul 11th 2012 edited
     
    Thanks. Can the DPM really be run under the insulation to the raft slab? Wouldn't it go above insulation for the raft slab?

    And what do you reckon about the piles? I don't suppose there's any such thing as insulating underneath the bottom of them. They're taking a lot of load and surely need to fully bear on the ground.

    I guess if we insulate the sides 2.5 m down they'll still act as thermal stores?
    • CommentAuthorShevek
    • CommentTimeJul 12th 2012 edited
     
    Here's a section through my footing and a plan view of the footing insulation arrangement.

    How do you actually arrange this stuff? What form does it come in? Do you then cut it up?

    Is my footing section right like that?

    The plan arrangement looks a bit congested due to the big piles in the middle.
      footing.png
      plan.png
  21.  
    Posted By: an02ewOk hope this works,


    an02ew, I have been working on a similar foundation scheme, with 200mm douglas fir posts on concrete pillars.

    The thing that I can't work out is how to get the DPM continuous where the concrete posts are?

    Is there a simple solution that I'm missing?
  22.  
    I am new to placing discussions
    Nice illustrations - how do you attach a PDF? In text format or HTML
    • CommentAuthorGavin_A
    • CommentTimeOct 20th 2012
     
    look below the smilies, click on the attachments button, chose file to attach etc.
    • CommentAuthorTimber
    • CommentTimeOct 20th 2012
     
    I don't have much to add to this regarding the foundations, but might I suggest that you consider fire risks and boundary distances? Timber cladding with EPS on the outside of a timber frame is a bit risky IMO and isn't something that I would build personally - especially if the boundaries are close.

    I don't want that to seem negetive, so sorry! Like the ideas so far though!

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