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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    Hi all,

    We have just started clearing our site and have run into a problem which may spell the end for our planned insulated slab foundation due to additional cost.

    The site has a lot of topsoil on it, approx 1.7m at the deepest point. Removing it is not a massive problem, but the level of the house cannot be dropped due to the need to get light into the interior over neighbouring buildings. Piling or pads have been suggested with ground beams and then the insulated slab on top. This would be the best suggestion, but I think it will be prohibitively expensive - basically like paying for 2 sets of foundations.

    Our architect is of the opinion that we should remove the soil, dig strip foundations and form a block basement. The site is dry and free draining sand, so we think full waterproof concrete would be overkill. We would not be able to complete the basement at this point in time, but would like to have the option at some point in the future.

    Basically, if we are going to have extra expense at this point, we would rather try and get a basement out of it. I am currently thinking of insulation to the base of the strip founds and up to ground level topped with a cast slab. Would this be possible to design in such a way as to mitigate thermal bridging to the basement below?

    Any suggestions for a cost-effective way around this problem, much appreciated.

    • CommentAuthorgyrogear
    • CommentTimeMar 16th 2016
    Posted By: divorcingjackWould this be possible to design in such a way as to mitigate thermal bridging to the basement below?

    No doubt - sure it can be done. EWI to basement wall, carried right on up the house walls also.

    However, my concern would be to insulate the basement slab also ! And put UFH in it - you will only get one chance to do that... So the basement is inside the thermal envelope - whether or not you (or future owners)choose to actually heat it... (which was not done in my house...)

    Ah, well there's a slight problem about continuing the insulation up - it's a twin wall timber frame, so the insulation is between the studs, not on the outside.

    This is the complexity, trying to join together 2 different building methods. My architect suggested just building the basement walls in blockwork and making the timber frame taller, so it would effectively be one structure with continuity of insulation. It would sit on the warm side of the blockwork wall.

    In fact, if we did that, would we need the external insulation to the blockwork? Would we just put the insulated slab at the bottom of the basement and have a suspended timber floor to the main living area?

    Have you thought what you would be using your basement for? If you see it as a living space then you'd want to look at bringing it within the thermal envelope. If it's for storage (and perhaps a nice root cellar and home for a chest freezer) then would it matter if it's outside the PH envelope?

    If you choose the latter, remember that the basement willl still be warmer than, say, an out building because it will be heavily insulated above and will have earth all around its walls.
    • CommentTimeMar 16th 2016 edited
    If I understand your proposal correctly, your architect is suggesting building the timber frame inside a blockwork 'bucket'? You'd need to tank the blockwork to keep water out, and then would you also be at risk of condensation on the inside of the blockwork and/or outside of the timber frame?

    If you build an insulated slab at the bottom of the hole and then externally insulated blockwork for the basement, I feel sure you could then plant a timber frame house on top, including a suspended timber ground floor. The connection between the blockwork and timber would need a bit of thought but I'm sure it's not rocket science. A bit of external insulation carried up the outside of the timber frame for a little way seems to answer most similar problems.

    edit: PS crossed with Pile-o-Stone; he makes an important point - you do have to decide now whether you want the basement to be 20°C or 10°C.
    • CommentTimeMar 16th 2016
    Posted By: divorcingjackThe site is dry and free draining sand, so we think full waterproof concrete would be overkill.

    Oh and who is 'we'? Is it the structural engineer who will be responsible for signing off this foundation/basement? If there's any risk of the water level ever rising, I suppose it will need to be strong enough to resist it, despite normally being very dry.
    If we manage to do it, the living room will eventually be living space for elderly parents, so we would need it inside the thermal envelope.

    djh - to be honest, there's been a lot of kicking around of ideas. We had a reasonably hideous quote for an insulated slab (34K!) and the news that there will be additional cost on top of this has not been a happy time. So, we're trying to get the best possible value out of the extra money that we have to spend.

    I think the architect was thinking that it may be the easiest way to maintain continuous insulation and airtightness - we would look at either spray-on tanking or an external drain/membrane solution (cost dependant). However, if you think it's possible to manage the joint efficiently with little thermal bridging, that might be the way to go.

    It's a bit of a kick in the teeth - they're not joking when they say getting out of the ground is the most expensive bit!
    "we" - architect, groundwork builder and myself/husband (no technical knowledge there beyond what we've read)

    We are emailing back and forth with the SE, so haven't received the latest suggestions from him yet. The last one was pads/piles with ground beams and then the insulated slab on top, but that just looks like money with not much return as opposed to possibly making the basement work.
    • CommentTimeMar 16th 2016
    I'm at home now. The Passivhaus Details book has several details of a timber frame on top of a basement so it's definitely possible. Admittedly the details in the book use a concrete slab as the roof of the basement rather than suspended timber; I don't know whether there's a fundamental reason for that or just Germanic preference/law. If your architect is designing to passive standard then he certainly ought to have a copy of the book.
    • CommentAuthorEd Davies
    • CommentTimeMar 16th 2016
    I've seen (and helped a bit with) a timber frame (oak post and beam then Pavatex and stuff) built on top of blockwork founds with what was then called Hanson Jetfloor, now Forterra Jetfloor:


    That would give a reasonably insulated version of your slab to build timber up from. Knowing what I know now I'd be a little doubtful of the cold bridges around the edge but that could be fixed with a bit of EWI on the blockwork.
    djh, I"ve just had a look at my copy of the details book and found that example, thank you for reminding me. Should have thought to look in there, really! The only added complexity is that we have a brick facing on some of the build, so not sure how that would work with the foundation for the outer skin and the recommended drain.

    Ed - someone else recommended Tetris, which is a similar system to the Jetfloor, but unfortunately you can't use it above an inhabited basement, due to the risk of the insulation melting in the event of a fire. Bit gutting really, as it looked really good.
    Hi Olwen, You don't need to remove 1.7m of topsoil to install a Passive Slab, you only need to strip a layer that contains the plant roots. Have you removed the soil already?
    Hi Viking,

    only just got your email and this - I've been out a customer site all day. In answer to your question, no, all we've done is cleared the site - there was some demolition and rubbish removal to be done.

    The 1.7m is all black earth, then subsoil, then medium sand. I was told that all the black earth has to go - is that not right?

    We are actually getting a quote for a block basement now, as we think it probably won't be too far off the piling/pads solution cost-wise and would give us a significant extra area that we could potentially convert to living space in the future.

    Intrigued about not having to remove the soil though ....

    Hi Olwen
    The loads in a Passive Slab are spread out at 45 degrees through the concrete, EPS and stone.
    A 50kN line load from an internal load bearing wall is reduced to a 4kN line load by the time its spread out at 45 degrees through the concrete, EPS and compacted stone, so very little load is transmitted to the soil below the stone.
    The load bearing capacity of poor ground is improved by control filling with 18-36 drainage stone installed in 50mm layers with each layer compacted with 6 runs of a 100kg plate compactor. Ground with very poor load bearing capacity is further improved with a layer of Geotextile membrane between the stone layers.
    Has your geo-technical report stated the load bearing capacity of the ground?
    • CommentAuthorDandJ
    • CommentTimeApr 19th 2016
    <blockquote><cite>Posted By: divorcingjack</cite>The 1.7m is all black earth, then subsoil, then medium sand. I was told that all the black earth has to go - is that not right?

    It depends. If your structural engineer says it has to go then I would say yes, it has to go!

    I think the general rule of thumb we use on site cost wise is that it becomes cheaper to pile when you get beyond 2m ish in the ground. I can't remember exactly but it is something like that
    • CommentTimeApr 19th 2016
    How about a semi-basement, standard Germany/Austria-style - the basement half a storey down (less heavy tanked construction), the house raised up a bit, catching more sun, better view over garden etc
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