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Posted By: dovemwOption 1 means removing about 800tonnes of clay from our restricted access site so is really a non starter and more expensive than piling.Oh well. We had to dig to 2 m at one end but only about 1.5 m at the other end (trees).
So we are where we expected with some form of piled foundation with insulation below the slab and possibly above with and option of using insulating connectors at the pile caps (Schoeck Sconnex).I'm not an expert but looking at p41 of https://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/How%20to%20Build%202023/HowToBuild-Ch7.pdf it suggests putting a ringbeam on top of the piles and then an insulated raft above that. It sounds like you have more sand than clay, but is there any possibility of heave? What is the bearing capacity of the ground?
We now need to include the piles into the PHPP model and we are trying to work out what the effect of the piles is on a slab with insulation below and do we need the thermal break devices at the pile cap or additional insulation above the slab. At the moment we are planning to model the thermal bridging of these 3 options to decide which way to go but the modelling isn't cheap
Posted By: dovemwWe are on a sandy clay so there is a medium heave potential but the quality of the sandy clay is variable across the siteApologies if I'm telling you things you already know. (1) The heave potential of clay depends on the particular type of clay, so you need to check what type you have. (2) A timber frame house tends to be a lot lighter than the average build, and a passive slab foundation is very good at spreading the load evenly over the whole area. So it's worth checking the numbers in detail since the engineers may be more used to 'conventional' heavy construction and make recommendations on that basis.
Posted By: dovemwWe have considered a ring beam and then an insulated slab over it but the ring beam itself is expensive (current conctete and steel prices) and ends up being a double up of structure. I am waiting on more prices but it looks like the insulated pile caps under the slab should be cheaper than the ring beam option.
Posted By: Peter_in_HungaryIMO it would be best if the piles and slab was treated as one without the complication of thermal breaks or alike between them. Piles and slab as one would be a 'standard' job for a SE without unknown or infrequently used features then with the slab design done you can get your PH designer / architect to do their thing on the slab.If you did this then the entire slab would need to be below the insulation, since it's thermally connected to the ground, so you would need insulation on top, followed by another slab. IMHO it's a very poor idea to get non-PH trained engineers to design the foundations and then try to patch up the design afterwards.
Another option would be to timber frame the whole lot with timber beams on the plies, probably engineered timber beams but e.g. green oak would also be an option. This is doable but not an option I would use, may not be much cheaper than concrete and almost certainly would have consumer resistance if the property ever needed to be sold.I looked at this option for our build. I didn't use it for two reasons (1) cost, and (2) depth of the structure combined with rules about keeping timber 6" above ground whilst achieving level access to the house. I wouldn't consider the green oak route under any circumstances myself.
Posted By: djhIMHO it's a very poor idea to get non-PH trained engineers to design the foundations and then try to patch up the design afterwards.
Posted By: djhIf you did this then the entire slab would need to be below the insulation, since it's thermally connected to the ground, so you would need insulation on top, followed by another slab.
Posted By: djhIMHO it's a very poor idea to get non-PH trained engineers to design the foundations and then try to patch up the design afterwards.
Posted By: Doubting_ThomasOn that note, have you tried Build Collective?Looks like a promising suggestion to me at first sight.
One trick I learned for finding architects, which might also work for engineers, is to approach one and agree to pay them to listen to/look at your design ideas and plans as far as you have them, and then for them to recommend say three other architects for you to approach to do the work.
Posted By: Peter_in_HungaryYes the entire slab would be below the insulation. Above the insulation would be no more than what you put down when you have insulation then UFH then floor finish.Given that the OP already rejected a ring beam on the piles supporting a separate raft above on grounds of cost, I suspect that a raft on the piles supporting a second slab above would also be rejected on cost grounds a fortiori. The slab would have to be strong enough to support the walls etc; it's not a simple screed.
Posted By: Peter_in_HungaryI don't see the need for a second slab if the first is a cast in one slab on the pilesThis is a PH construction, and thermal bridges matter.
Posted By: djhPosted By: Peter_in_HungaryI don't see the need for a second slab if the first is a cast in one slab on the pilesThis is a PH construction, and thermal bridges matter.
Posted By: Peter_in_HungaryI think we might be talking at cross purposes here.I don't think we're at cross purposes.
If the structural support is on the piles, both for walls and floor then the insulation is put on top of this raft / slab or what you want to call itI think this may be where there's a difference. How do you propose supporting the walls on the insulation above the raft?
Posted By: djhA raft is a concrete slab with enough reinforcement to be supported only at points - a typical use is over land over mineshafts, where there might be holes appear. Another use is a raft supported by piles.
Posted By: djhHow do you propose supporting the walls on the insulation above the raft?
Posted By: Peter_in_HungaryFor the walls the OP is intending to use straw bales wallsWhere do you get that idea? The OP said " a new self build PH standard timber frame project".
which I would have thought enough insulation without additional.I live in a straw bale PH, so I do know of what I speak, and no it isn't sufficient.
However If more or different insulation was needed between the bales and the raft (e.g. to prevent any damp from water ingress from seepage at the raft) then I would use 300 grade EPSIndeed but AFAIK EPS300 doesn't provide a suitably stable surface to build on top of. That's why we don't have it, for example, but have a concrete ring beam on top of EPS300.
Posted By: WillInAberdeenEvery 10mm that can be shaved off the slab helps, how thin can it go?Yes, my slab/raft is a good example of that. It varies quite a lot in thickness with a moderately complex design. In reality it varies in units of 100 mm since that's the thickness of the EPS used. It gets too complicated to think about different thicknesses.
Can it be thinner in lower-load places than others?
What would it mean if there were more piles spaced closer together, so the slab doesn't have to span so wide and is thinner, or vv with fewer piles?That is what you pay the engineer to determine!
Glass wool above the slab, instead of EPS belowCan you get glass wool in a 300 rating? Or even a 100 rating?
Can it really really not use timber :-) ?One big problem when I looked at it was cost, that may have changed, I don't know. A lot depends on the particular site; slopes make it easier; on our fairly level site another major constraint was keeping the ridge height down (we're next to a listed building) and adding 18" or so for the ground floor wouldn't have helped.
Ground level can be gently mounded beside 15% of the perimeter of a timber floor (iirc) to give landscaped level access, doesn't have to be an ugly ramp.
Posted By: djhPosted By: Peter_in_HungaryFor the walls the OP is intending to use straw bales wallsWhere do you get that idea? The OP said " a new self build PH standard timber frame project".
Posted By: djhHowever If more or different insulation was needed between the bales and the raft (e.g. to prevent any damp from water ingress from seepage at the raft) then I would use 300 grade EPSIndeed but AFAIK EPS300 doesn't provide a suitably stable surface to build on top of. That's why we don't have it, for example, but have a concrete ring beam on top of EPS300.
Posted By: Peter_in_Hungaryhttp://www.viking-house.ie/passive-house-foundations.htmlYes, I suggest you go back and read the whole thread again. We've already discussed all this. And yes, my house does have exactly that type of solution, but it is apparently too costly for the present situation.
shows a lot of PH designs, interestingly all without footings, just compacted hardcore i.e. all slab or raft construction with ring beams including the comment
" Viking Raft Slab and Viking Waffle Slab are suitable for soft ground conditions, the weight of the house is transferred throughout the slab, so we have solutions for most situations."
Depending upon the ground conditions perhaps a raft without piles might be an option. But that would be one for the soil engineer. I don't know if the SE has looked at a rafted solution (The OP just stated "So we are where we expected with some form of piled foundation)" Often I have found tech. people go with what they usually use and it is not until something else is suggested that they comment on that other feasibility.
Back to the original issue would the cost of and work involved in protecting the under raft insulation and the thermally broken piles be more arduous than above raft insulation even if another ring beam were needed. That would need costings.Indeed, which is why the OP is looking for suggestions for suitable engineers. I suggested Hilliard Tanner and Tom suggested Build Collective. I don't know whether the OP has contacted either.
