Redbeard

Thanks for the list @Vijay. My biggest concern is the anxiety and irritation that it's causing you. That may govern how much you try to speed it (them) up.

Appreciating that this is an evolving issue, do you wish to share the list of 'works set out above'?

Not an awful idea but edge restraint is critical, and that restraint must hold the sub-base and the grit-sand, not just the blocks, otherwise the sand just runs out and the blocks droop.

If you want to dot 'i's and cross 't's it may be worth asking Celotex (or the tool on Celotex's web-site) to do an interstitial condensation risk analysis (or someone on here who uses Ubakus - I am sorry, I don't) of that lay-up. If I am understanding it right you are describing a 'hybrid warm roof', with some insulation between the rafters and some above (with membrane, counter-battens and slate battens above that). The 'probably-get-it-right-without-doing calcs' rule of thumb is that two-thirds of the insulation value should be *above* the rafters and one-third between. You have the opposite, if I understood right. The theory is that, if you have it the wrong way round, there is a potential risk of interstitial condensation at the 'interface', because the 100mm is doing its job of keeping the heat away from the interface. Do it the other way, 50mm between and 100 above, and enough heat escapes thro the 50mm to keep the interface above the interstitial condensation threshold. That's the 'rule of thumb'. Much better is a calc.

...or a nice place to visit...

I don't know, but I want that word!

I'm not sure what the permanent solution would be. Permanent removal of the soil on the neighbour's land would require construction of another retaining wall at right-angles (and it'd need to be a hell of a lot more chunky than those slabs to meet Regs) and it would have exposed their footing wall. Subject to a structural assessment of that exposure, back-filling with pea-gravel might help the 'moisture management' but, like any French drain it becomes a maintenance liability in terms of potential 'silting'. This can be mitigated by layers of horticultural membrane, though. To get a really good idea of what is going on you really need to see in your neighbour's sub-floor void. If their 'arrangement' (solum - 'soil' under the suspended floor - about 300mm below joists, AFAICS) is the same as yours then their solum is against your back-room wall because of the height of the 'step' (the flagstone 'retainers'), so your chances of that bit of wall being, and staying, dry are poor. Interim solution for the rotten joist is simply not to 'pocket' it in the wall. Build a 'footing' out of your solum and sit the joist (protected by a DPC) on that.

I think your theory that the damp in the corner is being 'transmitted' via the neighbour's masonry is a good one. All of the 'solutions' which come to mind immediately are compromises rather than true solutions. I shall ponder a bit more.

Assuming your SE's spec and dwgs were submitted to BC at application stage then what was drawn and spec'd is what BCO will expect to see, not a not-as-specified alternative enacted by the builder. Is the builder in possession of the SE spec and dwgs? If not what went wrong and (come to think of it) if yes, what went wrong? Unless you can get your SE to say it's OK. (Which, given that... "Our structural engineer is not happy and says he will not stand behind it." does not seem likely) you arguably have to call a halt to all works forthwith until the builder has either re-done as per SE spec or come up with a proposal which, though different from SE's spec, will satisfy that SE and *get you BC sign-off*. (Arguably contractor should be liable for extra SE costs if incurred, too). All of this is a hell of a lot easier to say than to get done, and can be very much depend om personalities, so I wish you very good luck.

I have one very like that, and the price was exactly that. The quality is awful, but I expected that for £30! The thread was 'cut' (cast??) so badly that the paddle, when fitted, had about 30-50mm 'slop' at the 'business end (got round that by using the old but higher-quality paddle off my old mixer). The cable-entry 'grommet' is glued only (or un-glued now) - It does not have a groove, so now it's off there is little to stop strain on the elec connections. When it finally dies it will be either that or a burnt-out motor, whichever comes first. I only bought such a cheap one as I am no longer contracting and thought my use would be very sporadic. If it goes wrong it entirely serves me right for being a skinflint. It *will* mix concrete, but it doesn't like it, and it's a big strain on your back if your back feels strain. 1 jumbo bucket, 2/3 full, fine. 2, you're getting tired and irritated. 3 or more, or a more regular requirement to mix concrete, buy a second-hand Belle. (And yes, the bucket will spin, so you also have to use your ankles to 'pin' the bucket, hence not something to do regularly!

I think we need to know more about this before we express any opinion as to whether 25mm will make any significant difference. What is the width of the cavity? (So that we can work out, at least roughly, what the U value might have been intended to be.) What went wrong? You say full fill, so there is no *intentional* ventilation to the cavity. What about 'unintentional'?

"4) Outbuildings are not permitted development forward of the principal elevation of the original house. The term original house means the house as it was first built or as it stood on 1 July 1948 (if it was built before that date)" Castle-style porch?

So the builder has left a 100mm ventilation gap?! Guess you don't need to worry too much about the ventilation, provided he has built adequate 'ins' and 'outs'. Was the instruction to the builder clear? "Build me an extension which meets Bldg Regs ", for example? If so.... I like to have something to at least 'cloak' the thermal bridge of the rafters, so how about pull down the ceiling, add 50mm between and 25mm below the rafters (U val probably less than 0.13W/m2K compared with Bldg Regs target of 0.16. Then you know you have surpassed the Regs req't. Then you ask the builder to pay for re-boarding and plastering. By the way, It definitely isn't. In come circs the BCO might agree to accept it, but that just means you are getting away with more heat-loss than you should have. Not good.

Go for your own TP. Effectively you have already told us why you need to. One fewer thing to bite you in the future.

Thanks ST, I knew you'd know. I stand corrected. I somehow had in mind that XPS was OK. The rest stands, though!

Make the shape in XPS and lay up GRP over it, with whatever 'non-slipperiness' you want to incorporate? It works for boats (and indeed shower trays).

Strongly consider natural materials, such as wood-fibre or a proper insulating plaster like Diathonite - goes on very thick, usually sprayed! Remember that your stone building was probably built with materials - stone, lime mortar and lime plaster, which can absorb and 'breathe out' water vapour without damage. As far as possible you want to make the next layers in the 'sandwich' behave in the same way. Look into air-tightness with regard to 'solid' wall insulation and don't be fobbed off by 'it's OK - it's got a vapour control layer (VCL)'. Acquaint yourself with the Building Regs and in particular how adhering to the U value targets can, in some circs with internal insulation, cause 'interstitial condensation' - condensation on the old surface of the original wall behind the insulation. I'd suggest not getting talked into plastic insulation before you have had a very good look at all the pros and cons. Of *course* it can work, but knowing all the advantages, disadvantages and practicalities in advance gives you the best chance of making the right decision. Think v carefully when/if insulating suspended timber floors. It *can* work, but it can go wrong too. If you cannot get the ventilation (etc.) conditions right you might consider a solid floor, either with 'modern' (plastic) materials or with something self-draining like foamed glass aggregate.

Even if the drainage is functioning OK is there anything about the 'lie of the land' which is directing water towards those walls? (In one case the damp only seems to come in just inside the wall). Can it be 'encouraged away' by 'softening', re-directing, French drain? You might also consider this as bed-time reading (go to bed very early, though!): https://assets.publishing.service.gov.uk/media/5f05d211d3bf7f2be6e0217a/suspended-timber-floors-underfloor-insulation-best-practice.pdf If you can tick all the 'boxes' in that guide you are probably a long way towards being OK.

What you describe is a hybrid Warm Roof - some insulation between the rafters - not really a Warm Roof - and some over - the 'Warm' bit, tucking up the rafters in warm stuff. However, when you do a hybrid you need to ensure that there is not a risk of interstitial condensation at the interface between the 'between rafters' insulation and the 'above rafter' insulation. The 'rule of thumb' sometimes used is to have two-thirds of the R value above the rafters and one-third between, so it is not 'cold' enough to allow condensation at the interface. (A proper condensation risk analysis is better than a rule of thumb). Even if you could 'engineer' your 50mm uplift vis a vis your neighbour's roof that would mean only 25mm between (the one-third to the two-thirds represented by your 50mm). 75mm, even of PIR, may give you a U bvalue of around 0.3W/m2K, about half as good as the Building Regs U value target of 0.16W/m2K.

If it's to a SE's spec, usually on pre-cast concrete 'padstones' too, not just the brick.

Difficult to answer without knowing the SIPs designer's rationale/strategy, but yes, the bottom one could be the combined a/t and vapour-check plane. Although of course a/t layers can be external...

I have not come across @JohnMo's solution before, but there would normally in any case be a non-return valve performing this function. They can fail though or, as you describe (to my surprise), be left out!

There is no mention of insulation. Is that simply because you have not 'got there yet', or because you do not intend to include any? If you are planning to insulate the walls then your strategy for the 'damp' (have you established that it is definitely rising or penetrating damp, or could it be condensation?) needs to take that into account. If, for example, you used your 'liquid applied dpm' to 'dry' the wall, and then insulated internally, you would effectively have a vapour control layer (VCL) in the wrong place in the 'sandwich'. If you were not planning to insulate the walls please let us persuade you to do so!

Is the OSB your air-tightness layer, or is that dealt with by a separate membrane? If the former, yes, tape. If the latter, maybe not. (Unless you are a serious pessimist and decide to do both!)

I regret not, but there are compressed air staplers starting at under £50 in places like 'ToolFixstation'. Of course you'd need to buy or hire a compressor but they exist to buy at £100+, and presumably might be useful to you in the next stages of the job.