andyscotland

This is a very good point. I'd assumed as this is indoors then the source of the leak has been identified & fixed but you're right OP doesn't specifically say so.

I wouldn't necessarily expect corrosion/damp within the cable sheath to cause an issue of itself - the sleeving on the phase and neutral cores should in theory be enough to insulate them from the CPC and any contamination around it. However it's possible in practice that the inner insulation is degraded or cracked (especially if the cable has tight bends and/or is old) which has allowed liquid or salts to migrate into the gap and reduce the resistance. It's also possible the hydrochloric acid has damaged it - in theory I think that's compatible with pvc but I think it depends on the exact makeup of the plastic. I'd certainly agree that I'd start by suspecting this cable if I was fault-finding, unless I could prove it was fine. Yeah in that case I'd ignore that, it's a very acceptable result for a complete energised installation. Yep - faults like this will generally require a higher voltage to overcome the initial resistance of the fault. An insulation resistance test needs to be done at 250/500/1000V to give meaningful results (voltage depending on the characteristics of the circuit and whether any sensitive devices are connected).

If it's on the ceiling and relatively sheltered you might be ok, though I'd leave it as short a time as possible. If you don't need access through the front during that time, could you temporarily board up the opening with some OSB or similar? A couple of sheets would be fairly cheap compared to the cost of redoing the ceiling. If you do need access, maybe you could still put some boarding/protection as a "skirt" across the top section of the opening to prevent wind-driven rain etc from getting to the front edge of the plasterboard.

Personally I wouldn't spend too long chasing that - unless you confirm that it's entirely from the suspect circuit and there are no appliances of any kind connected (including sockets with built-in usb chargers etc). That sort of leakage would be quite normal once you have a few electronic circuits connected as they often intentionally leak to earth as part of electromagnetic interference protection circuits etc. It's nowhere near the 30mA that would be required to trip the RCD and well within the 30%-of-tripping-current safe limit whhich would be 9mA.

As @Mattg4321 says you really need to test the resistance between cores. Your continuity tests have proved that the copper is not broken/damaged. However that doesn't tell you anything about whether there's a section of insulation either damaged (e.g. by rodents) or bridged (e.g. by water). If you're only testing continuity of each core separately then the other cores are not connected to the test circuit, so even if there was a dead short somewhere the test current will flow entirely within the core you're testing and give you the continuity result you expect.

My wife has 🤣

Thanks @saveasteading. I wasn't so much concerned about heat loss as condensation risk at the bottom of the timber. But fair to say I've learned quite a lot in the subsequent 6 years! My experiment is going well so far, will just have to keep fingers crossed that it continues to do so...

I went with @PeterW's plan of laying a thinner section of PIR with 400mm rip of 18mm OSB on top to spread the load. I fixed the OSB down into the slab with hammer in fixings, with mastic around where they pass through the DPM. Then laid the VCL continuous across the whole floor (above the PIR, but the PIR is on the warm side of the insulation) and then screwed the sole plates down into the OSB. Can't comment on BC as I am not yet complete 🤣 and they've not been out to do any inspections as it's a "small" project. However I can say that the stud walls have now been there for 5 years 🤦‍♂️ and no sign of movement or issues.

I may be wrong, but I *think* it's essentially the same product, made by staff who originally worked on CT1 and then split off to do their own thing after the company was sold or something like that. I've used it quite a bit and it's worked well.

Absolutely, about all you can do at this point. Was more just noting for anyone that encounters the thread in future if they've not yet had work done.

Yep. Indeed - although, technically, an EICR is not as in-depth as the EIC that should be done for significant works and in particular doesn't involve anyone actually certifying that the work is safe and compliant. Generally speaking, yes - much more so than in the past - but not in every case. There are still faults that can exist and leave the installation in a potentially dangerous condition without immediately tripping a breaker. Well indeed, it's always sensible to assess the install for selfish reasons before you end up being responsible for fixing things you didn't break 🤣

IMO, you need one to know that the work was done properly by someone competent to do it. It should be a fundamental part of a board change to do the inspection & testing required to know it's connected up correctly, protective devices will function as expected, and the resultant installation is within safe parameters. If the installer has done those checks, why wouldn't they document them in the form required by BS7671? If they have not, then what other corners have they cut? Note: this is not about whether you get a NICEIC/ECA/Part P/whatever cert - I have mixed feelings about the value of the various pay-to-play registration schemes, and it's reasonable to avoid them if you are understand what you're getting/not getting/potentially saving by going that route. But the basic form of certificate is available free of charge and can be issued by any electrician and therefore should be.

In terms of a building control certificate, that depends on where you are in the UK. And as @nod says there will be sparks who are capable of doing a decent job but wouldn't for example be able to issue a Part P certificate (for English building control) due to not having paid the money/kept all the many things in date to be on the Part P register. However, you absolutely should have had an Electrical Installation Certificate with accompanying schedule of test results. There's no specific restriction on anyone issuing those - if you're competent to do the work you're competent to issue the cert. That is a fundamental requirement of BS7671 (644.1) and I would be concerned about anyone who did this kind of work without meeting that requirement.

All air contains moisture(*). What matters is the combination of how much moisture there is and how hot the air is. The warmer the air, the more moisture it can carry. If slightly warmer air is next to slightly colder air, it's easier for the moisture to stay as moisture and move to the warmer air, so everything stays in balance. Think of it like a big wedge made of toilet paper, the "cold" end is a single sheet thick, the "warm" end is several sheets thick. If you slowly pour water on the thin end, the paper will start to soak it up until it is saturated. If you pour more water on, the paper will draw it across to the thicker pile (even though that means the water is flowing upwards!). Only when all the paper is saturated will you start to see liquid water lying on top/overflowing the edges. But if slightly warmer air suddenly touches something cold, the air right next to the cold thing will be much colder and the moisture will have to condense into water. (Here imagine you rapidly press down with your finger on a wet bit of the thick toilet paper, so it is suddenly only the thickness of the "cold" end. If there is still some dry paper, you'll see it get wet. If all the paper is wet you'll see water leaking out) The moisture in the air under your house will mostly come from three places: the outside air, evaporation from the solum, and warm moist air leaking from your house. The outside air will be cold but by definition not be saturated (unless you are literally inside a rain cloud). So the air will be able to absorb some of the house moisture even as the house air cools. The solum (generally the coldest surface) then acts as a kind of buffer. If the air under the floor gets saturated (because it is cold and/or humid outside, or the house is humid inside), water will condense onto the solum and soak in making it damp. Nothing will evaporate from the solum because there is nowhere for it to go. But then on days when it is warmer/drier/you've had the windows open in the house, the air will not be saturated so the moisture in the solum will be able to evaporate. So - assuming your ventilation etc is as it should be - over time and across the seasons you will get a variation in dampness down there but it will balance itself out. * Well, not air in deserts or up mountains, but... If you use foil just below the wool, it will do very little because moisture can still get round it and will go to the places that are weakest. If you use foil across the whole loft (under the wool and over the joists) it will trap moisture at the top of the joists where they are cold and that will cause problems. You either need a fully sealed and continuous vapour barrier below the whole loft construction (e.g. between joists and ceiling) or you need to design the construction to breathe and then ventilate the loft, which is the standard detail for loft insulation in existing construction.

Condensation in buildings varies significantly over time depending on outside and inside environmental conditions (temperature & humidity) and the difference between them - and in this case also the temperature of the outside water supply and the amount of time that water is flowing/not flowing, since that will control the surface temp of the pipe. Small differences in any of those factors can be enough to make condensation appear or not at any particular moment. I'd be surprised if you'd get enough evidence over a few days/weeks to understand the long-term performance of that junction. Whereas there's plenty of evidence that the usual pipe lagging solution will work. Because the mineral wool is breathable, allowing a temperature and humidity gradient across it, so the condensation doesn't form at that interface. Big condensation issues in buildings are almost always caused by constructions that trap moist air in the wrong places.