Mike

+1. You need to identify the problem(s) and fix them. The IR thermometer should help to identify any gross air leaks that are in line-of-sight if you poke it through a down-lighter hole, but probably not not all of them due to the restricted vision. Yes, unfortunately this is likely to be necessary.

That poses an interesting question. It seems that it's actually France that has the higher heating requirement; the mean heating degree days (average figures for 2019-2024) are 2,080.81 for France and 1873.53 for the UK. That assumes that both datasets are population weighted - the UK certainly is and I'd be surprised if France wasn't - and also that the UK is still calculating as it did when it was an EU member.

A lot of that presumably due to the costs of the grid - a bigger country to connect, most of it away from the North Sea, and more rural areas too far away to connect (where main options are electricity, oil or propane).

Ignoring the saving on standing charges mentioned by @JohnMo - in fact ignoring standing charges all together - I just looked up the domestic rates here in France (first half of 2025, including tax): Electricity: €260/MWh Natural gas: €134 /MWh Ratio: 1.94 in favour of gas Compared to the UK (October to December 2025, North West region): Electricity: £271/MWh Natural gas: £63/MWh Ratio: 3.4 in favour of gas Which is undoubtedly a key factor in the popularity of heat pumps in France. Not to mention that gas has been banned for new developments for several years, and that replacement boilers (but not heat pumps) are hit with 20% VAT. It certainly looks that way.

It is indeed strange that you seem to be in a group of 1 with this one, but good to have your feedback and a resolution. It's unfortunate that it didn't go as expected.

It should either fall towards the unit, or towards to the wall vent - but you can also have some falling one way, the rest the other. So maybe you create a high point instead of a dip? It's only the exhaust that's susceptible to condensation, so don't worry about the intake duct.

I don't have roof vents, but yes, they need course mesh - see also this thread: You need to access them for maintenance, but that could be from inside (e.g. though a filer box) or outside. Wall vents are may be easier, but I guess you don't have that option.

You have an interesting building and some interesting challenges there! For the walls, consider lime-hemp (hempcrete), which can be applied directly to the wall in multiple layers, or cast against it using formwork. There are multiple examples of both on the internet, at least in French. Hemp handles moisture vapour very well, though not liquid liquid water, so you would need to make an assessment on the suitability and thickness. Your self-made insulating lime mortar sounds an interesting alternative. If you have lots of damp below the floor, you might want to consider laying ventilation pipes within the glass aggregate, connected to an extractor fan. If you search for hérisson ventilé you will find some examples. This may be enough to make the walls suitable for the hempcrete, if you think that it may be otherwise too damp. If the walls are still too damp for hemp, then your idea of expanded cork sheets sounds good, since cork won't be affected by moisture. I've used it myself in small areas - mainly on my chimney breasts - fixed with lime-based Isovit e-cork. You haven't mentioned how you are ventilating the building. I would suggesting using some form of continuous mechanical ventilation - dMEV / MEV / MVHR - to help control the internal relative humidity.

Not yet! Why not get some smoke pellets & test it yourself? Take a video of of any problems. And disable your smoke detectors...

Yes, that's the same 3m3/m2.h (and the same Lowe who first came up with the figure in 2000); it crops up in many places. The conclusion to that paper is interesting (though I know that it too has received some criticism): This paper has shown that previous analyses indicating that MVHR systems should only be installed in properties with an air permeability of 3 m³/m².hour @50Pa or less are based on outdated information and flawed assumptions using a modelling system which was never intended to be used to compare ventilation systems. A modern MVHR system will result in significantly lower CO2 emissions at any reasonable level of air permeability. However, an MVHR system’s primary function is ventilation and, in this capacity, it is far more effective at providing a good quality indoor air environment – regardless of external conditions – than natural ventilation. There is no clear up-to-date evidence behind the rule of thumb that says MVHR is inappropriate for dwellings with air permeability above 3m³/m².hour @50Pa. This analysis has shown that MVHR systems result in improved ventilation and lower carbon emissions for all levels of airtightness. There is a compelling case for MVHR systems to be fitted in all new dwellings and to be strongly encouraged in retrofits where significant reductions in energy demand are being targeted. It does underline, though, that the 3m3/m2.h 'guideline' is far from universally accepted and that it shouldn't be taken as definitive when deciding whether or not to install MVHR. Particularly if there are other reasons why MVHR might be desirable - to filter the incoming air or avoid external noise infiltration, for example.

Yes, the translation's not very good (it's tricky translating technical language, for AI as well as humans). The original French version of the video is here; the ones 'that consist of a simple grid' are regular trickle vents, the 'adjustable air inlets' are humidity-controlled - two different types that sound like one in the English version.

In a deep renovation that didn't warrant MVHR, I'd choose central MEV over dMEV. That is, one central fan that extracts via ducts from kitchen, utility bathrooms, WC (and potentially the hallway & elsewhere, depending on layout). The advantages include that the unit can be positioned somewhere noise-insensitive, there's only 1 exhaust duct to the outside (so no risk of a wind-tunnel effect from holes on opposite sides of the building), in refurb the disruption of installing the ducts likely isn't a problem, and a ceiling / wall vent is the only part you see in the rooms. You also have a single central control - which could be a pro or a con depending on your view point. This is where it's trickier. MEV has been common in France since rules brought in in 1982, and humidity-activated wall / window inlets were normally the go-to solution (a set-up known in France as VMC hygro B). However, now that new homes are more airtight (even those that are not class-leading), there there are reports that humidity-activated air entry is not providing enough ventilation for good indoor air quality - sometimes not enough to keep mould at bay either. Consequently some installers no longer fit them (e.g. see here on YouTube, in French). Instead, they favour permanently open trickle vents (VMC hygro A), so I'd suggest choosing that option. If not, be sure to get the CO2 sensor suggested by @JohnMo; it is, in any case, a good idea. The 3m3/m2.h is a guideline that originated from a paper published in 2000 that used SAP to consider the economics and CO2 emissions of ventilation strategies. Both the economics and the carbon intensity of electricity generation have changed considerably in the 25 years since then, SAP has been updated multiple times, and I doubt that it took into account the possibility of using heat pumps, so it's unlikely that an updated study would come up with the exact same figure today. I've seen 5m3/m2.h suggested in several places as the cut-off point, including on page 14 of this 2019 paper.

You're right not to get it - it's no different to any other underfloor heating. This has come up before:

As mentioned in another thread, text from a PassivHaus MVHR certificate says that 'In case the unit’s moisture recovery rate is larger than 60 % its airflow rate must be controlled based on the indoor air humidity, in order to prevent temporarily elevated humidity levels.' So yes, it seems that there is a potential for excess humidity in some circumstances so, to avoid problems, the unit needs to automatically switch to boost mode if humidity gets too high. If it's not doing that, I guess that there are at least 3 possibilities: it could be there's a control problem; maybe it requires (an additional / replacement) humidity sensor(s); maybe some of the exhaust air is finding its way back into the supply. +1

Yes, on the intake it'd definately easier to have one that doesn't need extra insulation. Mine was from Econology in France (site temporarily down). InovaTech in Germany sell a smaller one. So too does HomEvap, sold here in the Netherlands.

My Zehnder intake is around 10mm too - enough to stop leaves, bats & birds. I then have a filter box before the MVHR unit, with a large filter that can easily be changed, before the MVHR filter takes out anything that remains.

The deterioration of a fixed battery must have a negative impact on resale value; the economics of battery swapping might work if that is taken into account. Standardization would certainly help. The EU already has a directive (2023/1542) require that "batteries, as well as individual battery cells included in the battery pack, are readily removable and replaceable by an independent professional at any time during the lifetime of the product", so it wouldn't be a huge step to go further and specify standard battery pack sizes. A quick search also shows that the European Committee for Standardization is already drafting "Technical specifications for swappable battery system applied to L-category vehicles"

Or get a car where you can swap the batteries about as quickly as filling one with fuel, when they reach the UK: https://www.youtube.com/watch?v=WOsy_EvtHr4

I'd be with you for at least the fade-up feature, given a suitable controller to automate it. I hate dark days too, so if the alternative was a fixed CCT of 3,000 K or less, then this would be useful. However, with my personal preference for 4,000K I never have a desire to close the curtains before dusk. I can see the utility of this for some people. Though, as a night-owl, it wouldn't suit me at all, at least not until the last few minutes of the day. In short, I think that the value of changeable CCT lighting depends on how you like your lighting; I'm not a fan of cool lighting (except in limited circumstances) nor of warm lighting (unless it's in an old cottage or the result of real candles), but if you are, then go for it - in which case I'd recommend applying it to all lighting (that will be switched on together) so that you don't end up with an odd mix of colour temperatures.

I've used magnetic catches myself. Works well.

In addition to the above, lime is also strongly alkaline, which inhibits growth of mold & fungi.

That advice is for ceiling-mounted Coanda Effect vents; on a vertical surface things are different. Wall-mounted Coanda Effect vents are useful in large room used to direct air a long way across the ceiling of large rooms - but you don't what them in the position illustrated as that air will be directed down the ceiling & the wall. I'd be looking for a regular (not a Coanda Effect) vent, preferably with adjustable fins that give you the ability to direct the air flow. However check that you have enough space for a vent as drawn; it may be necessary to cut a hole through the RSJ for the duct (which would need to be part of the Structural Engineer's design) or to enlarge the boxing - or switch to a Coanda Effect vent on the horizontal part of the ceiling (probably the easiest option). From £1,651 +VAT at https://phstore.co.uk/zehnder-comfoair-160/ (20% cheaper than BCP - you may be able to negotiate it down further). Of course there are various other brands. For example those at https://database.passivehouse.com/en/components/list/ventilation_small which are independently tested to more accurate standards than normal (where a higher efficiency ratio = better economy).

My apartment isn't so different in volume, so overall no worries in principle. Ultimately it's about how much fresh air you need per person in a room. For example, at 'normal' fan speed a double bedroom requires from 15 m³ / person / hr; in a room measuring 3 x 4 x 2.5 that's 1 ach (air change / hour), but double the size of the room and that drops to 0.5 ach. Low fan speed is typically 40% below normal speed, so those figures become 0.6 & 0.3 ach. In short, in a smaller home the minimum acceptable ach will be higher than in a larger home. Probably not. I don't intend to be running mine lower than 0.33 ach; in your case that would translate to a little over 50m³/hr. The exception might be during an extended absence, but at least some controllers (including my Zehnder) allow a schedule to be set so that the average daily airflow could be cut well back by having it run for only certain hours per day; that's not recommended except for longer term absence. I have the Zehnder CA225 (similar to the CA220 that's sold in the UK) - 60 to 150 m³/hr according to the PH certificate & 50 to 200 according to Zehnder. That's partly because I also wanted to sling it from the ceiling where I have some more space. That doesn't seem unreasonable if you plan to use semi-flexible ducting, to keep the airspeed (and hence the noise risk) low. You could experiment using my MVHR calculator. It wouldn't be my first choice; I'd probably make that shower tray shorter to create a cupboard off the corridor. But in an enclosure with good sound insulation & anti-vibration mounts it should be OK in principle... Choose ceiling mounted Coanda Effect vents - Zehnder Luna, for example - and there should be no problem; keep them away from the walls by at least 350mm (750mm better, centre of room ideal subject to light fittings taking precedence).

+1 to plastering but, in case it's not obvious, done after first sealing the junctions with other materials & components

Not answering your question, but why would the alucald ones need replacing after only 9 years?