Danger! I like to see vapour membrane on the warm side of the insulation, otherwise moisture may pass through the insulation and condense on the inside of the dpm/dpc.

Would you consider 3 to 500mm eps beads under the floor boards with a sheet of poly as an air tightness layer also acting as a vapour barrier?

Posted By: j.neffNo need to worry:
I will "install ProClima SolitexPlus membrane over joists" for the exact reason: keep the moist air away from the insulation.

Err, that membrane is vapour permeable. So while it will stop air movement it won't stop water vapour. As Tony said, you need a vapour [impermeable] membrane on the warm side. And preferably something non-organic as the insulation so you don't have to worry about rot. I don't believe Solitex is 'intelligent' in the same way as Intello Plus.

The interior of a house has quite a lot of moisture (water vapour) inside it, since it's at a relatively high temperature and has sources of moisture within it. When the vapour cools to ground temperature it will condense.

Agree with Mike, and I'd add another insulant to the list to consider - foam glass such as technopor. It doesn't rot, it makes a capillary break; the only downside is that it's difficult to compact and get a level surface at the top. But you could use e.g. rockwool on top. It's good under a limecrete slab.

Posted By: GreenPaddyLift the boards, drape a VPM to support 2 layers of recycled glass wool

A VPM on the cold side of the insulation? And you got a warrant?

Posted By: Nick Parsons@djh, can you clarify your query? A current-Regs re-roof will have a VPM on the cold side of the insulation, and that's fine, so I'm not sure why you suggest it is not fine 'upside down', at floor level.

Sorry, I got confused. For VPM I understood vapour proof membrane. I suppose he meant Vapour Permeable Membrane? i.e. breathable.

Many thanks for all your comments. I will respond in more detail once I have compiled more details on my retrofit plans and strategies.

Before I respond to other comments, let me give you an overview of what I am planning to do for the retrofit. I kept my initial post short but now realise that more information is needed to prevent misunderstandings.

I am planning a Passivhaus standard EnerPHit ("Component method", i.e. considering U-values of heat loss surfaces rather than overall heat demand).
Indoor air quality and especially moisture will be managed by a MVHR. So humidity levels should be lower as in most other buildings relying on manual or uncontrolled ventilation. As far as practicable, I will make the building airtight to conserve energy and reduce the risk of interstitial condensation.

Insulation (mostly wood-fibre) will be added to walls, floors and attic floors up to 280 mm thick to reach a U-value of no more than 0.15 W/(m2.K). All build-ups will be vapour-open, i.e. using materials and membranes that allow moisture movement, should it somehow appear behind the vapour control membranes. So there should be no condensation issues as any slightly elevated humidity can escape, either to the outside or the inside.

As far as practicable, I am trying to eliminate or at least minimise thermal bridges, something I am working on at the moment, thus the delay in posting here. It is looking pretty good so far, which will hopefully reduce further the potential for moisture issues - unless I have missed a thermal bridge somewhere.
Although it is a lot of work, I find the results from Finite Element Analysis packages, such as THERM (https://windows.lbl.gov/therm-software-downloads) truly fascinating and how heat flows respond to changes in insulation and design.
For anyone looking to get into this, I can wholeheartedly recommend the online training course by WARM:
https://learning.coaction.org.uk/

Generally, I am trying to follow Passivhaus principles and those taught on the AECB Carbonlite Retrofit course, which I took about ten years ago (https://carbonlite.net/).

On this note, the pandemic stopped me in my usual profession as a contract mechatronics engineer in special-purpose machinery so I following what was then a pastime and trained as a Passivhaus Consultant (also with WARM, now Coaction). I am now doing to my own home what I advised others on with the difference of doing most retrofit work myself for a lack of suitably experienced contractors in this neck of the woods. I do notice my lack of experience in the details of building works so have been looking for good practice and solutions at this level.
I find resources such as the Retrofit Pattern Book (https://retrofit.support/) a great inspiration.

So I have had plenty of training in Passivhaus, retrofit etc. but it is a complex and developing field and I often feel I am still in the foothills of my journey to retrofit heaven. :))
I am open to have my views and training challenged even if just to test my understanding of building physics etc.

Posted By: GreenPaddyI think what you're describing, timber joists in an unventilated floor is against building reg's. Are you getting a building warrant for the works your doing? I assume you are, as I suspect it's warrantable.

I am curious to engage with my local building control officers on this as I have heard your concern mentioned previously. Before I do so, I will have to complete one of my current "pastimes": reading through almost 700 pages of the Scottish "Technical Handbook - Domestic" building regulations.
I would be surprised, if timber joists in an unventilated floor would be a problem if they are above a DPM and surrounded by vapour-open insulation. Otherwise I can't see why the exact same would be permissible on top of a concrete slab with DPM underneath (this is how the floor was built in a mountaineering club hut about 13 years ago, notably using vapour-closed PIR insulation - before my Passivhaus journey started).

My suggestion, as I've done it dozens of times, with warrant, is...
Lift the boards, drape a VPM to support 2 layers of recycled glass wool one under and one between joists, keeping at least 150mm clear of the solum. Make sure there's plenty of ventilation into the void. These wools are super-vapour-open, so will continue to act as the dehumidifer which it's been for the last 50 years.

This is pretty close to what I am planning only that I am using wood-fibre insulation. Like glass wool this is vapour-open but can also buffer moisture, if required, and is a natural material using less energy to produce (as far as I know).
If you seal the VPM to the DPC in the surrounding walls I am not sure why there should be any moisture risk to the insulation or joists. Keeping the solum moisture at bay is what the VPM is for.

If you put a VCL over the top of the joists, that will reduce the passive vapour removal from the house over that very large area. You will then need to design for another form of removal of moist house air. I often intall MVHR, but I don't know if you're going to that extent.

As mentioned above: MVHR will deal with indoor air pollutants and moisture.
I would never consider venting moisture through an insulated build-up for the risk of condensation on the cold side of the insulation. I agree that in a well ventilated crawl space or attic this risk is minimised for the supply of fresh (and generally drier-than-indoors) air but this approach would risk condensation in less obvious places.
This is what "intelligent" air tightness and vapour control membranes, such as IntelloPlus etc., can help prevent: keeping the moist indoor air away from insulation and, specifically, the cold side of the insulation.

Not convinced about the effort and cost of air tightening outside a cavity wall, even when it's cavity filled, as there are often large gaps in the fill. The cost of the self adhesive stuff is very high. What are you cladding over that layer with, that would stick to the airtight membrane? I'd put on EWI and re-render, which will give all the air tightening you'll need, especially compared to the rest of the leakage around the myriad of other junctions.

My wall cavity is hopefully well filled now given that I had it re-done a year ago. The installers appeared very competent and checked various places with a borescope. They were getting re-accredited on this job so were making doubly sure to get it right.

I admit, I have not looked up prices for the self-adhesive membranes yet. It is not essential. Normal membranes will do, too, just a bit more trouble to install, I expect.
On top of the membrane go timber I-joists with wood-fibre insulation batts in between, then OSB, weather protection membrane, battens and wood-wool boards and render or, in some places, PV panels.

The airtightness and vapour control membrane must be on the warm side of the insulation (at least 2/3rd of it) otherwise indoor air moisture can reach the cold side of the insulation and condensate.

Applying the air tightness membrane on the outside of the existing masonry wall should cover all the many junctions that would otherwise be a right problem if making airtight from inside, especially given that I am living in the house whist the retrofit is happening.

"Keeping the solum moisture at bay is what the VPM is for."

Sorry but no! Assuming VPM stands for vapour permeable membrane (mea culpa earlier), then vapour permeable means that it lets gaseous water through. Which way the vapour travels depends on the absolute humidity rather than relative humidity.

Posted By: Mike1Neither is either type a magic wand. My general scepticism of them was reinforced by a short but intensive experimentation with WUFI (https://wufi.de/en/) last year, in connection with my own current project in France. That suggested that - if the estimated internal temperature and humidity profile that I used is roughly correct - then addingany kindof vapour control membrane lead to anincreasein moisture build-up within the insulation

I'd be interested to see your WUFI calc if you still have it, or to understand what climatic conditions lead to that situation.

FWIW, although I have high-tech membranes in some places, the vapour control for my walls just consists of lime plaster on both sides. I expect there's some condensation in parts of the walls at certain times of the year, but hopefully not enough to cause rot.

And graphs showing the variation of moisture levels at the external face of the insulation over the course of several years.

Note that I started the modelling with no moisture in any of the materials (which is why moisture levels in year 1 are low); It's not often done that way, but I found that it gives greater clarity.

The upper one is when no VCL is used - there's an acceptable annual cycling of moisture levels.
The lower image shows what happens when a 'Smart' VCL is used - the moisture doesn't adequately dry out:

Thanks to a suggestion by djh, you can find larger images here, including one for Mento:

https://postimg.cc/gallery/N2ymhsw

Posted By: djhFirstly the wall itself looks pretty wet all the way through.

In the 'animations', the top of the green shaded area shows the maximum moisture content achieved over the modelling period. So the outer face is 100% wet when it rains but, one hour later, it may already have fallen to 90%, and by mid-day in summer may have fallen much lower. Similarly, the wall as a whole will reach 'high' maximum values at some points - in wet winter weeks when moisture from continuous rain is moving inwards and internal moisture is moving out - but that doesn't mean that the wall is permanently wet.

Our expectations of the 'wetness' of a wall are no doubt also strongly influenced by our expectations of the bit we normally care about - what is acceptable at the inner face of the inner leaf of a cavity wall (15%-ish when measured with a probe).

In fact it may be useful to know that the dark green line in the 'animation' graphs (which I said to ignore) shows the state of the Relative Humidity on the day and time when the modelling ended, at the point in time shown on the clock on the right-hand side of each image.

Posted By: djhI'd have thought the first thing to do was minimise the rain coming into the wall, with a suitable coating/paint etc

Yes, that's the standard advice, but not so easy when you have to convince the other co-owners of the building to agree to it in the first place (when the majority of other apartments don't even have an exposed north wall), and then for their successors to agree to maintain it for the next 50 to 100 years. Both seem unlikely!

Posted By: djhWith Intello it seems the minimum humidity is about the same as without a VCL, whilst the maximum humidity is about 10% lower with the Intello there? So I'm not clear why you think it's worse?

The maximum RH isn't the most important bit; what you need to see in the time simulations is that the maximum winter value declines quickly enough to an acceptable summer value, and that a similar cycle recurs each year (i.e. there is no year-on year accumulation of moisture). That last point is why I ran the 'Mento' simulation for longer; after 5 years the annual cycle hadn't quite stabilised (the peak value was still rising slightly each year, though it's not perceptible in the screenshot).

Beyond that, mould growth will only occur if 'high' RH and a sufficiently high temperature occur at the same time - so high winter RH doesn't matter if the temperature is too low for mould to start growing. WUFI comes with a plug-in that assesses that risk for a selected point in the structure - in my case at the external face of the insulation - using a traffic-light system, where green = no risk of mould; amber = potential for mould; red = high risk of mould. With no VCL the traffic light was green. With both Intello and Mento it was red, so in this particular case adding a VCL would likely be a serious error.

A comparison between the 'animation' screenshots suggests what's likely to have caused those results. In particular peak RH immediately behind the VCL is significantly higher. Since this is a build up caused by internal RH, that's not going to happen over a short timescale - something (the VCL) is causing a moisture build-up over an extended period, and that, in turn, is seems likely to be keeping the moisture levels at the external face of the insulation higher for longer (presumably into periods when the temperature is high enough for mould growth), even if the peak RH there is lower than it would be without a VCL.

Posted By: djhWhat's the internal humidity - what's shown appears to be very high?

The internal humidity screenshot shows my best-guess of what my internal RH will be, based on using MVHR. That's based on real life UK data (thanks!) which I've modified according to the difference in outside Absolute Humidity between the local UK and French climates, which then had to be represented as a sine curve; a gross simplification, but the only way WUFI can take it.

Despite the attempt at objectivity it's still an informed guess; hopefully one that's 'a little pessimistic' rather than 'very high', but I'd rather be on that side of the divide with so much at stake (particularly for the unventilated insulated ceiling not mentioned so far in this particular thread). I won't know the reality for another 2 or 3 years; I hope to start data logging at some point next year.

For the benefit of others, establishing exactly what the internal environment (temperature and humidity) should be when MVHR is involved is far from easy as there is a lack of published information on the topic. Though there is separate software that allows you to model the indoor environment, if you have the time and cash to do that.

Posted By: djhPlease read the page you linked to:"At the heart of the system are intelligent vapour control membranes such as Intello Plus and Dasatop, which adapt their vapour permeability depending on humidity levels. This helps prevent condensation and mould while allowing structures to dry out safely. These membranes are used on the warm side of insulation to provide airtightness and vapour control."versus"For the cold side, Solitex Plus and Solitex Fronta WA windtight breather membranes protect insulation from windwashing and external moisture, while maintaining breathability."
In particular the difference between"the warm side"and"the cold side"!

Thank you for forcing me to do something I meant to do for a long time: review the properties of the various ProClima membranes, at least the few I intend to use. I now see what you mean about SolitexPlus and DA vs. IntelloPlus.
My retrofit plans have been very disjointed over the last couple of years whilst having to deal with other "life events" so I obviously forgot some of the details over time - such as that SolitexPlus is a windtight membrane rather than a vapour control membrane comparable to IntelloPlus.

With the Ecological Building Systems build-up still in mind, I missed that this was specifically for a ventilated crawl space, not what I intend to move to now (base filled with hardcore, DPM, insulation, IntelloPlus, floor boards). So without the option for vapour to escape into the crawl space, the membrane with the best vapour diffusion towards the warm side will be necessary.

As I now found, vapour resistance and humidity-variable vapour permeance of IntelloPlus is incomparably better than any other ProClima membrane that I just looked at, so I totally agree now that this is the best membrane to place between insulation and timber floor boards.

For those looking for the details, here are a few membrane specification webpage links:

IntelloPlus
https://proclima.com/products/internal-sealing/intello-plus/technical-specs#sub_navigation

SolitexPlus (unable to find this at ProClima now)
https://www.ecologicalbuildingsystems.com/product/solitex-plus#technical-details

DA
https://proclima.com/products/internal-sealing/da/technical-specs#sub_navigation

Posted By: Mike1For the benefit of others, establishing exactly what the internal environment (temperature and humidity) should be when MVHR is involved is far from easy as there is a lack of published information on the topic. Though there is separate software that allows you to model the indoor environment, if you have the time and cash to do that.

The Passivhaus Planning Package (PHPP) VENTILATION worksheet provides "Average interior humidity during winter operation" figures for a given regional climate, building and occupancy. Although this is mainly to check that humidity does not go to low in particularly cold and dry areas, would this suffice for what you are looking for ?