....(Continued from first post) Would it be wishful thinking however to hope that rather than defining the internal plaster as the airtightness layer, the airtightness layer could be defined as a future layer of EWI adhesive over the outside of the cavity wall as discussed on this forum at
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14950&page=1 ? Or could a layer of a similar gunk there (Blowerproof for example - as seen at http://www.blowerproof.co.uk/blowerproof-airtight-membranes/) provide the air-tightness layer? At this current stage then we could leave the ceiling joists merrily leaking interior air into the cavity, in the hope that a later stage we will seal the external surface of the wall’s outer leaf (and seal the cavity at the top of the gables, and plaster or seal their inside walls in the loft). Even if current technology such as current EWI adhesives, or Blowerproof, doesn’t sound as if it will make the grade yet, perhaps we could be technological optimists and hope that future developments will make that feasible. That will save a lot of work now, and will get us back in business for the July wedding party. Bringing the underfloor void into the conditioned space of the house will also avoid the need to insulate the floor. Which means we can dismiss the idea of ripping it out and filling the whole underfloor void with concrete and insulation, as one or two potential passivehouse-minded advisers have suggested off-the-cuff as a possibility. That extra concrete and waste of a softwood floor seems like a lot of embodied carbon emissions, not to mention loss of the potential benefit of being plugged into the thermal mass of the earth below. Bags of Leca would be an alternative to concrete, but the same benefit would be lost.

Our (wet) heating system is currently a re-purposed hospital system with galvanised pipes and cast iron radiators – the builder in the 1950s was a keen re-user of architectural and plumbing bygones it seems, which was good for his pocket no doubt (and will also creditably have reduced the house’s embodied carbon I suppose, if re-used materials can be given a measurable credit). A largely-uninsulated 50 mm (2”) galvanised steel heating return pipe passes for about 4.5 m (15’) through one corner of the underfloor void. With its residual insulation removed this might perhaps I suppose ultimately be enough heat in the void, with the perimeter well-insulated at that stage. We currently have a passive ventilation stack with a humidity-sensitive extract in our bathroom, and another planned for our kitchen. The accompanying humidity-sensitive intakes have been unnecessary while the house is still draughty, but we are planning to install at least one into the underfloor void before sitting room floor replacement. (The distributors (BPD) of this Passivent iPSV system recommend just one humidity-controlled intake per habitable room). Our hope is that the gaps around the edge of the floor and skirting will provide sufficient passage of air from that intake below the floor up in to the sitting room above, precluding the need for us to install any floor ventilation grills. We have an enquiry pending with the distributors on this point.

As mentioned, beneath said suspended timber floor is a 100 mm (4”) concrete slab (rat pad?) which I suspect may have no membrane below it and be saturated. I have dug a 25 mm (1”) diameter hole right through it, and couldn’t detect any sign of a membrane below, but may have dug straight through the membrane I suppose. Probing down to 400 mm (16”) below the pad with a 13 mm (½”) diameter agricultural core augur a year or two later revealed a surprising and somewhat disturbing amount of void space immediately below that hole. The augur came up mostly with small lengths of clay coated with a little sandy residue, this residue being I guessed from a sandy layer just below the slab. Between the lengths of clay were void sections on the augur. But the hole I had dug in the concrete may have been made use of by vermin in the interim, making burrows, and hence voids, below, I suppose. Any suggestions for investigating further the presence or absence of a membrane would be gratefully received – dig a bigger hole perhaps, large enough to put a hand in, then dig the earth (or hard core) away from beneath the slab just around the hole, and feel beneath it for a membrane perhaps? Certainly if there is one, it hasn’t been brought up to be continuous with the internal wall’s visible asphalt DPC three brick courses above, but then in the 1950s I guess that might well not have occurred to those involved. Or would the probes of an inexpensive moisture meter held in contact with the top surface of the concrete give me the answers I need? Perhaps I really need to dig a number of holes anyway to determine whether there are masses of voids everywhere beneath the pad, and also by probing in them during a flood event discover whether they fill with water. In which case it would probably be wise to give up on bringing the underfloor void into the conditioned space of the house, and just install insulation below the floor one way or another.

Over the days following my digging of my hole in this concrete pad, a dark ring of moisture further saturated the concrete just around the dug hole. But I am not clear whether this means that the rest of the pad is drier than the moist ring, which means that the hole itself was then a more major moisture penetration, and that there is indeed therefore likely to be a still-intact membrane under the rest. Maybe it just means that more moisture is rising into the air in the hole from the soil beneath, and thence into the surrounding ring of concrete, which implies that the paler surrounding concrete is less saturated than the dark ring, but may still be damp. If a concrete pad is somewhat damp, but not fully saturated, does that mean it is acting as a reasonably effective vapour barrier I wonder? I presume not. In the months following, this dark ring of moisture has disappeared.

If the concrete rat pad (and its possible membrane, if any) is not acting as an effective vapour barrier then a vapour-impermeable membrane (thicker than 6 mil, and non-laminated, polyethylene perhaps, as suggested on this video https://www.youtube.com/watch?v=ypnhoM1BYvg) should presumably be put over the pad at this stage before we replace our lifted floorboards. So again any suggestions in this respect would be gratefully received. Further floorboard lifting will we hope be achieved without too much damage to them, as described by HandyAndy in the sixth or so reply of 6th March 2015 at https://community.screwfix.com/threads/lifting-floorboards-without-damaging-them.16635/ . By using a parallel punch to knock the nails through into the joists below, I seem to have avoided any significant damage to those boards removed so far. And for membrane installation I can just about slide around in the 270 mm void space on my back, with increasing feelings of claustrophobia, though working down there is certainly challenging.

We are hoping that simply taping our membrane to the DPM of the internal leaf of the external (cavity) walls, and if necessary at any overlapped joins in the membrane, will be adequate (But with say 200mm (8”) overlaps do these overlaps even need taping I wonder? Perhaps not.) We won’t be trying to make it an air barrier after all, not even at the inner leaf of the external walls, if our technological optimism about an external air-tightness layer on the outside of the walls is tenable. But perhaps we should be taping it meticulously to keep any wayward radon out, even in our near-zero- (or at least negligible)-radon area (according to Public Health England’s map)? And in the interim our membrane will have to be worked around all the air-brick-related holes in the inner leaf, so as not to impede ventilation for the moment. But we just have to accept that this latter type of minor additional hassle is the inevitable consequence of a step-wise rather than an all-at-once retrofit.

Should we bring the membrane up to the DPM at the top of the honeycombed pier walls, I wonder, in order to exclude any moisture which passes up from the concrete pad into those pier walls? Bringing the membrane up in this way wouldn't be as much of a problem as I had first thought. I had imagined that it would prevent conditioned air from circulating from one pier-wall division of the underfloor void to the next, so we would then need to install a humidity-controlled intake in each of the three sections of the underfloor void created by the pier walls. However I now see that of course the joists site on top of these pier walls, so that the between-joist spaces will still allow air and warmth to flow between the voids. Probably we should insulate the walls of the underfloor void internally at this stage, even though that ultimately may be redundant, in order to keep it warm enough down there until the external EWI (and air-tightness?) is done I suppose.

There is also the practical problem that the underfloor voids will in the interim (prior to EWI) probably lose so much heat laterally through their walls and through the foundations that they will potentially be cold, and thus temporarily liable to suffer from high relative humidity and concomitant moisture damage, even with the membrane in place and with the humidity-controlled intakes installed. And the under-hallway void will in the interim have no membrane, unless we start pulling that part of the house apart before the July wedding party too.

We will in any case also need to arrange a well-insulated pipe to bring some combustion air across this void beneath the sitting room floor, in the meantime, to the cut-out in the concrete kitchen floor at its junction with the sitting room floor void. This is needed for our ancient open-flued Vulcan Continental gas boiler which sits in a large cupboard in the kitchen, just behind the adjoining wall. It currently draws its air supply via this cut-out from the sitting room underfloor void, prior to wastefully pushing out masses of exhaust heat through its flue. It has lasted at least 40 years, so against its deplorable inefficiency must be offset the embodied energy (not to mention the cost) of the alternative of a new condensing boiler every 13-15 years or less, which considerably limits the carbon balance benefit of these more efficient but shorter-lived appliances, I believe. Once the house has eventually been insulated and air-tightened well it will need much less heating, so until then we’ll just live with our ancient boiler and hope it lasts that long. Being a simple beast, spares are still readily available, and our 97-year-old neighbour five doors down has one too which is still going strong (in a house built and at one time lived in by the builder of our house, her brother-in-law).

I guess that if we need to consult building control (?) about our proposed membrane, they may be nervous of the above-mentioned overall strategy. So I would hope just to stick to discussing the membrane with them, if necessary at all, and avoid going into any more details of future EWI etc at this stage. A sealable drain-with-flap from the underfloor void (above the membrane) to the exterior should presumably also be arranged, in case some mishap such as a major plumbing or roof leak causes bulk water to get into the void at any time in the future, and to pond in the membrane.

Or instead of a PE membrane, should we be considering a spray-on moisture-proofing of some sort for the concrete slab? That might be easier than trying to manhandle large pieces of a thick-ish polyethylene membrane down through a 30”wide space between floorboards and wall into the underfloor void, between 100 mm x 50 mm (4” x 2”) joists spaced at 420 mm (16 ½”) centres. Or should we invest in a ‘smart membrane’ which will pull moisture through to its underside? And would it be worth put in some more comprehensive underfloor heating pipes while we are down there I wonder? And instead of insulating externally down to the base of the foundations, should we just break up the full width of the concrete path which goes all the way round the house (and doesn’t subside on the other three sides) and put a skirt of insulation under its replacement? That would almost certainly be worthwhile along half of the north side of the house, where our neighbours have built a solid floor right up to our fence, which is only 1320 mm (4’4”) from our own house wall. So we could probably thereby plug into their warmer underfloor mass there, to reduce heat loss further in that direction.

With apologies for the lengthy explanations, hoping we are on the right lines, and welcoming any suggestions or thoughts from the assembled expertise on this forum. FosterTom according to my note you kindly offered a drawing in your forum post of Oct 28th 2016 about sealing and insulating crawlspace perimeters. I cannot immediately find that post at present, as I didn’t note which category or topic it was in. If your offer is still available, and you could let us have your email address again through this forum, or direct me to the topic and category of that post, I should be grateful.

Posted By: PlHadfieldwe are hoping to avoid lock-in / sterilisation

What do you mean by 'lock-in / sterilisation'?

Using the internal plaster as the basis of the airtightness sounds like a good plan.

Your stream doesn't sound to be too much of a problem, in fact it may be a help. You might think about a French drain when you install the EWI, but you will need a place for the French drain to drain to and that sounds like it might be difficult to find.

The concrete pad under your floor is below the DPM/DPC and so potentially damp. You'll need to think carefully about what to do in that space; how deep is it?

It might be worth making or hiring a blower fan to get an idea of existing airtightness and where the major leaks are.

Posted By: PlHadfieldDo you have any ideas on how I can convert jpg photos for attaching?

You need to edit them. I run linux and would use gimp or ffmpeg. Typically, find a photo editor and save the photo with reduced resolution.

PS If you want quotes to come out in blue, choose Html formatting. Choose Text formatting if you want links to work. You can edit a post to change the formatting.

Posted By: Nick ParsonsOnly if you are completely in control (and I really do mean 'completely' and 'in control'!) of the EWI could you hope for (even if not guarantee) there to be sufficient coverage of adhesive for it to function as an air barrier.

(Edit: The above quote was part of a whisper from Nick Parsons, which I hadn't realised was a whisper, so replied publicly. Moreover neither Nick nor djh had seen the second, lengthy original posting at the time they made their comments above, as I had mistakenly whispered it. I am a beginner, but learning now.)
Thanks for this Nick. Indeed if we sell the house in a few years time then inevitably we won't be in control at all of what happens to it. But we will have no further direct interest at that point. I didn't meant that we were expecting to leave it and still retain an interest.

We aren't contemplating an all-at-once, whole-house, retrofit, for all the reasons that the 'Europhit' project gives, including that it is probably most economic to upgrade elements of the building as they wear out. So I believe that step-wise retrofits are well-regarded now, and inevitably people will sell and leave houses when these step-wise retrofits are part-way complete.

There doesn't seem to me to be any way of avoiding that. The only ray of hope which I see, to mitigating the problem of handing over which you rightly point out, is to have the sort of national database of addresses, and the measures taken there, such as is proposed by the Bonfield report (if I understand it correctly). Then new owners would have the benefit of high quality advice about the measures taken on the building to date, and the potential future measures, including access to the entire retrofit plan for the house, such as it is. Hopefully they would also by that means have access to advice about which expert practitioners, such as qualified retrofit coordinators and other expert AECB members (including yourself), will know enough about retrofit to advise on the next steps.

It seems to me that we each have to do what we can, in the situation in which we find ourselves and in the face of climate change, even if we aren't sure how long we it will be before we sell a house and move on. And it seems to me that when we are upgrading parts of the fabric for other reasons than energy upgrades we should do our best to avoid locking in poor air-tightness and insulation. Do you think I am correct in these respects?

Posted By: djhPS If you want quotes to come out in blue, choose Html formatting. Choose Text formatting if you want links to work. You can edit a post to change the formatting..... Typically, find a photo editor and save the photo with reduced resolution.

Thanks djh, I have converted my previous comment to html now. And I will have a go at editing my photos as you suggest.

Posted By: djhYou will need to decide, plan and install your underfloor insulation before putting in a vapour barrier, of course.

(Edit: djh hadn't seen the second (part of the) original posting at the time he made his quoted comment). What we are proposing is not to insulate the floor at all in fact, but to insulate the perimeter instead, and to bring the underfloor void into the conditioned space of the house by closing all but a humidity-sensitive intake ventilator (or three) and then ventilating that void to the room above. The vapour barrier, if appropriate, would just be on top of the underfloor concrete rat pad (or 'oversite' as I believe it is sometimes referred to), to reduce the amount of moisture evaporating from it into the void space and thence into the house.

Russel Hayden is relying on the EWI adhesive on his project:

https://thebox-haus.weebly.com/posts/more-tape-please
https://thebox-haus.weebly.com/posts/rockwool-redart

(Note my comment... have to say I get a tad annoyed with blogs that leave the comments open and no-one ever responds... anyway it's an interesting blog).

As Nick implies, continuous beds of adhesive is not standard practice. Indeed, I've heard it can make things more difficult in certain cases, with less even walls. No chance of S+C parge coat before the EWI?

Good to hear of another Leicestershire member! There aren't many of us. I'm near Melton. Sounds like we have a lot of similarities - 1950s CW build, lots of clay! At least our floors are solid

Hi, I am in a very similar situation to you: extending and thermally upgrading a 1945 (very early ) cavity wall build. Living room: suspended wood floor. Kitchen: concrete on hardcore on clay soil, not comfortable at any time of year.
Although it was quite expensive, we decided to have the kitchen floor dug up, have a new load bearing floor, 300mm XPS and 75mm screed (with UFH) floor on top put down. Absolutely no regrets of spending the hard earned cash on this, even with the UFH still not connected it is quite comfortable already! I very much doubt whether you get the same with putting EWI down to footings level (if you can achieve that neatly without destabilising the walls), as it will remain a massive heat sink. In fact, we are thinking of not trying to insulate the suspended wooden floor but doing away with it completely and give the living room the same treatment.

For the airtightness layer we were advised to go for the inside (render/plaster) layer. Given the poor detailing of our cavities and their terminations around (historic, bricked up) doors and windows, I think it would have been very hard to achieve an airtighness layer on the outside. Even though our 50mm cavity is filled (blown) with mineral wool, it is by no means airtight. Admittedly it is finicky to get a good continuous airtightness layer between ceiling and floor, this is where I have to take it room by room. And since it is not visible I can slap on the parge coat myself :-)
Good luck! Bart

The whole point of a whisper is that nobody else hears it :)

Thanks for making it public; I was wondering why there was only one part!

edit: you might want to fix the broken blowerproof link.

I don't think there's any chance of a membrane underneath your rat pad. 1"-2" of sand blinding maybe. The voids in the clay may be due to the clay shrinking as it has dried out through the years, after having been built on when wet and expanded. Hopefully you're not anywhere it could be subsidence?

You've come up with a bunch of fairly complex possible proposals and some fairly fundamental questions. Given that, I think you're likely to experience a fairly steep learning curve over the next few months, and likely to change your thoughts as to the best plan. So my advice would be to put your floor boards back down and get your sitting room back for your pre-wedding party. Spend the time up to then learning more and planning but not doing anything to the house yet, oh and planning for your wedding! The alternative would be to hire a professional that you trust and follow their guidance as to what to do, but finding such a paragon might be difficult.

Posted By: PlHadfieldI am hoping that by plugging into the thermal mass of the ground below it will not in fact be a heat sink but a source of thermal mass, along the lines championed by FosterTom on this forum. It is also the approach taken by the hugely respected Building Science Corporation in the USA, and of their legendary Joe Lstiburek in particular, who says 'don't insulate the floor, insulate the perimeter, and bring the underfloor void into the conditioned space of the house.' That is the approach I have been favouring.

I don't think perimeter insulation is the ivory-tower disruption-irrelevant money-no-object ideal solution, but it is a solution good enough to achieve Ph levels of performance, therefore it's good enough in my project. It seems like it should be default for retrofits where disruption is to be kept to a minimum.

Posted By: PlHadfieldLikewise, very good to hear. I have spoken to one or two members of the Institute of Sustainable Development at De Montfort University in Leicester about the possibility of getting together a retrofit group for the east midlands, along the lines of the excellent Manchester-based Carbon Coop of which I am a member. The sticking point seems to be that there are few experienced AECB members, or retrofit designers, coordinators or contractors in the area, and none in Leicestershire as far as I can determine. Nottingham, nearer to you than to us, has one or two, but that is rather at the northern end of the East Midlands area. We need to crank up the 'Midlands Engine' (our equivalent of the 'Northern Powerhouse'), in this respect.

Yes, plenty going on in Nottingham which is most definitely East Midlands, although to an extent it feels like it has lost momentum somewhat.

I too have been mightily impressed with what Carbon Coop are doing - the grants they appear to be finding, excellent practical training (it looks like). Not decided to become a member due to the fact I'm not in Manchester!

There are some Leicestershire practitioners on the design side, but not many. There's a Ph in Rutland, a self build by the architect. A high performance build in Pickwell, not Ph as far as I know. Not so much on the retrofit side.

Posted By: PlHadfieldI wonder how feasible it would be to seal the cavity at the eaves however in advance of that, at the time of the EWI. In other words will it be feasible to seal the top of the cavity before, and separate from, the full roof treatment some years or decades later. Can we hope to make the top of the cavity wall airtight simply by stripping the eaves tiles and felt, and sealing whatever is nominally closing the cavity at that point (gappy slates I suspect, which could just have a parge coat too perhaps)?

Is it filled? I think you can get most of the way there if so, but sealing the top is also important. See my eaves detail thread. http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15484&page=1 .

Also https://thebox-haus.weebly.com/posts/the-zen-art-of-taping

I think that's backwards. You need the advice first....and think again after getting the advice and agreeing a broad plan.

Thanks for this djh, from nearly two years ago. Our hope at this stage however is that we do what we can below the floor while its edge boards are up (still! We borrowed a neighbour's house for the bride and six bridesmaids to dress in for the wedding in the end), avoid spending the £6k+ on a professional plan in case we don't stay long in the house, get the floor back down, and then, as things become a little clearer about our circumstances, decide whether we are going to stay in the house for a reasonable time. At that point we can take a deep breath and spend the money to have the full stepwise plan made (or I can perhaps invest in the PHPP software and go on the course on how to use it myself). But we are hoping that at that point whatever we have done below the floor will be enough to preclude the need for it to be pulled up again. Psychologically having spent this time with the edge boards up (largely due to other calls on our time), we cannot really contemplate having to pull them up again in the future. And we can’t really see that we should need to do so, as long as what we do down there is un-objectionable.

So as readers may or may not remember, or may have seen again earlier in this thread, and with acknowledgment of your contrary suggestion djh, we envisage such a stepwise retrofit plan being potentially drawn up and gradually implemented in future, if we stay, but not yet. In outline we intend that the plan (if we stay) will include external wall insulation (so as not to reduce our living space), down to the footings, and the bringing of the crawlspace (the underfloor void) below the suspended timber floors into the conditioned space of the house, thereby tapping into the warmth of the earth below, by building up a column of warming subsoil below the house over several years (we are on 200 feet of clay here, above the bedrock). This is something which is next-to-impossible to model iiuc, but which I believe is recommended in all cases by the leading building scientists in the USA (Joe Lstiburek’s Building Science Corporation) as well as being championed by fostertom on this forum. So while not imagining that this is necessarily a panacea, we envisage making it a given for any future retrofit plan (albeit with a possible retrospective measure for the floor, which I will mention later, if it turns out not to be as beneficial as had been hoped).

So our plan is that instead of insulating the floor of our ground floor sitting room itself, we will just make airtight the insides of the external walls of the crawlspace there, from floor level down to the concrete rat pad, in case the future plan specifies that the internal wet- plastered walls’ plaster layer should be the/an airtightness layer for the whole house (as seems quite likely in the light of your comments about it djh). We can do this airtightness work we believe, while the edge floorboards are up in that room for our electrical wiring work there, with relatively little risk or effort, and as a no-brainer, without committing at this stage to the expensive, professionally-produced retrofit plan. We anticipate the eventual insulating the perimeter of the crawlspace externally, along with the full external wall insulation of the house walls, as a future step (the air bricks having been sealed). By monitoring the RH and T below the floor we hope to see if that works. If over several years after the external wall insulation the floor does not warm up as much as hoped, probably because of some unforeseen aquifer in the clay subsoil whipping away the warmth as groundwater flows through it, then we are hoping that it may be possible to pump in some insulation under there through the air bricks. We plan to plug the air bricks reversibly in the meantime, with monitoring in order to keep an eye on what is happening down in that crawl space, and if it proves necessary with a floor grill and fan (or two) blowing air from the room above down there to assist with conditioning it, at least until the external wall insulation is done.

So we decided to extend the potentially airtight layer of the wet-plastered wall downwards from floorboard level in this way, by means of a 5mm approx. sand/cement parge coat (is that thicker than necessary?) on the 13-18” of wall height). Our thinking is that no amount of air-tightness is too much, and no number of air-tight layers is too many, in any part of any building (unless not worth the effort/ cost; but our proposals involved no great deal of either). The tricky part was to avoid having our parge coat bridge the existing damp proof membrane, which is located below a horizontal mortar joint in the brick of the inner leaf down there, and is of what I imagine to be a 1950s asphaltic material of some sort. To avoid bridging it, I decided to use two strips of Contega Solido SL tape, one above and one below the dpm, attached above and below a 3/4" wide strip of modern dpm plastic, at right angles. The tape stuck much better to that than to the narrow (and in places non-existent) visible edge of the existing asphalt dpm. We taped it so that the edge of the new ¾” dpm strip sits just on top of the existing dpm’s edge and projects rather floppily out into the crawlspace, and the tape stuck to the brick above and below held it in place (patent not yet applied for). Where necessary, I chamfered the existing mortar above the existing dpm so that the edge of the new piece of dpm could sit with ¼” or so of overlapping contact on top of the edge of the existing one. All this I did before the parge coat of course, so that the tape was subsequently parged over. By this means we separated the two parts of the parge coat, namely above and below the existing dpm. So we hope that no moisture bridging will occur, and that any existing horizontal gaps between the existing dpm and the brick or mortar of that inner leaf will be sealed by the two strips of tape adhering to the narrow strip of new dpm material. Maybe someone can think of a better way that this bridging could have been avoided and airtightness achieved.

As mentioned earlier in this thread our concrete rat pad below all this is above a granular layer of some sort it seems, but has no membrane beneath it as far as we can tell (and judging by the helpful comments from others here about its presence being unlikely under a 1950s concrete pad, for which many thanks). Meanwhile I had imagined that that concrete pad could and probably should be overlaid, while we have the boards up, with a vapour barrier, at no great risk of unintended consequences (and at no great cost), and with a benefit in keeping moisture out of the crawlspace and hence the house (see below however). I have covered half of the pad with 1200 gauge (300 micron) polythene (which I haven’t made any attempt to make airtight), and am now in the process of covering the other half. I had imagined that a vapour barrier here was as much of a no-brainer as the afore-mentioned bit of air-tightness work. But part way through this as we now are, I have realised three things that I might have realised earlier.

The first of these is that as mentioned in other recent threads on this site, the surface below a floor is often, especially if of earth, not in fact a source of moisture as might at first be imagined (and as I had imagined), but a sink for it, even in winter. So what about concrete? Is a concrete pad with no membrane going to be a source or a sink below a suspended floor? And if the answer to that question depends on the detailed circumstances such as the nature and thickness of the concrete and/or of the granular layer below, then can any of the wonderfully capable building scientists on this site point me to some information about what it depends on, and how? If it is likely to be a sink for moisture in winter then I should presumably cease covering the rest of the pad with polythene and remove it over the half I have covered. Any thoughts?

The second thing I have realised is that a vapour barrier which is not fully airtight, or has occasional air gaps, is usually fine, but only if it is in fairly full contact with a solid and fairly airtight surface. Otherwise moisture-laden air will move around in the spaces behind/below it and be continually finding the gaps in the vapour barrier, carrying its moisture through with it. As I had not grasped this simple fact of building physics until recently, I had not been attempting to make my polythene vapour barrier airtight at its edges and overlaps. So if the answer to the question about our concrete pad being a moisture source or a sink is ‘source’, then I am after all correct to continue to cover it with polythene, but I believe I will need to seal all the edges and overlaps with tape. This I think I can still do, fortunately. Had I realised this at the outset, I might not have needed to create an additional airtight layer by parging the inner sides of the crawlspace, as the about-to-be-airtight polythene vapour barrier layer will suffice. However I won’t feel too bad about this, as the parging hasn’t been a huge job, and you can never have too many air-tight layers iiuc, as long as they cost little or nothing, and it will probably be trickier to achieve a good degree of air-tightness with a membrane than with parging.

It had also seemed like another no-brainer to insulate the inside of the crawlspace, around the external walls, while we were doing this work, with (a 13-18” height of) 4” of insulation, which unlike most internal insulation would take up none of our living space after all. And it seemed that the correct place to put that bit of insulation was outside the vapour barrier, between it and the wall. Cost negligible, and minimal chance of the vapour barrier being the cold layer where the moisture would condense, as it was on the warm side of the insulation. In fact while we were about it, why not put (as I did) a 4” depth of insulation onto the rat pad around the external perimeter, around 3’ wide, to give the heat from below the pad a greater thickness of soil horizontally through which it has to travel to escape to the colder soil outside the perimeter of the house? Duh. I forgot that the mineral fibre loftroll which I used for both these insulation jobs (as I had it left over from some loft work) will be rendered much less insulating once it has become somewhat moist and humid, as it is liable to become down there below the dpc/vapour barrier. Still, I guess at least it is doing no harm apart from a minimal waste of money, unless anyone can suggest otherwise. And ultimately once/if the external wall insulation is done and extended down to the footings as we envisage, then the 4” of (13-16” vertical height of) internal insulation on the crawlspace’s external walls will be a negligible bonus anyway, even if effective. The 3’ wide roll on the concrete pad’s external perimeter might have been of more significance however. If we stick with the poly membrane down there, because you experts here tell me that our concrete is not a moisture sink but a source, so the poly is worth keeping, then in that case should I pull out that mineral fibre roll and put it above/inside the membrane? A bit of condensation between a 4” thickness of vapour-permeable mineral fibre insulation (in a conditioned crawlspace) and a polythene vapour barrier, with no contact with any biodegradable materials, is perhaps not a problem after all, and should be much less subject to a reduction in its effectiveness due to moisture, I would have thought. Or should I replace it with something else (XPS? - but getting pieces of rigid XPS down below the joists there will be hugely more difficult than the flexible loftroll was, as it will necessitate cutting them into small pieces and then, once down there, sealing all the joints between them with sealant. Is there another more flexible, or loose, insulation capable of maintaining its insulating characteristics in moist environments?).

One final question of principle: below our patio door I have taped some air-and-vapour tightness membrane to the hardwood sill, but need to fix a batten to support the ends of the floorboards there. The easiest way of doing this would be to fix the batten to the sill with nine or ten screws or nails through the air-tightness tape (Contega Solido SL). But will the screw or nail holes through the tape compromise its airtightness significantly I wonder? There is an alternative way of supporting the batten for the floorboards, but it is a considerable amount of extra work, so if the screw or nail holes will not be a significant problem then I will go for that.

Anyway, first things first, do I pull out my vapour barrier membrane I wonder, in the expectation that my concrete pad is a moisture sink rather than, as I had imagined, a source?

(And who was it who said that my ideas would probably change....? They were clearly correct. Still, you do what you can, and can afford, in the circumstances, and with as much help from your friends as you can get).