My understanding is that SIPS are already vapour-tight so no further barrier is needed (as it wouldn't do anything). Same thing for ICF construction, too.

I could be wrong, though.

Paul in Montreal.

yes, the SIPs 'wall' is not 100% PU (or other insulant) but bridged by timber and OSB spigots and joints.

I think the orig. comment is largely "marketing-speak", and not approved detail?
Tony, where did it come from?

Posted By: fostertom Except for gross momentary internal peaks - cooking, bathing, tumble drying - ventilation does nothing to lower internal RH - that's a popular but outdated fallacy.

So all the threads on all the forums where people have high humidity in their houses, often after insulating and draughtproofing old houses, and where ventilation, often just opening the windows more, improves the situation are lies? And humidity-controlled ventilation systems must be so obviously wrong that they'll never catch on. Methinks there's more going on than you recognize here.

Once indoor air is identical with outside air (by means of ventilation exchange), the only difference being the temp that each is currently at, then it's that temp difference that causes interstitial condensation

And how does this happen exactly? Temperature does not affect water vapour movement.

http://www.conservationphysics.org/vap/hansen-moisture-transport-NSB14.pdf

It's been said before, but there's a lot of good stuff on that site.

Both of you are right!, It comes down to where you measure the RH. Measure the RH on the inside of the insulation then ventilation does reduce it in winter, but measure it on the air that exit though a gap in the insulation then it does not.

Posted By: Ed Daviesif the air inside the house is drier than it would be with less ventilation

Does 'drier' mean RH, or abs concentration?

Posted By: Ed Daviesadditional ventilation will bring it progressively closer

'closer' becomes 'equalised' as a result of only a modest amount of bulk ventilation exchange, so it's a red herring to imply a progressive lowering of internal RH by adding more and more ventilation.

I won't answer blow-for-blow, but re-state hopefully clearer.

Modest ventilation is essential, to effectively equalise inside with outside air - it can't possibly do more than that. Ventilation should be just enough to achieve that, no more. As outside air becomes inside air, it's the same air, just raised to higher temp, and that lowers its RH below what it was while outside, even tho abs water vapour concentration remains same.

Then intermittent water vapour is added - cooking, bathing, tumble drier - but it only takes 1 or 1.5max airchanges to re-equalise inside with outside. The 'bit extra' of ventilation rate needed to cope with those spikes is in fact lower as a result of casual (furnishings) or deliberate (Warmcel or other organic insulation) hygroscopic buffering - the in-the-air concentration peak is lower and is spread over longer time.

As far as mould or interstitial condensation is concerned, intermittent RH peaks are irrelevant, except insofar as they slightly bias internal RH upward, long-term - but the ventilation should be sufficient (but modest) to make that bias almost negligible.

Going back to the OP, I suppose if you're assuming worst-case hopeless tenants who prevent ventilation while generating lots of water vapour, then yes a VCL may be key to preventing interstitial condensdation (but irrelevant to surface mould). Then, installing tamper-proof MHRV (as distinct from more preventable forms of ventilation) may lessen or eliminate the necessity for VCL - but foolish to so generalise - calc it in WUFI. That can be the only half credible meaning of that OP.

Posted By: djhthe temperature does not influence the rate of diffusion

Do you mean temp, or temp difference?

Posted By: djhIf the abs water vapour concentration inside and out were indeed equal then there would be no diffusion

Now you're giving me hope of clarification - surely on GBF I've been put right, in terms that 'vapour drive' is always from warm side to cold side, and only extreme (and rare) low abs vapour concentration on the warm side, relative to high abs concentration on the cold side, could reverse that. That also seems to figure in basic theory diagrams in the excellent-textbook WUFI Help. I could never make sense of that tem-driven 'vapour drive', but admit I've parroted it here.

So, trying to understand what's happening ...

You're saying that:
Typical modest rates of ventilation do not near-enough replace inside air with outside;
Inside air is not equal to current outside air, except warmed up and intermittent extra water vapour peaks added (cooking, bathing) until re-equalised;
Typical ventilation is merely dilution of inside air, not near-enough complete replacement;
Inside air permanently (or usually) has much higher abs water vapour concentration than the outside air it's being diluted with by ventilation.

I want to know:
Where does this perpetual addition of water vapour inside, come from? Is it just those intermittent peaks (cooking, bathing) plus perpetual occupants' respiration?
Surely that is intermittent enough to be near-completely exchanged away in between the peaks, by typical modest ventilation?
And, if outward-diffusion is driven entirely by difference of abs water vapour concentration, not by temp difference, then all the outward-diffusion must be due solely to that perpetual addition of water vapour inside.
In other words, in a house that's heated and ventilated but unoccupied, so no addition of water vapour happening inside, outward-diffusion would stop eventually, as inside air would indeed eventually become equalised by the ventilation with outside air, except warmed up?

Looks good Dave - apart from your mention a few posts above I wasn't aware of this - maybe a ready made alternative to seeking wisdom from GBF!
Pity it doesn't go into u/ground heat and water flow.

No

Not sure I understand that !

I thought that the dew point was a *temperature*, namely, that at which the humidity in the air will condense out, into "wet water..."

(I obviously understand the need to prevent that happening !)
So to prevent the air temp getting that cold, I thought I would warm the CS, with incandescent lamps...
But I find that as the temperature increases, so does the RH !

(I am in the process of concreting the CS floor (it is just earth...)).

I suppose that until the floor is finished, the humidity will just keep coming up, from the ground (not to say, out of my concrete...).

I don't really like the idea of a wet crawlspace...
===========
So I found this chart:

The CS currently is at 17.8°C and the RH is 85%
(The house is at 19.5°C and 62% RH)
The CS dewpoint is 15°C

Per the chart, the RH should be the orange line = 40%
Therefore I deduce that my CS RH is far too high
However, since the CS temp is 2°C above the dewpoint, "I should be OK, as condensation will not occur"

My local weather is: temp 11°C, 83% RH, increasing to 97% tomorrow...

Thanks to anybody for telling me if I am still on the rails here !

gg

Posted By: gyrogearI thought that the dew point was a *temperature*, namely, that at which the humidity in the air will condense out, into "wet water..."

You're kind of right, but so was Gotanewlife. He meant the point at which the RH reaches 100%. For a given mix of steam and air there's a corresponding temperature, but you can also just reduce the amount of steam.

(I obviously understand the need to prevent that happening !)
So to prevent the air temp getting that cold, I thought I would warm the CS, with incandescent lamps...
But I find that as the temperature increases, so does the RH !

(I am in the process of concreting the CS floor (it is just earth...)).

What you're seeing is 'buffering'. Earth, specifically clay, is one of the best humidity buffer materials. BTW, notice that those perforated bricks need to be UNFIRED. i.e. clay, not pottery.

I suppose that until the floor is finished, the humidity will just keep coming up, from the ground (not to say, out of my concrete...).

Exactly, so I hope there's a good DPM under the concrete.

I don't really like the idea of a wet crawlspace...
===========
So I found this chart:

The CS currently is at 17.8°C and the RH is 85%
(The house is at 19.5°C and 62% RH)
The CS dewpoint is 15°C

Per the chart, the RH should be the orange line = 40%
Therefore I deduce that my CS RH is far too high
However, since the CS temp is 2°C above the dewpoint, "I should be OK, as condensation will not occur"

My local weather is: temp 11°C, 83% RH, increasing to 97% tomorrow...

Thanks to anybody for telling me if I am still on the rails here !

I think you're probably on the rails, but I don't understand the chart. What is the vertical axis showing and what material(s?) is it for. Psychrometric charts are normally pretty curves, sometimes with hysteresis loops.

Condensation in the crawlspace doesn't really matter, unless it's on wood - typically the joists. That's why insulating between and under the joists is often used.

edit: corrected Psychrometric.

Many thanks to djh and torrent99 for those explanations !
===================
"What is the vertical axis showing and what material(s?) is it for ?"

Apologies, I must have amputated the legends without noticing:
the L/H side is temperature in °C, and RHS is in °F.
I "suppose" (= "just assumed...") the chart is for AIR...??
=========================
"What you're seeing is 'buffering'. Earth, specifically clay, is one of the best humidity buffer materials. BTW, notice that those perforated bricks need to be UNFIRED. i.e. clay, not pottery."

Good news to hear about the CLAY for buffering...
I did a sort-of DIY proctor test on my mud floor, and it looks like half-cay, half-sand.
Guess this is thus LOAM.
I have been steadily digging it out and replacing it with limecrete, so there is NO vapour barrier underneath...

So without abusing of your time and patience and good-will, to what extent could a limecrete slab (thick) ALSO provide buffering ?

gg

Thank you for the links, djh, very pertinent, I will have a lot of reading to do over the weekend !

In particular, the notion of "relative volume" is neat and will certainly help me get a better understanding of the subject...
==========
The limecrete floor started off as a simple project to stabilize the CS mud floor (which was pretty awful), using lime, raked in to the surface.

My motivation was simply to "sanitize" the crawlspace as I did not want such a horrible zone under my house (the CS had quite obviously at some time previous been used for storing firewood (!). Removing the rotted firewood was a horrendous experience, as you might imagine. Plus bare copper pipes had been condensing into the floor for years...

Somewhere in the process I discovered that when the infloor electric radiant was running, the CS temperature increased by 2°C. I did not like the idea of this, so got sidetracked into insulating the CS walls (100 mm of XPS).

From there things evolved towards "conditioned crawlspace".
However, I did not like the idea of a floor membrane, so decided on a slab.

Basically due to lack of access, and bad logistics (impossibility of ready-mix...) & to cut a long story short, I finally ended up going for an "ad-hoc" limecrete slab, using in-situ loam.

So as you "encouragingly" suggest, I expect the slab to "breath water", in both directions.

When I'm finished, the CS will be conditioned with house air, which will be ejected outside.

So that is the current status: Works in Progress... I have 4 square meters of floor complete, and the remainder (44 sq.m) in various stages of completion. A single-handed CS dig in 32-inch free height is a pretty laborious project (-:

If there is anybody out there building CS, just have a thought for the poor owner, a few years down the line...

gg

thank you !

I sure deserve it, LOL !!

ha ha !