Very difficult to use buffering to increase humidity to that extent without spilling water on the carpets.

V interesting - so internal doesn't track external unless you're in an empty room looking at nothing but plasterboard before hitting a VCL.

Posted By: goodevansall the plaster, brick, straw, wood has it's own balance point depending on the average RH values - and it's going to be slow release (but alot of it)

Fits perfectly with the benefit of not having VCL (i.e. breatheable construction) - it dries out inward just as much (or almost) as outward.

In my playing with WUFI, I found my test cases of breatheable construction (you know my favourite recipe) actually performed worse (more interstitial condensation, more danger to the OSB 'tea cosy') when an internal VCL was added - but not so worse when it was 'intelligent'. The VCL was obstructing the vital inward drying, the intelligent one less because basically it disabled itself half the time (behaviour well worth paying premium for?!).

Posted By: tonyVery difficult to use buffering to increase humidity to that extent without spilling water on the carpets.

Trust me, I don't spill water on the carpets, and I'm not allowed to spill wine

Posted By: djhI just moved a meter to on top of a floor supply valve, which should be fairly close to the main supply conditions.

19.3°C and 27% RH for the incoming air and about -2°C and 63% outside. I've now put it elsewhere on the landing.

edit: Oh internal humidity is always fairly constant. Normally 40-45%, occasionally up to 50% and 37% is as low as I've seen it.

More tomorrow.

DJH,
This is a gem...
http://www.jti.se/uploads/jti/JTI_Rapport_63.pdf

It even gives data on how long it takes small samples to reach EMC. For these loose samples the time is measurable - confined, on mas,s in a low draft environment it will take longer. The only problem is the pages are a little jumbled up.

Posted By: goodevansConcrete looks like its EMC varies between 1% and 2% for the 37% to 80% RH range - that 1% diffence in moisture content is weight based so a single 20kg 140mm thick medium density block would gain/lose 0.2 litres. I imagine screed would be the same.

I don't have any screed, or blocks or bricks. The only concrete is in the slab, which is below the chipboard and the floor coverings (carpets, bamboo & vinyl) so I suspect we can ignore it.

the WUFI validation source is here: https://wufi.de/en/wp-content/uploads/sites/11/2014/11/Mundt-Petersen-Harderup-Validation-of-a-One-Dimensional-Transient.pdf

the data I was looking at was figure 7 however the same data appears in figure 8 - it is unclear which is correct. one purports to be on the other side of a bathroom waterproof membrane the other not. Unfortunately none of the charts show the internal and external data on which the testing was based so I can not infer any lag times.

Yes, that looks like a misprint. I'll read the paper later and see if it makes sense.

I've just found this reference which may be applicable to DJH - it involves straw and earth plasters ...
https://tallerconco.org/wp-content/uploads/2017/05/An-experimental-investigation-on-equilibrium-moisture-content-of-earth-plaster-with-natural-reinforcement-fibres-for-straw-bale-buildings.pdf

That's one of the ones I had found but haven't read yet. :)

DJH, What is the mass of the straw in your walls by the way (back of envelope calc)?

Each bale is 20 kg. And there were 400 bales, so 8 tonnes in total. There'll actually be a little less to allow for wastage.

This is a gem... http://www.jti.se/uploads/jti/JTI_Rapport_63.pdf

It even gives data on how long it takes small samples to reach EMC. For these loose samples the time is measurable - confined, on mas,s in a low draft environment it will take longer. The only problem is the pages are a little jumbled up.

Excellent; I haven't seen that before. I'll add that one to my list of references, thanks.

I've just looked at the humidity sensor in the house here - it reads 18.1 degC and 22% RH. Mind you the ACH per hour is probably well above 1.0 at the moment given the wind speed outside (v. draughty house).

That seems fairly consistent with my reading for my supply air, though I haven't checked its sanity.

Posted By: goodevansSo lets say you have 7000 kg of straw , and say half of it is on the warm side of the walls, and internal RH between 40 and 80 between the seasons with giving a moisture content range of say 7%. 7000*0.07/2.0 = 245 litres buffer. lets split the year up and say for 25% of the time the moisture is coming out the walls - that gives an average of 245*(365*0.25*) = 2.7 litres a day. And if your walls are very vapour open then perhaps at dry periods such as now you may be releasing double that.

I think 7000 kg is probably a low estimate but it's within 10% I suppose. The straw varies uniformly from room temperature at the inside to external temperature at the outside (except for the differential across the lime). It's very vapour open and also capillary active. The capillary activity serves to limit the moisture content at the cold side, if memory serves. The internal RH is fairly constant, and the internal temperature varies fro 20°C in winter to about 26°C in summer. The external RH averages about 80% IIRC and the temperature varies seasonally.

Your original post was that maybe 6 litres per day was being released - with ordinary daily activity's plus what's coming out of the straw alone I think you can get to those sort of numbers - not to mention what's coming out of the hard dense stuff with a 1% MC range.

I ignored the buffering in the Warmcel in the roof because I think at the moment the Intello will be vapour closed. I think it's supposed to open in the summer when the top of the roof is hotter than the underside.

So perhaps you are buffering and as a result your build keeps the humidity at a nice 37% for a few days when otherwise it would be like this house down to 22%. Lets see what happens in summer when its 28 degC and 80% humidity and a nice cool 24 inside (you may have data for this) - I bet it doesn't get foggy - and the vapour's got to be going somewhere. For you - plenty of room in the straw for other mortals it'll go into the plasterboard, walls, joists etc etc.

It certainly disappears from our shower rooms. We're typically happy to run the house at say 26°C rather than try to get 24°C during summer.

Experience tells me houses aren't that foggy in the summer even with a sealed MVHR house on a hot humid day.
Just in my proposed relatively standard house my trusses will buffer 277 litres (22*12.6) between seasons because I have 3 tonnes of truss (and the trusses hardly feature for the loads on the piles).

If you have some data we can see what the summer and winter lag is (either time constant or half life) - together with an estimate as to how much moisture is buffered in your house as a total we should be able to backtrack for a rate of release and takeup.

I don't really have any useful data, because what I have is from when the render and bales were completing their drying out. I must actually do something.

https://www.labenvironex.com/en/environment/bacteria-and-mould-analyses/moisture-sources-in-houses/

Bit related..

Posted By: goodevansI've just looked at the humidity sensor in the house here - it reads 18.1 degC and 22% RH.

Completely separate question, but are you comfortable at that sort of RH? There were teething problems with the ventilation system at the office I worked in just after it opened that meant the RH dropped to 30% and below and lots of people were complaining of dry eyes and itches etc. It didn't bother me especially but did convince me the guideline was sensible.

This is very wrong (except may be in summer).

RH cannot be used to drive thinking. Partial vapour pressures determine the direction of moisrure movements. From a area of higher to lower pvp. Outdoors the pvp is prety much always lower than indoors if the house is heated and lived in and as has been said before pvp is very temperature dependant.

Try this to help your thinking, plug in some numbers from your home:-
https://www.rotronic.com/en-gb/humidity_measurement-feuchtemessung-mesure_de_l_humidite/humidity-calculator-feuchterechner-mr

I think that is the difference in partial vapour pressures that drive vapour movement

Dew point does come into it as that is where liquid water annoyingly appears out of cold air but generally as this happens vapour is transported from areas of high partial vapour pressure to the sink point (the place where condensation is happening or to other places of relatively lower pvp

From that, it's interesting to see that the question is very much open, the answer unconfirmed - 'this supports an argument that ...'. It amazes me that 'the answer' isn't self-evident, given proper understanding of the phenomena in play - that it isn't simply lack of the latter that's causing confusion.

For my money, it must be gradient in PVP that causes the molecules of individual gases to move through a mixture of gases - aka diffusion. The reason being that the second P means Pressure - which seems self evidently a phenomenon that causes something to move. Why should simple concentration of water vapour content itself cause something to move - except inasmuch as that concentration is proxy for Pressure? (which it is).

Potential gradient - sounds good but what is the nature of that Potential, that is causes things to move? Leaving aside electric field (good point) quantified as Electromagnetic Potential, this Potential must exert some kind of force. What is it? Gravitational Potential, Electrostatic Potential? Crowded jostling concentration might cause Mechanical Potential - why wouldn't you call that Pressure?

All those Tom, they're just different kinds of potential energy. Most of physics boils down* to the world trying to arrange itself into the lowest potential state it can find.

Try here if you like this kind of stuff:
https://en.m.wikipedia.org/wiki/Chemical_potential#Electrochemical,_internal,_external,_and_total_chemical_potential

*sorry!

Better not sidetrack the thread, which is actually not about diffusion at all, its about humidity buffering.

Posted By: WillInAberdeenporous materials that bridge across the construction will absorb water from high RH areas (outside) and desorb it in low RH areas (inside) so act to pump water into the house

I've been thinking about this as well - and this has the ring of truth about it also. I am assuming that the moisture that ends up in wood as a result of the humidity in the air is in liquid form and can distribute itself by capillary action or other measures. The moisture content of wood (and it seems most materials) is largely controlled by the RH of the atmosphere around it. If the RH is out of balance with the moisture content of the material then the material will release or liberate vapour to reach equilibrium.

Here's a thought... I surmise that vapour permeability tests take place in an environment where the temperature is the same each side of the test material. Under these conditions the RH and the vapour pressure are strongly related i.e. high RH is high vapour pressure. The test can not, and does not, distinguish between vapour diffusion through connected air pores or the capillary movement of moisture content from the wet side to the dry side - there is just one number (e.g. the Sd value).

Lets take an extreme example: a sheet of osb boarding up a broken window in a heated house (which is sheltered from the rain). Lets say is 6 Deg C and 100% rh outside and 20 DegC and 50% rh inside. Classic vapour pressure calcs will show that moisture will be driven from inside to outside because the inside vapour pressure is 11.68hPa and the outside vapour pressure 9.34hPa. But this can not be right - the moisture content of the osb overall will be appropriate to the average rh value inside and out (75%) the outside layer of the osb skin will have a higher mc value and the inside surface a lower mc - what will be happening is the OSB will absorb moisture from the outside and evaporate moisture on the inside in an attempt to reach its equilibrium moisture content - against the vapour gradient.

But this is an extreme case - if the osb is insulated on one side or the other (or both) the temperature will be more or less the same on both sides of the OSB and normal vapour pressure rules apply. (until condensation occurs then a whole raft of other phenomena come into play).