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    •  
      CommentAuthordjh
    • CommentTimeMar 1st 2018 edited
     
    The present cold weather is a good opportunity to explore humidity buffering a bit more, I think. Using a calculator like http://www.dpcalc.org/ suggests that at present

    ext temp -2°C and RH 60% implies a dewpoint of -9°C, which given my internal temperature of between 20-21°C implies an internal RH of 12-13%. But the actual RH is 37%. It has been decreasing by maybe a half to one percentage point per day over the past few days. So clearly there is some considerable humidity buffering going on, contrary to thoughts that MVHR means the internal humidity just tracks that outside.

    Now I've found https://www.lenntech.com/calculators/humidity/relative-humidity.htm and that suggests that I need internal gains of 250 mL/hour to attain 40% RH or 200 mL/hour for 37% RH at my current ventilation rate of 0.2 ACH.

    Does anybody have any idea how to estimate internal gains and/or how to estimate the humidity being supplied by my walls and furniture*?

    *Actually, it won't be furniture so much as the wool carpets I expect.
    • CommentAuthortony
    • CommentTimeMar 1st 2018
     
    Very difficult to use buffering to increase humidity to that extent without spilling water on the carpets.
    • CommentAuthorgoodevans
    • CommentTimeMar 1st 2018
     
    Yep there must be some buffering - and a lot of it - but perhaps that shouldn't be a surprise. I would guess that all 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) - I need to do some calcs to see how much moisture is in those materials per % RH for a typical building (in litres) and how fast it can be drawn out. From there we can see how much vapour might be coming out of the fabric.

    It might be worth placing the sensor in the MVHR supply stream to check that the sensor can record lower RH values. It should be rock bottom in that air stream.

    I was looking at the WUFI validation data the other day - one of the tests had a humidity sensor just the other side of plasterboard and before the VCL - it looked like even here Wufi modeled a 'half life' for humidity change of the order of a few weeks (and the model appeared to be a good match to reality once it had settled down).

    For juggling between different humidity measures I have found this site easy to use...
    https://www.cactus2000.de/uk/unit/masshum.shtml

    I don't suppose you have long term external and internal temp/humidity data that I can play with.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 1st 2018
     
    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?!).
    • CommentAuthorgoodevans
    • CommentTimeMar 1st 2018
     
    I agree Tom - in-fact perhaps winter is the ideal time for drying a heated house (or at least the inside half of it).

    So far the only material I have numbers on is wood - in my house each truss will weigh 140kg - lets say that the humidity ranges from 37% to 80% - the equlibirium moisture content of the wood would be approx 7% to 16% for that range - I make that 12.6 litres of moisture per truss. I'll see what the EMC values are for other materials to see where we get to.
    •  
      CommentAuthordjh
    • CommentTimeMar 1st 2018 edited
     
    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 :bigsmile:
    •  
      CommentAuthordjh
    • CommentTimeMar 1st 2018
     
    Posted By: goodevansYep there must be some buffering - and a lot of it - but perhaps that shouldn't be a surprise. I would guess that all 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) - I need to do some calcs to see how much moisture is in those materials per % RH for a typical building (in litres) and how fast it can be drawn out. From there we can see how much vapour might be coming out of the fabric.

    Well, there's no brick in the building but apart from that I would agree. I don't believe there's much moisture held in things like plaster, though I need to check for the lime plaster. I've always worked on the basis that straw probably behaves much like wood but on somewhat faster timescales because of the increased surfaces. Your suggestion has motivated me to find a few extra papers, but I haven't read them yet. I'll post details if they're any good.

    It might be worth placing the sensor in the MVHR supply stream to check that the sensor can record lower RH values. It should be rock bottom in that air stream.

    Good idea, but not terribly practical at the moment although I do have a longer term plan to do exactly that. And others in the other main ducts so I can tell when the duct heater is on and what its doing and what the external temperature is etc. In the meantime I just moved a meter to on top of a floor supply valve, which should be fairly close to the main supply conditions. In doing so, I realized that the meter is normally sat on a bookshelf amongst a load of paperbacks so they might well be affecting its microclimate. I'll put it somewhere else for a while once I've measured the supply air.

    I was looking at the WUFI validation data the other day - one of the tests had a humidity sensor just the other side of plasterboard and before the VCL - it looked like even here Wufi modeled a 'half life' for humidity change of the order of a few weeks (and the model appeared to be a good match to reality once it had settled down).

    Gypsum plaster I presume? With no service cavity? That result does surprise me. Was the plasterboard paper-faced?

    For juggling between different humidity measures I have found this site easy to use...
    https://www.cactus2000.de/uk/unit/masshum.shtml

    Thanks for the pointer.

    I don't suppose you have long term external and internal temp/humidity data that I can play with.

    Not particularly. I'm somewhat embarrassed by my lack of recorded data. I had all sorts of grand plans but ... other distractions.I have a bunch of temperature and humidity sensors embedded in the walls and roof, connected to a logger. I have hourly data from them from 2015-11-11 to 2016-03-28 but then the battery ran out. The logger didn't restart when I changed the battery and I haven't investigated since :shamed:

    Since then I've just used various non-recording meters to get an idea of internal and external conditions. Again, I have plans to build a recording system one fine day...
    •  
      CommentAuthordjh
    • CommentTimeMar 1st 2018 edited
     
    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.
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2018 edited
     
    Concrete 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.

    for concrete source... google "concrete emc rh" and look at the google books reference (second entry down).

    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.

    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

    DJH, What is the mass of the straw in your walls by the way (back of envelope calc)?
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2018
     
    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.
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2018
     
    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).
    •  
      CommentAuthordjh
    • CommentTimeMar 2nd 2018
     
    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.
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2018 edited
     
    So 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.

    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.

    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.

    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.
    •  
      CommentAuthordjh
    • CommentTimeMar 2nd 2018
     
    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.
  1.  
    DJH, you started a similar thread a while back: http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14682

    At that time you were apparently extracting 9kg/d of water without humidity change. Has the straw dried out since then, or did it just reabsorb the water in the spring?

    I suggested an alternative theory, which might or might not be the explanation:
    " AIUI any porous 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, against the prevailing vapour pressure gradient, a capillary action similar to many biological systems. I don't know how to estimate how many kg/d this brings into the house but sure someone will."
  2.  
    •  
      CommentAuthordjh
    • CommentTimeMar 2nd 2018 edited
     
    Grr, second time of posting. An extremely annoying feature of either FF or this forum, not sure which, is that it depopulates input fields if you go somewhere else and then hit Back. Oh well.

    Posted By: WillInAberdeenDJH, you started a similar thread a while back

    Ooh, so I did. Same title too. How embarrassing! edit: I changed the title of this thread so we can identify them more easily.

    At that time you were apparently extracting 9kg/d of water without humidity change. Has the straw dried out since then, or did it just reabsorb the water in the spring?

    I think it was still drying out at that time.

    I suggested an alternative theory, which might or might not be the explanation:
    " AIUI any porous 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, against the prevailing vapour pressure gradient, a capillary action similar to many biological systems.

    Ah, now that is an interesting theory, which is worthy of further study, I think.

    Thinking cap on.
    •  
      CommentAuthordjh
    • CommentTimeMar 2nd 2018
     
    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.
    • CommentAuthorgoodevans
    • CommentTimeMar 2nd 2018 edited
     
    Posted By: djhare you comfortable at that sort of RH?
    Strangely no problem at all - the only disadvatage is the static charge off the man made carpet.

    I read that a dry atmosphere could cause problems with flying dust etc - I have a dust mite allergy (but low level) and a very dusty house - (hoovered out a pc today after 1 year of use - there was so much dust in it the cpu heat sink fins were completely full of dust - no air flow at all) - even so no adverse symptoms (except when hovering which generate a sneeze or two without fail).

    I didn't bother checking the RH sensor when this subject first came up because I thought it was in error as it had been stuck on 33% for a few days earlier in the year- it looks like that was probably the true internal RH at that time. Perhaps it wasn't windy (thus reducing the air changes) and the RH reflected a longer term winter month humidity.
    • CommentAuthortony
    • CommentTimeMar 5th 2018
     
    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
    •  
      CommentAuthordjh
    • CommentTimeMar 5th 2018
     
    Posted By: tonyThis is very wrong

    No, it's not. We're citing actual facts that are not consistent with what you claim is happening. Incidentally, it isn't partial vapour pressure that determines vapour motion; it's the concentration gradient, i.e. difference in specific humidity.

    The reason you're confused is that you're considering only vapour movement. We are also discussing liquid water movement and that can be driven by RH in hygroscopic materials.
    • CommentAuthortony
    • CommentTimeMar 5th 2018
     
    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
    •  
      CommentAuthordjh
    • CommentTimeMar 5th 2018 edited
     
    tony opined: "I think that is the difference in partial vapour pressures that drive vapour movement"

    As I said, I think you're wrong. Fortunately these people agree with me. Or rather I agree with them:
    http://orbit.dtu.dk/files/103642170/hansen_moisture_transport_NSB14.pdf

    "The concentrations proved to be more equal on either side of the specimen than the partial vapour
    pressures. This supports an argument that it is concentration difference that drives diffusion of gases.
    Isothermal diffusion cannot be tested experimentally in this way, but it is reasonable to assume that
    concentration is the driving potential. The close equality of the concentrations makes it unnecessary
    to invoke temperature difference as a third possible potential for driving diffusion."

    The paper was written to record experiments that specifically set out to determine what the driving potential is, to resolve disagreements between published literature:

    "The purpose of this paper is to test – with a focus on the best possible accuracy of the experimental
    set-up and method – if either the partial water vapour pressure, or the water vapour content adequately
    explains the observed diffusion in a temperature gradient, without invoking temperature as a necessary
    participant in driving the process."

    Dew point isn't particularly relevant for what we're discussing, I don't think.
    •  
      CommentAuthorfostertom
    • CommentTimeMar 6th 2018
     
    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).
  3.  
    Diffusion (more usually convection in gases) is driven by Potential gradient, for which Partial Pressure is a good approximation in many but not all situations, especially not with large temperature differences. Concentration gradient is a better approximation. If there is a strong electric or gravitational field, then all bets are off.

    Adsorption/buffering/capillary transport in porous materials is empirically related to Relative Humidity, not directly to Partial Pressure or Concentration. That's what this thread title is about.


    I read through this paper and its spot on with the qs in the OP. For their example house, about one-third the moisture generation came from the occupants, and two-thirds from seasonal humidity storage in the timber and concrete. Over the first few years this fell to half-and-half from seasonal storage and from occupants, directionally as DJH reported in his house. Lots of numbers to help calculations.

    https://www.labenvironex.com/en/environment/bacteria-and-mould-analyses/moisture-sources-in-houses/
    •  
      CommentAuthorfostertom
    • CommentTimeMar 6th 2018
     
    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?
    • CommentAuthorgoodevans
    • CommentTimeMar 6th 2018
     
    Sorry I've been away a few days - to within a few mm of a gnats whisker you are all correct.

    Tony - you are right RH is no good at working out in which direction vapour goes. However...

    Partial pressure, volume mixing ratio, specific humidity and absolute humidity measure exactly the same thing - just in different units. If the vapour partial pressure is say 25.335hPa then -regardless of the temperature and RH there will be 15.7g/kg specific humidity and 15.951g/kg absolute humidity and 25.004 units of volume mixing ratio (i'm ignorant on these units). Psychometric Charts often show more than these unit sharing the right hand side.

    If the density of air didn't change with temperature then the g/m3 would also be treated the same - however as the density does not change significantly over the temperature ranges we are talking about it can also be used instead of partial vapour pressure. But the convenient thing here is air flow devices such as mvhr usually gives air flow in m3/hour so we can work out the extraction rate of moisture.

    At the end of the day we are talking about gas diffusion - i.e. any gas will try to diffuse from a high concentration to a low concentration. And the concentration is determined by partial pressure/volume mixing ratio etc etc.
  4.  
    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.
    • CommentAuthorgoodevans
    • CommentTimeMar 6th 2018
     
    Will, That is a cracking paper about moisture in houses - in short, unless your house is built in plastic, there will be a sizable chunk of moisture coming out of the fabric of the house over the winter months. It does explain djh's observation that his RH is not as low as he expected and I'm cogitating on the effectiveness of VCL's as advertised - (in effect a VCL forces any summer stored humidity to have to pass to the outside only) and where I have been surprised is the quantity of water stored between seasons - particularly in heavy stuff such as concrete, blocks, and plaster But a timber framed house stores a massive amount in the wood.
    • CommentAuthorgoodevans
    • CommentTimeMar 6th 2018 edited
     
    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).
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