Posted By: fostertomThanks Viking

To add to that, when water vapour is squirting through that pinhole in a VCL from inside to outside, if it finds something cold to condense on after squirting through to outside, then water is continually taken out of the vapour phase there, as it becomes liquid. This has the effect of keeping the water vapour concentration low, outside, relative to the inside concentration, so the PVP differential is maintained and never equalises, so the supply of water vapour to the condensation surface keeps coming, and liquid water keeps on condensing out.

This is how a pinhole functions as a very active one-way pump of water vapour, and can pass great quantitie, that create much liquid water.

And...(I am still foolishly trying to keep up with Tom)

If the cold something it condenses on is plastic, or is not significantly hygroscopic and vapour permeable and capillary open, the liquid water will not be dispersed but will pool somewhere, and will probably stay in that interstitial space, and cause lots of damage to the insulation value of the wall, cause mold to grow and/or damage the structural integrity of the wall.

But if that cold something is natural and IS hygroscopic etc, then that water will be dispersed throughout that element and will travel to the outside (and perhaps the inside) by vapour permeability and by capillarity, and may well evaporate safely. Not compromising the structural or thermal integrity of the element, or going moldy. And this is achieved by natural, minimally processed materials such as clay, straw, timber etc. in principle available locally almost everywhere.
Of course, the challenge is to ensure by design and implementation that not too much water gets in from inside, or outside.

Peter

Hi Tom

We have a humidity monitor on our FiWi HRV http://www.viking-house.ie/fine-wire-hrv.html which keeps ventilating til the internal RH% drops to 40% @ 20 degrees, so with an outside temperature of 5 degrees @ 75%RH, how low would the internal RH% have to drop to stop the PVP flow?

Posted By: fostertomSounds like you're "keeping up" well! And I'm no expert, just ivory-tower extrapolation from A-level Physics 43yrs ago!

All you say is right, but to be pedantic ...

Posted By: Peter Clarkif that cold something is natural and IS hygroscopic etc, then that water will be dispersed throughout that element and will travel to the outside (and perhaps the inside) by ... capillarity

by capillarity Yes, by vapour permeability No. Only if conditions become right (the summer after?) and that dispersed water evaporates will it

Posted By: Peter Clarktravel to the outside (and perhaps the inside) by vapour permeability

And if there's a strong inboard VCL (regardless of pinholes) that water won't

Posted By: Peter Clarktravel to the ... inside

And once you're using

Posted By: Peter Clarkhygroscopic and vapour permeable and capillary open

materials, a strong inboard VCL is unnecessary anyway, provided you organise them into a breatheable sequence, obeying the 5:1 ratio of inboard to outboard water vapour resistivities.

That is a complicated post Tom, i don't know whether you are saying something important or merely being pedantic as you suggest.

If some water condenses inside an external wall on a cold winter night, it may very well start to evaporate the next morning and not have to wait until next summer?

When it does, getting to the outside will be aided by the hygroscopicity and capillarity of the wall element, these properties of the wall will mean that water, having condensed, will be taken up by the wall material and distributed throughout instead of being pooled or concentrated in one place. Therefore when the time to evaporate comes, it will be ecvaporating over a wider surface.

If we do not use a VCl and do have a 5:1 ratio, it seems highly likely that any vapour will move in the inward direction, yes. i do think it is possible that some evaporation may occurr inwards sometimes, in extreme conditions. I cannot decide whether that might be a good thing - helpingto dry out the wall, or a bad thing - damaging the inward wall element - osb ETC.

I don't think it works that way - AFAIK it's as I said - you'll never get internal concentration of water vapour (regardless of temp/RH) down quite as low as external, when your only means of doing so is by importing that same external air.

Isn't external 10 degree air @ 65%RH much drier than 20 degree 65%RH internal air. So I'm not sure I agree with your statement ;

Posted By: fostertom"So the incoming air will have exactly the same concentration of water vapour molecules as outside"

We know that warm air can hold more water vapour and if you have warm humid air it will dump moisture as it cools down. Isn't this why we see fog in the morning around rivers, the fog disappears when the air heats up.

Can somebody explain the Relative in Relative Humidity? is it relative to the temperature or relative to the maximum vapour the air can hold at that temperature?

The way I was looking at PVP was if I reduced the RH% in a house using FiWi HRV or whatever to a level where the building fabric can easily deal with the PVP moisture load, we were safe enough. There is a big difference in the amount of moisture in 65%RH 20 degree air than in 45%RH 20 degree air.

Posted By: Viking HouseI was told (now I'm a builder not a scientist) that if you lift the temperature of 10 degree 70% RH external air by 1 degree then the RH drops to 63%. Now this is the bit I don't understand, what if I rise the temperature of the 70%RH external air by 10 degrees, does it then become to 0% RH air?

So if what I am told is true then 40%RH 20 degree internal air should balence 85% RH 10 degree external temperature air as the PVP would be balenced!

What you've been told appears to be wrong, but only a little. But the answer to your question is a big no. The most important point is that RH doesn't follow a straight line; it isn't linear. You need to do complicated sums or look it up on a graph.

You can read about relative humidity in lots of places and which one is best will depend on your background and what style of writing you like. So searching with google is probably best. "Dewpoint" might be a good keyword.

Looking for articles with graphs is probably a good idea. I also find it helpful to play around with calculators like this one http://www.decatur.de/javascript/dew/index.html or this one http://www.dpcalc.org/

Using those I believe that if you lift the temperature of 10 degree 70% RH external air by 1 degree then the RH drops to 66% or so. While 40%RH 20 degree internal air balances 76% RH 10 degree external temperature air. Note that the problems with dry internal air occur when the external temperatures are much lower; for example, raising 100% external air from 0 degrees to 20 degrees drops the RH to 26%.

If you raise the temp of 70% 10 degree external air by ten degrees, it drops to 38% RH.

Thanks guys, that's excellent information, I understood the principle but its great to see the figures, great mix of practical and science here.