Green Building Forum - Article challenging the benefit of breathability Wed, 03 Jun 2026 06:07:04 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87619#Comment_87619 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87619#Comment_87619 Mon, 23 Aug 2010 09:53:18 +0100 fostertom
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.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87623#Comment_87623 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87623#Comment_87623 Mon, 23 Aug 2010 11:19:47 +0100 Peter Clark 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]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87642#Comment_87642 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87642#Comment_87642 Mon, 23 Aug 2010 13:39:43 +0100 fostertom
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.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87774#Comment_87774 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87774#Comment_87774 Wed, 25 Aug 2010 00:31:48 +0100 Viking House
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?]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87793#Comment_87793 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87793#Comment_87793 Wed, 25 Aug 2010 11:05:23 +0100 fostertom
However, your MHRV is basically trying to fill your interior with the same air as outside, albeit at raised temp. So the incoming air will have exactly the same concentration of water vapour molecules as outside. But when it arrives, there's almost always more water vapour being added, from interior sources. So I'd say that your humidistat can run the MHRV for as long as it likes, but internal concentration will always be higher than outside, so there will always be a PVP gradient from inside to outside.

When a lot is being generated inside e.g. while showering, then that gradient will be v high locally, so any pinholes in the bathroom wall's VCL will be pumping masses of water vapour into the insulation - which the puny MHRV airflow will take a long time to dilute away.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87822#Comment_87822 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87822#Comment_87822 Wed, 25 Aug 2010 16:39:50 +0100 Peter Clark 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.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87863#Comment_87863 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87863#Comment_87863 Thu, 26 Aug 2010 01:59:45 +0100 Viking House Posted By: fostertomPVP is proportional not to the difference in RHs, but to the difference in absolute concentration of water vapour molecules, in the 'from' and 'to' zones. Whether that's concentration by volume, by mass or by moles or something, my A-level Chemistry is too rusty. That concentration, for given RH at given temp, can be looked up in psychrometric tables/charts.

However, your MHRV is basically trying to fill your interior with the same air as outside, albeit at raised temp. So the incoming air will have exactly the same concentration of water vapour molecules as outside. But when it arrives, there's almost always more water vapour being added, from interior sources. So I'd say that your humidistat can run the MHRV for as long as it likes, but internal concentration will always be higher than outside, so there will always be a PVP gradient from inside to outside.

When a lot is being generated inside e.g. while showering, then that gradient will be v high locally, so any pinholes in the bathroom wall's VCL will be pumping masses of water vapour into the insulation - which the puny MHRV airflow will take a long time to dilute away. I 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!]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87873#Comment_87873 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87873#Comment_87873 Thu, 26 Aug 2010 09:15:59 +0100 fostertom Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87874#Comment_87874 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87874#Comment_87874 Thu, 26 Aug 2010 09:19:34 +0100 fostertom Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87883#Comment_87883 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87883#Comment_87883 Thu, 26 Aug 2010 10:21:20 +0100 Viking House 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.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87897#Comment_87897 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87897#Comment_87897 Thu, 26 Aug 2010 11:01:15 +0100 fostertom Posted By: Viking HouseCan 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?It's the latter.

If your MHRV is set to keep on motoring till internal RH falls to a low figure, then you shd I think look again at that principle of control. The MHRV can keep on running forever but it can never lower internal water vapour molecule concentration to below whatever the concentration currently is in the outdoor air that it's importing (unless your MHRV is actually an air conditioner actively removing water vapour from the incoming stream). Once internal water vapour generation is added into the picture. internal concentration can only be higher than current external.

The incoming water vapour molecule concentration translates into a certain RH, depending on current internal temp. Let's say that's 50%. You say
Posted By: Viking House
We have a humidity monitor on our FiWi HRVhttp://www.viking-house.ie/fine-wire-hrv.html" >http://www.viking-house.ie/fine-wire-hrv.htmlwhich keeps ventilating til the internal RH% drops to 40% @ 20 degrees
but actually a humidistat senses RH irrespective of temp. So if your MHRV is set to keepsgoing till RH falls to 40%, it can keep going ineffectively forever - until either internal temp rises (which automatically results in lower RH for unchanged water vapour molecule concentration), or external/incoming water vapour molecule concentration drops.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87917#Comment_87917 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87917#Comment_87917 Thu, 26 Aug 2010 13:06:15 +0100 djh 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.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87926#Comment_87926 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87926#Comment_87926 Thu, 26 Aug 2010 13:31:58 +0100 jules
The key issue is the absolute amount of water in the air, and the relative humidity is the ratio of that amount to the maximum amount that the air can hold (warmer air holds more).

DJH has explained this above very well, but here are the actual numbers: hoping the formatting works, the numbers below show the maximum amount of water that air at a particular temp can hold - add more water or lower the temp and water will condense out:
-10 ºC – 2.1 g/m3
0ºC – 4.8 g/m3
5ºC– ~7 g/m3
10ºC– 9.4 g/m3
15ºC– ~12 g/m3
20ºC– 17.3 g/m3


As I said, RH is the ratio of the actual water content to the relevant number above.

So using djh's example - air at 10C containing 6.6 g/m3 water is at 70% RH (ie 6.6/9.4 x 100). Raise the T of that air to 20C, and its RH drops to 6.6/17.3 x 100 or 38%. Cool it to about 4C (eg on a cold surface) and RH will reach 100%, and you'll see condensation.]]> Article challenging the benefit of breathability https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87950#Comment_87950 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=4579&Focus=87950#Comment_87950 Thu, 26 Aug 2010 16:59:26 +0100 Viking House