Green Building magazine: Article challenging the benefit of breathability

31 to 60 of 224

- CommentAuthorMike George
- CommentTimeSep 24th 2009 edited
I can see why some would call this a warm roof, though strictly speaking I don't think it is. I'll see if I can find a good definition somewhere

They seem to be advocating something similar to the between rafter scenario I gave above but have a condition for both VCL [warmside] and breather membrane [coldside]. Also a recomendation for a Condensation risk analysis. Personally I think this one's a bit risky especially if any rainwater gets in and exsacerbates the condensation risk.

Also don't forget there is much more aiflow over a roof than under a sub-floor
- CommentAuthorskywalker
- CommentTimeSep 24th 2009
Mike

Lukewarm?

Anyway we are off topic, it's your thread, I have so many questions on this topic I do not know where to start.

S.
- CommentAuthorTimber
- CommentTimeSep 24th 2009
I read the article.

Thought it was pants.

Nuff said

Timber
- CommentAuthorMike George
- CommentTimeSep 24th 2009
Posted By: skywalkerMike

Lukewarm?

S.

LOL
- CommentAuthorMike George
- CommentTimeSep 24th 2009
Posted By: TimberI read the article.

Thought it was pants.

Nuff said

Timber

No constructive criticism then?
- CommentAuthorCWatters
- CommentTimeSep 24th 2009 edited
Anyone want an FAQ on warm roof vs cold roof. Note this has nothing to do with the temperature of the loft space...

Warm roof
All rafters/joists are on the warm side of the insulation. (eg where condensation isn't a risk), Great for barn conversions where exposed rafters are required. Generally recommended for replacement flat roofs. Fewer gaps so insulation is more efficient.

Cold roof

Insulation is either between rafters or between the floor joists (eg of a loft). The two cases are different but the principles are the same - in both cases part of the structural timbers are on the cold side and must be ventilated.

Insulation between the floor joists: The whole loft is ventilated at the eaves allowing air to flow across the loft above the insulation.

Insulation between the rafters: The cold side must be ventilated. This can be done two ways depending on the type of membrane used. If the membrane is not vapour permeable (eg most old membranes) then a 50mm ventilated cavity under the membrane must be formed. Vents at the eaves and ridge must be inserted through the membrane. If a vapour permeable membrane is used a ventilated cavity is not required. However an unventilated cavity may still be required! If you want to fully fill the rafters you must check that both the insulation and the membrane manufacturers are happy with this.

There are a bunch of other issues. For example it might be important that the membrane sags between the rafters... This allows water to run down the outside. If the membrane doesn't sag water can collect just above the tile battens causing them to rot. So if you fully fill the rafters with insulation you may need to use counter battens on the outside to raise the tile battens off the membrane.

In all cases a VCL on the warm side of the insulation is used.
- CommentAuthorfostertom
- CommentTimeSep 24th 2009
Posted By: CWattersIn all cases a VCL on the warm side of the insulation is used
Not necessarily - think breatheable. It's well tried, in UK at least.
- CommentAuthorTimber
- CommentTimeSep 24th 2009
Mike - Yes and no.

The article mainly focused on masonary constructions to which I have no comment. As for timber frame, all insulation materails should be breathable to ensure longevity of the timber structure.

I think that some people confuse a breathing wall with breatability. i.e a wall with a vcl on the warm side of breathable insulation will allow the wall to breathe (out). A breathing wall (in my view) either uses a vapour perimable vcl or no vcl at all.

All walls need to be able to breathe (unless it is a true warm wall) but a breathing wall is not always required.

Timber
- CommentAuthorfostertom
- CommentTimeSep 24th 2009
Posted By: Timbervapour perimable vcl
These are expensive and sophisticated - think of all the extra work drilling holes all over a bucket! (how do you do a 'wink' smiley?)
- CommentAuthorali.gill
- CommentTimeSep 24th 2009
I believe the point timbers making is that a vapour control layer is not actually a fully fledged vapour barrier, there is still some element of vapour passage through a vcl.
i'm sure we had a discussion about this before so the thread should show from a search otherwise i'll try and find the doc i referenced before.
- CommentAuthorMike George
- CommentTimeSep 25th 2009 edited
Posted By: TimberMike - Yes and no.

The article mainly focused on masonary constructions to which I have no comment. As for timber frame, all insulation materails should be breathable to ensure longevity of the timber structure.
...unless you live in Washington

Seriously though, I would not use PUR in a timber frame building, as there are easier [and some would say more effective] ways. Though as you say this is not the focus of the article. Neither is the intricacies of VCL's and their position or omission in such structures.

It's about the alleged positive effect that breathability has on internal air quality.
- CommentAuthorskywalker
- CommentTimeSep 25th 2009 edited
Not having read the article I am at clear disadvantage!

If the subject is focussed we are asking/debating a fairly tight question:

Does a truly breathable structure (no effective vapour barrier in the commonly accepted sense), where the passage of moisture freely through all elements of the structure is facilitated by the materials used, provide better air quality than a truly sealed structure (an effective vapour barrier in the commonly accepted sense), where the passage of moisture is freely through all elements of the structure is resisted by the materials used; where all other factors affecting air quality are equal & within proscribed limits for occupant health.

Or something like that.

S.
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
Posted By: TimberA breathing wall (in my view) either uses a vapour perimable vcl or no vcl at all
Posted By: ali.gilla vapour control layer is not actually a fully fledged vapour barrier, there is still some element of vapour passage through a vcl.
There's a real absence of VCLs (as in membrane) that have a known, real, but very weak water vapour resistance, such as would be needed inboard in a breathing construction, where the outer layers are vapour-open and the inner layer needs to be about 5x as resistive - i.e. still weakly resistive. The 'vapour check' resistance of e.g. foil-backed plasterboard, though far from being a reliable full vapour barrier, can vary by a huge margin depending on detailing and careless installation, but always much too strongly resistive for breathing construction. So, timber, what do you have in mind as such a 'vapour perimable vcl'?
- CommentAuthorTimber
- CommentTimeSep 25th 2009
Refective breather membranes actaully have a fairly high moisture vapour resistance.

Tyvex Reflex, for instance, has a Mu value of 480, where-as OSB has a Mu value of around 180 (ish).

Therefore, you could actually use Tyvex reflex breather membrane as a vcl, IF your outermost material had a Mu of 100 or less.

For instance, most wood fibre insulation boards have Mu of less than 100, so OSB (180) and tyvex reflex (480) taped and jointed etc on the inside. 300 mm of warmcel and then 50 mm of wood fibre insulation board (<100) on the outside, job done!

Timber

P.s. getting back to the origional topic of air quality. Don't know to be honest.

What really needs to be done is that a 'breathing' wall needs to be put into a Hygrothermal test chamber and the amount of moisture that passes though the wall monitored. The one could see, in reality, how much moisture vapour really does pass though a breathable wall. That could then be compaired to moisture vapour carried in ventillation systems.

Timber
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
Yes, even the main sponsors of breatheability, Excel/Warmcel, don't seem to have any solid research to back it up. I've asked, and all they have is a v dubious study that Wimpey did on 2 comparison houses - shades of Actis!

Anyway, if it's true that there's 100 x as much water 'bound' in the structure of an average house, as there is within its internal atmosphere, then the whole idea that interstitial condensation is about controlling the internal water vapour's passage through the structure, must be nonsense - it's trivial compared with the seasonal ebb and flow of what's already in the structure. The Euler diagram approach, as embodied in BuildDesk etc does seem to give reliable enough results - but that must be for the 'wrong' reasons. A lot of work in Hygrothermal test chamber, as Timber suggests, and/or whole-season simulation in WUFI or similar, using actual weather data and internal climate data, could lead to a radically different understanding.
- CommentAuthorskywalker
- CommentTimeSep 25th 2009
I'm almost speechless.

S.
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
But not quite, surely - give us a clue?

Posted By: skywalkerDoes a truly breathable structure (no effective vapour barrier in the commonly accepted sense), where the passage of moisture freely through all elements of the structure is facilitated by the materials used, provide better air quality than a truly sealed structure (an effective vapour barrier in the commonly accepted sense), where the passage of moisture is freely through all elements of the structure is resisted by the materials used; where all other factors affecting air quality are equal & within proscribed limits for occupant health.
No - that's not the purpose of breathability.
- CommentAuthorskywalker
- CommentTimeSep 25th 2009
???
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
Great debating technique!
- CommentAuthorPeter Clark
- CommentTimeSep 25th 2009
Posted By: fostertomNo - that's not the purpose of breathability.

My understanding of this:

The purpose of a â€˜breathable wallâ€™ is to protect the structural and thermal integrity of the wall, and to avoid mould growth within the wall that might compromise internal air quality.
To achieve this it is best if the materials of the wall are hygroscopic, vapour open and have good â€˜capillarityâ€™.

This means that moisture can and will enter the wall, from outside or inside, and may condense there (but this is reduced by the hygroscopicity), but it can also get out again.
To minimise the moisture from inside, the gradient in diffusivity of materials from inside to out is used (5 fold?). To minimise moisture from outside entering the wall, overhangs, drip details, cladding and renders with ventilation and capillary closed (but vapour open) paints etc. on the external face.

See this document in general:

http://www.natural-building.co.uk/PDF/Case%20Studies/Breathability_in_buildings.pdf

Especially the discussion starting â€˜So, for example, a material such as mineral wool insulation is indeed very vapour open...â€™ on pages 13 and 14.

Peter
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
V gd summary Peter, except breathability doesn't require hygroscopicity (or caplllarity) in its materials.
Hygroscopicity is something else, very beneficial, which doesn't benefit the interior (or anything) if there's a significant VCL between it and the interior.
So the hygroscopicity thing requires breathability, but breathability doesn't require hygroscopicity.
Not sure how capillarity is ever beneficial - that's materials' ability to attract, soak up and transport liquid water - is that ever good? Whereas hygroscopicity is materials' ability to do the same for water vapour.
- CommentAuthorali.gill
- CommentTimeSep 25th 2009
I received an email from WUFI regarding workshops in English which those with deep pockets and a serious interest in this subject might be interested in.

We'd like to give you some more information on the English WUFI seminars in
Holzkirchen in December.
You can now download the detailed program for the different workshops from our
homepage:
WUFI Standard Course:
http://www.hoki.ibp.fhg.de/download/temp/WUFI_Standard_Course_2009.pdf
WUFI Advanced Workshop:
http://www.hoki.ibp.fhg.de/download/temp/WUFI_Advanced_Workshop_2009.pdf
WUFI-2D Course:
http://www.hoki.ibp.fhg.de/download/temp/WUFI_2D_Workshop_2009.pdf
WUFI-Plus Course:
http://www.hoki.ibp.fhg.de/download/temp/WUFI_Plus_Workshop_2009.pdf

Until 16th of October you can register for a reduced registration fee:
Each two-day workshop
Normal registration: Euro 680,- (US-$ 850) Early registration: Euro 580,- (US-$ 725)
Package of 2 workshops
Normal registration: Euro 1200,- (US-$ 1500) Early registration: Euro 1000,- (US-$
1250)
Package of 3 workshops
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1650)
- CommentAuthorPeter Clark
- CommentTimeSep 25th 2009 edited
Posted By: fostertombreathability doesn't require hygroscopicity (or caplllarity) in its materials.

Does it depend how you define â€˜breathabilityâ€™? You can say, as you are, that â€˜breathabilityâ€™ means only vapour openness, but in practice is this not always enhanced by hygroscopicity and capillarity? Hence Neil May referring to:

â€˜All three of the â€œbreathabilityâ€ qualities make a significant difference to the performance and therefore the design of insulation in timber frames and roofs..â€™

and

â€˜a material such as mineral wool insulation is indeed very vapour open compared to plastic insulations, particularly closed cell insulations such as polyisocyanate boards. It is not however â€œbreathableâ€ in the same way as timber or clay, or indeed as natural fibre insulations.â€™

Posted By: fostertomSo the hygroscopicity thing requires breathability

Neil May again
...it is assumed by many people that a paintâ€™s vapour permeability seriously affects the hygroscopic performance of the rest of the wall. In practice this may not actually be the case.

Posted By: fostertomNot sure how capillarity is ever beneficial - that's materials' ability to attract, soak up and transport liquid water - is that ever good?

Poor neil is working hard here:

All natural fibre insulations dry quickly, because of the better capillary mechanisms.
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
All good points - some terminology to be sorted here, but understanding's improving, every time we hash over the subject. I say GBF is my University of the Air!
- CommentAuthorPeter Clark
- CommentTimeSep 25th 2009
Posted By: fostertomunderstanding's improving, every time we hash over the subject

completely agree Tom, always a pleasure to hear what you have to say.

Peter
- CommentAuthormarktime
- CommentTimeSep 25th 2009
Tom, if the dew point exists within the external brick for an internal insulation solution, the capillary mechanism of the clay brick is surely important. Or am I missing something?

Or perhaps the water vapour never gets a chance to condense as there is some subtlety in latent heats i.e. it would have to lose heat and therefore increases the energy in its surrounding medium and so perhaps it never condenses but is merely dispersed hygroscopically?
- CommentAuthorskywalker
- CommentTimeSep 25th 2009
Don't know what happened with the ? mark thing other than a pile of post dumped on my keyboard by - -

er

Me.

Thanks for the response Tom just one more myth ruled out so for - very good.

Getting my head round all the links & thinking like a water molecule.

I'll be back!

S.
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
marktime, yes, brick cav walls are weird - for a start, there's a routinely-done breatheable wall for you, to those who say it's unnecessary, or unsafe. Seems the outer leaf of a cav wall is usually left to its own devices - however sopping wet it gets, doesn't matter as it's not touching anything.

Interesting about latent heat - never occurred to me that interstitial condensation raises the temp locally, which you'd think wd postpone the dewpoint hence the onset of condensation, but it's the condensation that caused the temp raise, which .... etc. Does the Euler diagram method take this into account? I can't work out whether this would fuzz out, or sharpen the diagram crossing point at which condensation occurs.

Such temp rise resulting from release of latent heat on interstitial condensation would increase the wall's rate of heat loss, by creating a refreshed, prob quite steep temp gradient in the wall's outer reaches. Another good reason to avoid it, even if it's in a 'safe' position e.g. within a cav wall's outer leaf.
- CommentAuthorCWatters
- CommentTimeSep 25th 2009
Posted By: fostertom
Posted By: CWattersIn all cases a VCL on the warm side of the insulation is used
Not necessarily - think breatheable. It's well tried, in UK at least.

I'm happy with the idea of no VCL on a brick or stone wall or even on something like an oak frame but I think it would be very risky not to use a VCL on a modern softwood timber frame house. That was one of the lessons learnt when modern timber frame construction was first done in the UK...

This is bit of a long read..

"Learning the lessons from systemic building failures" - NHBC

http://www.nhbcfoundation.org/LinkClick.aspx?fileticket=%2ByHbjZ7R3QU%3D&tabid=339&mid=774&language=en-GB

Breathable is ok provided it's done right. I'm concerned that breathable construction is becoming fashionable without people understanding the issues.
- CommentAuthorfostertom
- CommentTimeSep 25th 2009
Posted By: CWattersI'm happy with the idea of no VCL on a brick or stone wall
See post above - an unexpected reason why 'brick or stone' maybe shouldn't be regarded as 'safe'.
Posted By: CWattersor even on something like an oak frame
Why would that be 'safe'? Seems v vulnerable, to me.

Posted By: CWattersvery risky not to use a VCL on a modern softwood timber frame house. That was one of the lessons learnt when modern timber frame construction was first done in the UK...
That was only because the vapour resistance gradient was the wrong way round - ply/felt outboard, plasterboard inboard. As no-one wanted to rethink the moderately-resistant ply/felt outboard, it became necessary to put a really strong resistance inboard - hence the arrival of such VCLs in UK practice. But all that was needed, then as now, was an inboard to outboard resistance ratio of about 5 to 1, which is readily achievable if the outer layers are made vapour-open, then a strong VCL becomes not just unnecessary, but gross overkill, with many disadvantages. And the whole myth that the condensation problem is about separating internal atmospheric water vapour from the fabric, arose from that - well that's my view of it.

Internal membrane VCLs are in my opinion a primitive obsolete solution that originated I believe in 1920s USA, based on an erroneous idea of what's going on with water vapour in buildings. They're almost impossible to install as effectively as theory assumes, and it's beyond belief that they're supposed to survive intact for the life of the building. Breatheable construction may not be any better understood, but it seems to work, is not impossible to install, and isn't liable to early breakdown - it's simple and robust. And has other fringe benefits that are becoming increasingly relevant and beneficial.

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Green Building magazine: Article challenging the benefit of breathability