Posted By: SteamyTeaSo well understood

I wish

Simply put, the more you put in the less effect it has until a step change happens.

Nearly everything in nature follows some sort of exponent curve as there is a time element involved (generally one pert of the exponent, the 't'). The other part of the exponent is the descriptor (often unitless) and describes the system you are testing/observing. That is the bit you changes.
So take a very simple function y=2^(t*0.5) and plot it in Excel.
Play with the variable and see what happens.

Referring back to Viking's comment re. the buffering effect of plasterboard (or lack of) has anyone any suggestions for a practical internal lining to the underside of a habitable roof space (insulated with cellulose). I am beginning to wonder if a decent grade of ply would be OK but could look a bit rustic. Any other boards that would take a breathable paint finish spring to mind ?

Posted By: Cav8andrewReferring back to Viking's comment re. the buffering effect of plasterboard (or lack of) has anyone any suggestions for a practical internal lining to the underside of a habitable roof space (insulated with cellulose).

Fermacell is gypsum fibreboard, so should be relatively vapour open & a reasonable humidity buffer.

David

Posted By: Cav8andrewReferring back to Viking's comment re. the buffering effect of plasterboard (or lack of) has anyone any suggestions for a practical internal lining to the underside of a habitable roof space (insulated with cellulose). I am beginning to wonder if a decent grade of ply would be OK but could look a bit rustic. Any other boards that would take a breathable paint finish spring to mind ?

If you look back at the Fraunhofer report showing the difference between the humidity spikes in a room with no lining and lined with plasterboard, you'll see that the moisture buffering effect of plasterboard is almost as good as the wood based products. But the moisture buffering effect of the wood based products is substantially reduced when used behind plasterboard.
Based on the Spokane and Tsongas study where they opened up the walls of 250 houses in America and found no decay. Based on the Fraunhofer report above. Based on the long discussion I had with the Irish WUFI expert who does the calculations for Joseph Little.
My conclusion; Diffusion is not so important, air-tightness is, install a humidity activated ventilation system. Our "Breathing Window" has flatter humidity spikes and troughs than the wood-fiber wall because the fans ramp up when there's a humidity or CO2 spike and you need ventilation anyway! If the Humidity in a house is kept below 50% in Winter then the Partial Vapour Pressure pushing Water Vapour through a wall in an effort to equalise is reduced by 30-40%.

To go back to the beginning of this topic:

Posted By: Mike GeorgeThere is much discussion here and elsewhere relating to 'breathability' of structures. Critical to how this 'mechanism' works or [or does not work] is the placement of materials based on their relative Vapour Resistance Factor [Mu value]. The understanding of this is however confused by the way in which values are sometimes stated:

Mu values (no units)
Vapour Resistance (MNs/g) or even
Vapour Resistivity (MNs/gm)

are all used as a measures of a materials reluctance to allow water vapour to pass through it/them.

A good document explaining the conversion between these can be found here http://www.builddesk.co.uk/files/BuildDesk_UK/Home/Software%20support/Vapour%20Resistances%20and%20Mu%20values.pdf

This is all very muddled

I find the BuildDesk article unhelpful!

Posted By: fostertomFor a start, could our boffin types take a look athttp://www.springvale.com/uploads/resources/brochures/roofshield.pdf" >http://www.springvale.com/uploads/resources/brochures/roofshield.pdfin which EPS100 permeability is quoted as 0.010 – 0.024 mg/Pa.h.m.
My first question - does mg there mean miligrams, or meter-grams?

I think milligrams.

This way of writing the units is very confusing, I think. A Pa is a N/m² so a Pa·m is a N/m which is a rather odd thing. Better to move the 1/m on the bottom to a metre on the top. I.e., mg·m/N·h. If you push on one milligram of water vapour with a force of one newton for an hour you'll move it one metre through the material. Or, if you push on one gram of water vapour with the same force and for the same time you'll move it one millimetre.

Further, I'd prefer µg·m/N·s (microgram-metres per newton-second) because the hour is not an SI unit¹. Note, this unit is different from there's by a factor of 3.6.

Their thermal conductivity units (W/m²K) on the line above are just wrong.

¹ The hour is a unit “accepted for use with the SI” (along with the minute, angular degree, nautical mile and a very few other units) but if you're going to be talking about milli- or micro-whatevers anyway it seems silly to introduce strange units arbitrarily.

Mike, I've just come across a publication about something else (straw bales, written in Dutch!)

http://www.orioarchitecten.nl/publicaties/pub_Strobouw-Praktijkhandboek-Duurzaam-Bouwen.pdf

But on p6 it includes a graph from a book by Minke that gives various mu values for earth and lime materials and similar. It's in English. Just thought you might want to add the numbers to your data set.

Hi all, first post coming up.. I’ve just stumbled across this very old thread searching for Mu values for my collection. They are so hard to get hold of! Would it be helpful if the data sources people have found are recorded somewhere?

Hi all,

I'm jumping into this conversation late, I'm flat out with compliance stuff at work so haven't had a chance to read the full thread yet but it seems very interesting.

Some will know that I work for a company that manufactures wood fibre - I've been here for 3 years and I'm still learning how the 'breathability' and sorbative qualities work within various wall build-ups.


Main takeaways - there are 4 critical aspects:


1. Internal VCL location - and attention to detail in taping and sealing.

In our standard constructions, we move the racking layer inside the timber frame. In Germany, they specify OSB3 taped and sealed to work as both vapour control and airtightness layer. That doesn't work in the UK as the OSB3 here is very low quality and made to inconsistent standards. Much as our ethos is to remove unnatural materials from our build-ups, in the UK we have to specify Durelis VapourBlock or Smartply Airtight which use plastic layers to create consistent moisture and air resistance.

No matter how good the VCL properties, how well fitted and taped, moisture will always get into the insulation layer. We've seen this even with test walls built under ideal conditions with impeccable build standards that could never be repeated in the real world. And that is before you factor in builders and homeowners screwing things into the walls and adapting the house later.

If the external wall finish allows less chance to remove moisture, (e.g. TF with stone cladding and 'ventilated' airspace) then ideally the internal VCL will actually be a 'smart' moisture-vapour variable membrane which will restrict moisture entering the insulation but still allow it to 'breathe' back into the room. This works well when paired with...


2. Sorbative insulation materials.

Glass fibre and rock wools are vapour open but not sorbative - moisture is held between the fibres rather than binding with them which offers more chance for interstitial condensation to form in cold weather in combination with poor external airtightness. Natural, Sorbative materials, wool, hemp, woodfibre, straw, cellulose etc can buffer moisture during periods of high humidity and actively release it when conditions allow. Unnatural vapour-open materials can work, but detailing and build quality is very important and relies heavily on factors 1 & 3.

There is a paper somewhere published by one of the heritage groups, based on a long-term study of woodfibre internal wall insulation in a property with a history of dampness. To cut a long story short, in the areas covered with wood fibre the remaining damp structural timbers dried out quicker than in areas left deliberately exposed. Varies probes and sensors showed that the sorbtion of the woodfibres was drawing moisture away from the wet timbers. Presumably, this would happen with other natural materials too. It wasn't part of the study, but my gut feeling is this wouldn't happen to the same extent with a vapour open but non-sorbative insulation.

(I'm sure I've got a copy saved somewhere but I'm blowed if I can find it at the moment - does anyone else know the one I'm thinking of?)


3. Vapour open outer layer or insulation. (Breathable warm construction)

In our constructions this is normally a woodfibre sheathing that is both waterproof and insulating, keeping the TF and insulation warm and reducing the opportunities for interstitial condensation as the insulation is less likely to get to temperatures where moisture can condense. Moisture can be easily and actively transported away when conditions allow.

'Traditional' UK TF construction with the racking boards outside the frame can create a vapour barrier at the coldest location.


4. External airtightness

Of the 4 this is probably the most critical. Cold air entering the insulation zone allows any moisture present to condense. I spoke to somebody at a show this year who was adamant from his experience that the dew-point could never form in the centre of insulation, (which a lot of computer models show) but is always formed on the edges exposed to wind wash. From my roofing days, I can say that I saw more signs of condensation on the tops of rafters with insulation between them than anywhere else. I remember taking tiles off in winter and finding the fibreglass had a layer of ice on the top but the rest was fine.

I disagree with my own company's main assertion on this point - their focus is on the internal racking layer providing both VCL and air tightness. I think the real reason their build-ups work so well in real-life long-term studies, (in areas with higher internal humidity and fewer opportunities for drying than the UK) is that the external woodfibre sheathing reduces wind wash and cooling on the internal insulation layer.

Our woodfibre sheathing is not specifically tested for airtightness, but the dense boards do significantly reduce airflow into the structure along with the insulating qualities mentioned previously.

As ever the edges, and intersections are always the areas of higher risk and detailing is critical - much as I'd prefer not to see plastic in the structure, if I were building with this system I would place a carefully sealed UDB membrane between the sheathing and the timber frame for peace of mind.



P.s.

Ross87 - data sheets for all of our insulations have the Mu values stated and are available on our website. Just look for the green leaf... ;-)

Dulux SD Data

Fabulous experience/info sgt_woulds - thanks.

I use OSB3 racking, as the airtight layer but sufficiently vapour open, outboard of the Warmcel-filled stud (95 or 145), but with EPS EWI (150 or 100) outboard of that, so the OSB3 is not 'at the coldest location'. V interesting that OSB3 quality-controlled for stated airtightness does exist, and UK's (Irish Smartply and Scottish Sterling) uncontrolled ditto is the aberation, not the norm. I take it that UK OSB3's moderate vapour permeability is trustable (i.e. vapour-open is Good), that it's the airtightness that's variable? (If not rendered, I also have wind barrier outboard of the EWI).

Seems it's possible to download WUFI v7 beta for free at the mo, so am hoping to find time to investigate such things, inputting variations in the OSB3's properties, see whether fatal or not.

AFAIK so far, in UK but maybe not inland Europe, internal vapour barrier e.g coated Smartply Airtight is counterproductive - free re-drying both inward and outward works much better. Yes I get interstitial condensation in the very outermost bit of the EWI, but it's harmless, well away from any timber.

Posted By: fostertomSeems it's possible to download WUFI v7 beta for free at the mo

@fostertom or @Mike1 have either of you actually managed to download the beta of WUFI v7?

Whenever I try the link to the zip file it just redirects to the homepage.

Ah, too late, never mind.

When I have the time and attention to actually get into it then I'll do things properly and apply for a trial...

Thanks for confirming.