justifying 'air infiltration model IBP'

[Joseph Little]

Hi
I'm involved in an expert witness case where the opposition have claimed:
'WUFI cannot calculate the effect of 5% of a good quality vapour control membrane being missed or any other percentage other than 0% or 100%. Gaps and tears etc are localised and therefore different from reducing the vapour resistance of a membrane by 5% or another chosen percentage.

I am claiming that the 'air infiltration model IBP' (which we have used in our simulations) does allow you get a sense of discontinuities in the vapour control membrane (I don't know why the opposition have mentioned 5% of a gap specifically).

How would you explain the rigour and validity of the 'air infiltration model IBP'?
Many thanks.

[Daniel]

Hi Joseph,

the air infiltration model is primarily related to the moisture entry into an assembly which has no "damage leakges" but a normal air tightness level. No building is perfectly tight and the remaining small impferfections are covered by the three air tightness levels A (passive house level) B (air tight building accoriding to German requirement) and C (air tight but not tested by blower door - approx. practice level in Germany today).

More details can be found in the online help or in the following publicatins:
English: http://www.brikbase.org/sites/default/f ... uenzel.pdf
German: http://www.wufi.de/ibp/publikationen/ko ... r_germ.pdf

Of course it is also possible to use the model to consider higher air permeabilities. Therefore you simply increase the Q50 value (assembly permeability). The problem is, that there is no clear relation between the % of missing vapor retarder surface and the component permeability. If the vapor retarder is partially missing - but the OSB or gypsum board is rather air thight, this situation could remain unproblematic. If you get strong air flow through a small hole it could cause problems... That's why we relate the infiltration rate to the compomenent permeability which depends on several influencing factors and should be mesured in case of doubt...

best regards
Daniel

[fostertom]

At recent Training, I understood Christian to say that this whole infiltration/air permeabilty question was of great importance, that it is being researched and worked upon by Fraunhofer, that new more quantifiable approaches are being designed to replace or supplement the preliminary IBP approximation, and that all this will be the next big line of development for WUFI.

Did I get that right, or is it just wishful dreaming by me?!

[Daniel]

Dear Fostertom,

at the moment we cannot do intensive research on this topic. The model is based on lab test in the US as described in the publications. Also comparisons to field tests showed good agreement between measurement and simulation.

So the model is definitely not preliminary - but its and engenieer model to improve the moisture safety of lightweight structures whith an approach which should be slightly on the safe side concerning the moisture amount for the structures on the three different air tight levels. The German wood protections standard DIN 68800 requires either to use constant values to consider for infiltration moisture (100 resp. 250 g/m²a) or to use a transient model like the one in WUFI. Also in the the new WTA guideline 6.2 for simulation of wooden structures this approach is implemented.

So at the moment we are quite confident with the model. But of course further field tests are always welcome for additional validation and maybe refinement of the values.

best regards
Daniel

[fostertom]

Well, the IPC model may correspond to measured reality under assumed 'typical' conditions, but I have everyday, non-exotic conditions for which it cannot be right.

AFAIK, IPC looks at

1) small all-over leaks in the air barrier due to joints (e.g. taped seams) and fixings (e.g. staple holes);
2) larger spot-leaks around windows etc;
3) the air permeance of the body of the air barrier itself.

I'll bet that IPC assumes that 'typically' in all of Air Tightness Levels A, B and C, 1) and 2) will be dominant and 3) will be very small, assuming a commercial air membrane. So IPC largely discounts 3) and assumes a 'typical' ratio between 1) and 2). Of these, concentrated 2) is the 'heat leakage' one, and dispersed 1) is the critical one, of interest in WUFI, as far as 'moisture-risk leakage' is concerned;

But when I can be confident that

1) there are negligible small all-over leaks in the air barrier due to joints (e.g. taped seams) and fixings (e.g. staple holes);
2) there is a 'typical' ration of larger spot-leaks around windows etc;
3) but that the body of the air barrier itself has moderate air permeance,

then Air Tightness Levels A, B or C bear v little correlation to 3) above. I am now interested not in 1), but in 3), as the critical one, of interest in WUFI, as far as 'moisture-risk leakage' is concerned. But IPC does not gauge 3), as the critical leakage type, because its assumes 3) is very small.

The common construction that I have in mind, uses OSB as air barrier, gapfilling glued and screwed to studs etc at all joints. I can be confident that 1) is negligible, and I need to know the air permeance of OSB, as 3). There is almost no data on the latter, and probably little quality control of air permeance between batches of OSB. Yet there are reports of excellent air tightness using this easy-to-do, foolproof, long-term robust method of air tightness.

I want to see WUFI taking 3 direct, separate inputs, of

a) materials' air permeance data 3), and
researched criteria for separate input of grades of
b) window etc leakage 2), and
c) staple-hole etc leakage 1).

I don't want to have to rely on an approximation, based on 'typical' assumptions, that Air Tightness Levels A, B or C correlate with WUFI-critical 'moisture-risk leakage'.

Who better than Fraunhofer to start research and modeling on that basis?

Meanwhile, surely all new materials tests should include air permeance? Starting with Smartply OSB, currently under test, I believe!

[Daniel]

We speak about a 1D model - of course it must assume a typical ratio between 1) and 2). There is no other possibility to do that. However in practice this is also the relevant proceeding. If you have bigger leakages you should seal and not simulate them!

The air permeability of the air barrier itself is of little relevance and can be covered by small standard amounts of infiltration moisture (let's say max. 50 g/m²) - this is implicitly considered in the model.

No manufacutrer of other materals will be willing to pay for air permeance mesurements.

However: all your points could also be of interest. If you can find some funding possibility for more detailed research on these points this is very welcome, and we can start immediately!

best regards
Daniel

[fostertom]

Daniel, perhaps this seems like a minority concern, but many designers and builders want to move away from reliance on mainstream plastic membranes and tapes, even (or particularly) 'intelligent' ones, and rely, principally, on OSB or similar, instead:

there are reports of excellent air tightness using this easy-to-do, foolproof, long-term robust method of air tightness

This http://www.linkedin.com/groups/OSB-is-n ... S.60928033 might be of interest.

This growing approach does demand development of the WUFI Infiltration Model, and data on air permeance. You doubt that anyone would pay for air permeance tests, but this company
http://www.spanotech.be/sites/default/f ... %20res.pdf#
has invested in being able to provide just this assurance, and I know other OSB manufacturers are thinking the same way.