Materials' air permeance data?

[fostertom]

I don't see 'air permeance' or perhaps 'air diffusion resistance factor' in the materials data - is that correct?

I understand it now seems that moisture transport by bulk air leakage through the layers is so significant that it can dwarf moisture diffusion, capillary action etc.
And that the next stage for WUFI is to model such air leakage rather than just inserting arbitrary 'sources'.
I'd think that with data for each material's air permeance, air leakage driven by wind pressure and stack effect could be quite readily modeled, leaving only the joints to be estimated by a 'source' or similar.

I typically specify OSB with all joints glued and screwed, as air barrier layer. It makes a very robust, easy to get right barrier, located in the middle of the wall build-up, so well protected from later puncturing. Reports from Ireland show excellent whole-building airtightness using this method. But I would be glad to see tested confirmation of both the OSB's air p[ermeance, and of the airtightness of glued and screwed joints.

I understand Fraunhofer is currently testing Irish Smartply OSB, which I specify by preference. Is Smartply being tested for air permeance? If not, could that be added to the current tests?

[Daniel]

Air permeance of the materials itself is not included to the simulation. The permeance of materials like gypsum board, OSB etc. is very low and the resulting moisutre entry on this way should be negligably low. Thus they are suitable as air barriers...

However this moisture entry is included in the air infiltration model which you can use under moisture source. It takes account for the air flow through all moisture relevant leakages depending on the whole building envelope air tightness, the temperature caused pressure differences and the potential flow path defined by the user.

Concerning measurements of specific materials please contact the companies directly.

Daniel

[fostertom]

The permeance of materials like gypsum board, OSB etc. is very low and the resulting moisutre entry on this way should be negligably low. Thus they are suitable as air barriers.

That is interesting, because there is much doubt and controversy about whether for example OSB is an adequate airtight barrier (the board itself, not the joints), and people saying it least has to be 18mm OSB not 9mm.

For example during blower door tests people have taped a piece of polythene to an OSB 'wall' and have seen the polythene balloon slowly outward due to air being pressured right through the OSB. Over large surface areas, that kind of weak airtightness can seriously degrade a building's airtightness figure.

We are told that Fraunhofer research is showing that this kind of all-over slow leakage transports much water vapour into a wall's insulating layers. We are told that a Fraunhofer priority is to find ways to predict and quantify such moisture transport (alongside diffusion etc), instead of just inserting a guessed air leakage or moisture source.

That will surely include figures for the air permeance of the materials, as well as the joints, so I would expect Fraunhofer to be gathering that data for materials being currently tested.

[Daniel]

It is an impossible attempt to simulate the real multidemsional air flow through imperfections of strucutures.

The IBP air infiltration model approach ist transient and quantifies the moisture entry depending on the different factors
- air tightness of the envelope
- pressere differences accoriding to the hourly climate conditions
- temperature level in the structure at the relevant positions.

The amount of moisture is based on measurements by Lstiburek in the US like I mentioned before.

If you find a more accurate model or more detailed investigations - this information is very welcome. But in fact hardly any studies worldwide deal with moisture leakages - and from my point of view the model helps a lot to ensure a sufficient drying potential. Three comparisons with field tests in the past three years showed very good agreement between the mesured and modeled moisture conditions including air infiltration.

Concerning OSB you're right that a too thin board can provide holes through the complete board thickness.

Daniel

[fostertom]

The IBP air infiltration model approach ist transient and quantifies the moisture entry depending on the different factors
- air tightness of the envelope
- pressere differences accoriding to the hourly climate conditions
- temperature level in the structure at the relevant positions

Yes, and seems a good approach, except for one thing. Using 'air tightness of the envelope' gives no clue about where the leakage is.

The envelope leakage might be harmlessly through window seals, not at all endangering the wall sandwich, yet we're assuming that there's a little or a lot of moisture-laden air being transported through the entire wall sandwich area.

Or the leakage might be through one area of the walls, with a particular construction, but not at all through another section. One of those constructions might be at risk from such air/moisture transport but the other much more robust and immune - and we don't know which case it is.

Or assuming that the leakage is slight because evenly spread, we might miss that it's all concentrated in one endangered area.

Is it possible, with the IBP model, to make judgements as to how the total 'air tightness of the envelope' is distributed amongst
a) harmless window seals
b) joints between panels, nail holes, faulty tapes etc
c) the panels themselves?

Concerning OSB you're right that a too thin board can provide holes through the complete board thickness

So as well as judgements about a) and b) it would be very helpful to have tested data about c). For example, the moderate but useful air resistivity of blown-in cellulose fibre, at different densities, and allowing for the way it fills all gaps and odd spaces most effectively (assuming steps taken to prevent it slumping in time).

[Daniel]

Please check here:
http://www.brikbase.org/sites/default/f ... uenzel.pdf