Hygrothermal Source & Clipping

Everything concerning hygrothermal Sources & Sinks within the assembly
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sdoggett
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Hygrothermal Source & Clipping

Post by sdoggett » Tue Mar 11, 2014 4:27 am -1100

WUFI Support Team:

Source term cut-offs at ‘free water saturation’ (default) or ‘max water content’ typically result in a high ‘unreleased moisture source’ fraction when employing the ASHRAE 1% wind-driven rain source. For example, consider an EIFS wall assembly with EPS insulation and a liquid-applied WRB over glass mat gypsum sheathing. The calculation status indicates a moisture source of 0.07 lb/ft-2 and an unreleased source of 1.235 lb/ft-2. It is assumed that the source strength is reduced if the moisture source is expected to exceed the Free Water Saturation – likewise if Wmax is selected rather than Wf.

Please elaborate on the fate of moisture designated as ‘unreleased moisture source’. Does the calculation consider, in any manner whatsoever, the ‘unreleased moisture source’ moisture transport to adjacent layers in direct contact with the WRB (i.e. EPS to the exterior; gypsum sheathing to the interior)? Or, as assumed, is the moisture source strength simply reduced and the intended 1% fraction becomes something considerably less?

I understand, that the user must determine whether the unreleased moisture affects the calculation validity. But the design assumption for the 1% wind-driven rain fraction is that: A) it is present; and B) it must therefore be considered in some manner within the one-dimensional construct, especially those where materials are in direct contact with one another where capillary transport would be in play.

In short, it appears that the cut-offs reduce the actual risks and are therefore unrealistic. Conversely, ‘no clipping’ assumes unreasonably high burdens and are similarly unrealistic. At a very minimum, these options leave the user making some very difficult assumptions that affect reliability.

Are there user-defined values for common assembly types that provide more realistic results?

Is it ever valid to assume ‘No Clipping’? Is so, what are those conditions? Does the lack of ‘diverging results’ prompt the user to move in the direction of increasing the source strength or possibly considering the use of no clipping? Alternatively, should we consider expanding the source over a wider area but still use the default value of free water saturation’?

Daniel
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Re: Hygrothermal Source & Clipping

Post by Daniel » Wed Mar 19, 2014 11:18 am -1100

Dear Sdogett,

the idea of the clipping function is the following: rain water penetrates the assembly and is offered to the materials a the interface beteween Cladding / EIFS and the substrate. If both materials provide no liquid transport (like EPS on the one side and WRB on the other side) the moisture will run off from the systems and hardly any water is absorbed.

However if you have absorptive materials like OSB, Gypsum board or whatever, the moisture in the defined source area will be absorbed until for example free saturation is reached. Afterwards the excessive water amount again will run off from the system.

So you can influence the amount of water remaining in the system by the chosen source area. Choose this area thicker including the gypsum etc. more water will be absorbed. If you limit it to the EPS or WRF hardly any water is absorbed. Maybe it could be also realistic to introduce a storage layer inbetween EPS and WRB which stores at least a thin water film which afterwords contributions to humidification by diffusion transport... However we have no predefined values for that. Try to check if you can get more information from US or Canada research institutes as the mentioned measures are rather uncommon in Europe.

best regards
Daniel
Dr.-Ing. Daniel Zirkelbach, Deputy Head of Department Hygrothermics, IBP Holzkirchen

sdoggett
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Re: Hygrothermal Source & Clipping

Post by sdoggett » Thu Mar 20, 2014 2:50 am -1100

Daniel,

Thank you for the reply. In my particular example, placing the wind-driven rain load beyond the liquid-applied WRB (into the gypsum sheathing) may not be realistic. But doing so offers results similar to "no clipping". At the same time, it is not realistic to assume that all surfaces will drain (drainable EIFS with grooved channels still has significant surface contact between the EPS and WRB/sheathing).

Can you elaborate on the potential use of a moisture storage layer? Your comment strikes at the core of what is required... the ability to have a moisture storage film that then contributes via diffusion.

"Maybe it could be also realistic to introduce a storage layer inbetween EPS and WRB which stores at least a thin water film which afterwords contributions to humidification by diffusion transport..."

To what extent can we rely on air layers to achieve this? Adhered EIFS will have a thin standoff from the sheathing (<1/8", 0.3175 cm); so we have often used this as the moisture storage layer. This seems more realistic than placing an arbitrary amount into the sheathing.

Daniel
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Re: Hygrothermal Source & Clipping

Post by Daniel » Thu Mar 20, 2014 7:38 am -1100

It is not really possible to tell you what properties would be realistic. Let's assume that on a not absorbing surface about 150 g/m² can remain befor run off. And the whole thing on both sides than you would get 300 g/m² in a gap.

You could use for example mineral fibre insulation put in the 1 % source and limit it to an amount which coressponds to 300 g/m². For a 1 mm thick layer this would mean 300 kg/m³.

I'd say this should be on the very safe side for your described situation...

Daniel
Dr.-Ing. Daniel Zirkelbach, Deputy Head of Department Hygrothermics, IBP Holzkirchen

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