Lime plaster properties & rain water absorption factor

[Arcanne]

Dear,

I use WUFI to simulate the behaviour of lime plastered straw bales walls in different french climate conditions. The aim is to give appropriate building systems in response of vapour diffusion and driven rains solicitations.

As component, I use :
2,5 cm lime plaster (outside)
35 cm straw bale
2,5 cm lime plaster (inside)

I use the following location: Brest (Bretagne, Nord West France, from Meteonorm Database). The exterior mean humidity never sinks below 80 % and driven rains go from 250 mm/year to 500 mm/year depending on the orientation.

With different kinds of lime plasters from WUFI database, the results are very similar: straw bale is getting wet (>90 %) almost all the year (without cladding taken into account), except for "Exterior plaster A (4 layers)" (IBP) and “Rehabilitating plaster (high porosity)” (TU Wien) which show good results. Strawbale houses were instrumented in this region, showing no humidity hazards. So I was wondering :

- What is the the “Rehabilitating plaster (high porosity)” made of?
- What is the the “Exterior plaster A” (water-repellent system) made of?
- Should I define plasters with 3 or 4 separately layers (ground layer, final layer, etc.), with each specific properties?
- Are the capillary transport coefficients defined good enough to take driven rains into account?
- Rain water absorption Factor is 0.7 for a short building: Is this value appropriate for a lime plaster wall?

Plasters that I used:
Lime mortar (TU Wien)
Hydraulic lime mortar (TU Wien)
Hydraulic lime mortar coarse (TU Wien)
Lime plaster (for salt extraction) (IBP)
Lime plaster (stucco) (IBP)

Thank you in advance for you answer

Kind Regards,
Bruno JARNO
Association ARCANNE

[Thomas]

- Are the capillary transport coefficients defined good enough to take driven rains into account?

Dear Mr. Jarno,
it is one of WUFI's main purposes to compute the water uptake due to driving rain, and it has been used successfully for this purpose in a large number of cases, so we are quite confident that WUFI is able to compute the absorption of driving rain with reasonable accuracy under normal circumstances.

- Rain water absorption Factor is 0.7 for a short building: Is this value appropriate for a lime plaster wall?

That depends on how exposed your wall is to driving rain (open or sheltered situation?), but in most cases 0.7 should be a reasonable assumption. Usually, the calculation results should not depend very much on this value anyway. (If they do, it means that your construction is a borderline case and may fail in a rain-rich year while it would be okay in a dryer year.)

- Should I define plasters with 3 or 4 separately layers (ground layer, final layer, etc.), with each specific properties?

If the material database contains data for different layers, as with the "Exterior plaster A", then the plaster should be built up from these layers. Otherwise (ordinary lime plaster, lime cement plaster etc.) one layer is sufficient.

- What is the the “Rehabilitating plaster (high porosity)” made of?
- What is the the “Exterior plaster A” (water-repellent system) made of?

I'll ask colleagues who should know...

straw bale is getting wet (>90 %)

What kind of '%' is this? Relative humidity or mass-%? And which material did you use for the straw bales? WUFI's material database has no straw bales, so you must have substituted a different material.

A straw bale wall with lime plaster has recently been investigated at our Holzkirchen test site (exposure to the copious driving rain in Holzkirchen). The investigations included moisture measurements and comparisons with WUFI calculations. I've been told that the calculations are in good agreement with the measurements (relative humidities permanently above 90% seem to be normal under these demanding climatic circumstances). I have asked for a preview copy of the report, and I will tell you how the calculations were done.

Regards,
Thomas

[Arcanne]

Dear Thomas,

I thank you for such a quick answer.

I got no doubt, that Wufi calculations are in good agreement with what measurements could be in almost all the case, but I was just wondering, whether it could work with the specificities of straw bale wall (capillary contact between plaster and straw bale) . So it’s very good news!

In my calculations, straw bale is getting wet >90 % relative humidity. In this thesis about humidity in straw bale walls: http://www.jakubwihan.com/pdf/thesis.pdf criteria for straw decomposition are assumed to be : >84% during a long period (low rate of straw decomposition) and >98% (straw decomposition), though it depends on temperature too.
The author, Jakub Wihan, used Wufi to simulate straw bale walls in response of water diffusion, so I took his straw bale properties to create the material in the database.

In Bretagne (North West France), the climate conditions give a high relative humidity level combined with mild temperatures, so it may be more unfavourable than in Holzkirchen. Anyway, I am very interested in getting the results of your investigations at Holzkirchen, or at least to know the hypothesis and materials used for calculation. Would it be possible?

I have a few additional questions about database materials:
How can I know which lime plasters in database are hydraulic limes and which one are air limes (carbonation with air)?
Is it possible to know the granulometry of the sand used in lime plaster from database, as it should have an influence on the porosity and on the capillary transport?

Thank you very much
Bruno