Coatings and cavities

Everything concerning the adequate application of WUFI in Ireland & UK depending on local conditions e.g. architecture, building codes, standardization, laws et cetera.
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Coatings and cavities

Post by paulprice » Mon Jan 24, 2011 7:12 am -1100

Hi Joseph and Calina,

Playing around with the wufi parameters makes for interesting results but being confident that the parameters are correct is a challenge even if they are available.

As clear in "Breaking the Mould V" and from course, driving rain penetration is key to weather a wall fails. However, finding sd values can be difficult and estimating adhering fraction of rain is not given as an option for sealed or painted surfaces.

What values for sd and a.f.r. are sensible for coatings such as paint?

I found this article for built up layers of paint saying sd of acrylic latex is about 0.6m and siloxane is about 0.2m. At least I think that is what it says! ... 63-68).pdf

Also in cavity walls what level of air change is sensible to assign. I also wonder how to model the very low density polystyrene which is now in our filled cavity. Are there any figures for cavity fill EPS? I think there must be some convection/air change as well?

Thanks, Paul

Calina Ferraro
WUFI International Support Team
WUFI International Support Team
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Post by Calina Ferraro » Thu Feb 03, 2011 10:56 pm -1100

Hi Paul,

In response to your questions, I agree that it is difficult to find concise information.

For any information regarding material properties, your first approach should always be to contact the manufacturer and see if they can provide the information to you. It's likely they can't, but it's still worth checking and at least it will alert them that this is something they should be testing their product for. In lieu of manufacturer's data, the next best option is to look for research papers (which it sounds like you've done).

Paints, Coatings and Sd-values
I checked the paper you referenced and yes, depending on the number of coatings of paint, it appears the Sd value for acrylic is 0.4-0.8m, and for siloxan is 0.16-0.28m.

Here is another article:, which doesn't include siloxan, but agrees with the range for acrylic and silicate in the article you found.

Generally, the same advice applies here as with any other unknown value. Try both extremes of the range. If your build-up succeeds with both, you’re fine. If it fails at one end or the other, it may too risky anyway. Remember the Sd-value is dependent on the thickness of the layer, and therefore very dependent on the workmanship as well as the material properties.

The Sd value relates to vapour diffusion only. To simulate the affect of liquid moisture transports you have to look at other values:

Adhering Fraction of Rain
Note: this value was previously called the “Rain reduction factor,” so if you’re looking for it in the WUFI help file, try that name.

The adhering fraction of rain value refers to how much of the driving rain hits the surface and is available for absorption (i.e. how much hits the surface and does not splash off). This is marginally related to the surface structure, but more strongly related to the orientation or shielding of the surface.

Absorption Coefficient
To simulate the effect of impregnation or coatings on rainwater absorption, it’s better to change the absorption coefficient (A-Value) of the outer layer of wall. For a masonry wall, you should create a thin layer of the masonry material at the exterior surface representative of the depth of penetration (~1-10mm).

There is very little information that I could find for representative A-values of impregnation. There is one response to another question on this forum ( ... drophob%2A) which suggests a value of 1 kg/m^2/sqrt(hr) is a representative number to see the effect of impregnation/hydrophobization. The units must be converted to SI (divide by 60: 1/60 = 0.01667 kg/m^2/sqrt(s)) before entering it into WUFI.

Cavity Ventilation
Cavity ventilation is another area where research has been done, but it’s difficult to get exact figures because it depends on a number of different factors. We have a research paper from Canada that suggests cavity ventilation airflow is on the order of 5-25 ach and another one from New Zealand that is showing up to ~50ach.

We have yet to find one that measures the air flow if this cavity is filled with blown bead, however, it would clearly be significantly less than these.

The questions you’re asking are difficult to answer. The values you are looking for are currently being tested and researched around the world so there is limited information available. If the Fraunhofer Institute has done specific research or has further comments on any of these items, perhaps they can add a response to this thread with their input.


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Post by paulprice » Fri Feb 04, 2011 11:02 am -1100

Hi Calina,

Many thanks for taking the time to reply in detail to the tough questions.

Good to know the values for coatings now. (BTW that link to the other article does not seem to work for me.) Trying extremes of range makes sense to see what happens with leaks.

Will try the A-value adjustment and see what happens. Had not realised you could model the outer layer of wall in that way.

Thanks again, Paul

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Re: Coatings and cavities

Post by mattclubb » Mon Dec 05, 2022 12:23 am -1100

12 years later, I am also looking for an answer to the question of Cavity Fill EPS Beads.

Has there been any further analysis on this topic?


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